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General Electronics Measurement

32 Posts authored by: StevenLee Employee

What is a floating power supply output?

First let me tell you that a floating power supply output is NOT what is shown below in Figure 1 (haha).


Now some background: "earth ground" is the voltage potential of the earth. To greatly reduce the risk of subjecting a person to an electrical shock, the outer covering (chassis) of most electrical devices is internally connected to a wire that is connected to earth ground. Most devices connect to earth ground through their power cord. The idea here is to ensure that all surfaces a person can touch are at the same voltage potential - earth ground.

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As long as that is true, the person can freely touch things without the risk of getting shocked due to two of the things he touches at the same time being at different voltage potentials, or one of the things being at a high voltage potential with respect to the earth. If the voltage difference is high enough, the person could be shocked.
Earth grounding the chassis also protects the user if there is an internal problem with an electrical device causing its chassis to inadvertently come in contact with an internal high voltage wire. Since the chassis is earth grounded, an internal short to the chassis is really a short to ground and will blow a fuse or trip a circuit breaker to protect the user instead of putting the chassis at the high voltage. If you touched a chassis that had a high voltage with respect to ground on it, your body completes the path to ground and you get shocked!


So to protect the user (and for some other reasons), the chassis of Agilent power supplies are grounded internally through the ground wire (the third wire) in the AC input line cord. Additionally, most if not all of our Agilent power supplies have isolated (floating) outputs. That means that neither the positive output terminal nor the negative output terminal is connected to earth (chassis) ground. See Figure 2.


Figure 3 shows an example of non-floating outputs with the negative output terminal grounded.
For floating DC power supplies, the voltage potential appears from the positive output terminal to the negative output terminal. There is no voltage potential (at least, none with any power behind it) from either the positive terminal to earth ground or from the negative output terminal to earth ground. A power supply with a floating output is more flexible since, if desired, either the positive or negative terminal (or neither) can be connected to earth ground. Some devices under test (DUT) have a DC input with either the positive or negative input terminal connected to earth ground. If one of the power supply outputs was also internally connected to earth ground, when connected to the DUT, it could short out the power supply output. So power supplies with floating output terminals (no connections to earth ground) are more versatile.
If the outputs are floating from earth ground, we need to specify how far above or below earth ground you can float the output terminals. Our power supply documentation provides this information. For example, most Agilent power supply output terminals can float to +/-240 Vdc off of ground. You will frequently see the following in our documentation:
Also, some power supplies have different float ratings for the positive and negative output terminals. For example, for Agilent N5700 models rated for more than 60 Vdc, the following note in the manual means you can float the positive output terminal up to +/-600 Vdc from ground or the negative output terminal up to +/-400 Vdc from ground:
The output characteristic table may list this as “Output Terminal Isolation” as shown below which means the same thing as maximum float voltage:
Figure 4 shows an example of floating a power supply to 200 V above ground. The power supply output is set to 40 V.
You can see from the last example that you have to take the power supply output voltage into consideration when ensuring you are not violating the float voltage rating. If you exceed the float voltage rating of the power supply, you are potentially exceeding the voltage rating of internal parts that could cause the internal parts to fail or break down and present a shock hazard, so don’t violate the float voltage rating!

A two-quadrant power supply is traditionally one that outputs unipolar voltage but is able to both source as well as sink current. For a positive polarity power source, when sourcing current it is operating in quadrant 1 as a conventional power source. When sinking current it is operating in quadrant 2 as an electronic load. Conversely, a negative polarity two-quadrant  power source operates in quadrants three and four. Often a number of questions come up when explaining two-quadrant power supply operation, including:

  • What does it take to get the power supply operating as a voltage source to cross over from sourcing to sinking current?
  • What effect does crossing over from sourcing to sinking current have on the power supply’s output?



For a two-quadrant voltage source to be able to operate in the second quadrant as an electronic load, the device it is normally powering must also be able to source current and power as well as normally draw current and power. Such an arrangement is depicted in Figure 1, where the device is normally a load, represented by a resistance, but also has a charging circuit, represented by a switch and a voltage source with current-limiting series resistance.




Figure 1: Voltage source and example load device arrangement for two-quadrant operation.


There is no particular control on a two-quadrant power supply that one has to change to get it to transition from sourcing current and power to sinking current and power from the device it is normally powering. It is simply when the source voltage is greater than the device’s voltage then the voltage source will be operating in quadrant one sourcing power and when the source voltage is less than the device’s voltage the voltage source will be operating in quadrant two as an electronic load. In figure 1, during charging the load device can source current back out of its input power terminals as long as the charger’s current-limited voltage is greater than the source voltage.


It is assumed that load device’s load and charge currents are lower than the positive and negative current limits of the voltage source so that the voltage source always remains in constant voltage (CV) operation. A step change in current is the most demanding from a transient standpoint, but as the voltage source is always in its constant voltage mode it handle the transition well as its voltage control amplifier is always in control. This is in stark contrast to a mode cross over between voltage and current where different control amplifiers need to exchange control of the power supply’s output. In this later case there can be a large transient while changing modes. There is a specification given on voltage sources which quantifies the impact one should expect to see from a step change in current going from sourcing current to sinking current, which is its transient voltage response.  A transient voltage response measurement was taken on an N6781A two-quadrant DC source, stepping the load from 0.1 amps to 1.5 amps, roughly 50% of its rated output current.



Figure 2: Keysight N6781A transient voltage response measurement for 0.1A to 1.5A load step


However, the transient voltage response shown in Figure 2 was just for sourcing current. With a well-designed two-quadrant voltage source the transient voltage response should be virtually unchanged for any step change in current load, as long as it falls within the voltage source’s current range.  The transient voltage response for an N6781A was again capture in Figure 3, but now for stepping the load between -0.7A and +0.7A.




Figure 3: Keysight N6781A transient voltage response measurement for -0.7A to +0.7A load step


As can be seen in Figures 2 and 3 the voltage transient response for the N6781A remained unchanged regardless of whether the stepped load current was all positive or swung between positive and negative (sourcing and sinking).


While the transient voltage response addresses the dynamic current loading on the voltage source there is another specification that addresses the static current loading characteristic, which is the DC load regulation or load effect.  This is a very small effect on the order of 0.01% output change for many voltage sources. For example, for the N6781A the load effect in its 6 volt range is 400 microvolts for any load change. In the case of the N6781A being tested here the DC change was the same for both the 0.1 to 1.5 amp step and the -0.7 to +0.7 amp step change.

There are two more scenarios which will cause a two-quadrant power supply transition between current sourcing and sinking.  The first is very similar to above with the two-quadrant power supply operating in constant voltage (CV) mode, but instead of the DUT changing, the power supply changes its voltage level instead.  The final scenario is having the two-quadrant power supply operating in constant current with the DUT being a suitable voltage source that is able to source and sink power as well, like a battery for example. Here the two-quadrant power supply can be programmed to change from a positive current setting to a negative current setting, thus transitioning between sourcing and sinking current again, and its current regulating performance is now a consideration.  Both good topics for future postings!

Typically, power supply users require only positive voltage for their testing as opposed to negative voltage. However, there are instances where you may need negative voltage from a power supply. Driving devices such as operational amplifier and transistors will sometimes require negative voltage biases. It is important to keep in mind that when we use terms like positive voltage and negative voltage, they are relative to a reference point. Voltage cannot be defined in absolute terms but instead is measured relative to a voltage reference.


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Most power supplies will not let you program a negative voltage value. This means that if you need a negative voltage, you would have to program a positive value and reverse the output to achieve the voltage that you need. This could be a bit troublesome especially if you are using a power supply to frequently power devices require both positive and negative voltage values. Keysights new E36300 quickly and easily solves this problem.


You can create up to +25 volt and -25 volt DC outputs using Channels 2 and 3 of Keysights new E36312A or E36313A bench power supplies. They can also be configured to track each other.


All three outputs of the E36312A and E36313A bench power supplies are electrically isolated from earth ground.  By connecting the output terminals as shown in the diagram below, you will use Channel 2 as your positive voltage source and Channel 3 as the negative voltage source. In the figure below, we provided an example of +18V and - 18V DC supplies.



You can use the front panel voltage knob or numeric keypad to adjust the output voltage of Channels 2 and 3.


If you need symmetrical output voltages, you can use the track mode feature, so that both DC outputs track each other.  Then, you will only need to program Channel 2 to 18V and Channel 3 will automatically be set to 18V.  Enable tracking mode by pressing Output Settings > Operation Mode >  Mode Tracking.     


Note that the front panel meter will always read positive voltage and current values for Channels 2 and 3. 


For other tips, read our application note Speed up Your Test with an Upgraded Bench Power Supply.


Our "Power Up Your Bench Contest" Week #6 winner is Jon Snowman.


Here is Jon's story:



I’m an engineer who was inspired at high school to learn electronics, and I have continued it as a hobby ever since. I am therefore immensely passionate about helping schoolchildren become as engaged and excited about science, technology, engineering and maths (STEM) subjects as I am, and to encourage them to take them up at university and in their future careers.



Engaging schoolchildren with STEM subjects is considered difficult, since they can appear too academic with too few practical elements and demonstrations. To address this, I began designing and building VertigoIMU - a compact inertial and GPS datalogger which gives physics students previously impossible insight into physical systems. From the ground up, I have captured the schematic, designed the PCB and fabricated prototypes to be tested by the school.


VertigoIMU comprises a 9 degree of freedom IMU (3 axes of each of acceleration, gyroscope and compass), a high precision 10Hz GPS unit, barometric pressure, humidity and temperature. Data is logged to a microSD card for analysis.



Some examples of where we have successfully deployed VertigoIMU:

• On a rotating bicycle wheel to demonstrate the equations of circular motion such as centripetal acceleration (a = w^2 x r).

• On the GreenPower competition vehicle – an electric vehicle built by the students – to examine the lateral forces on the wheels to inform decisions about tyres. A plot of acceleration whilst being driven in a circle is shown below.




Next Steps

VertigoIMU prototypes are being tested at the school with great success. However, the principal complaint is that battery life is not long enough. Since the GPS must maintain a lock before datalogging commences, and between datalogging runs, an optimised ‘standby’ mode is required, in which the GPS retains lock and all sensors are initialised and ready.


Longer battery life is essential so that students can capture exciting data with VertigoIMU, especially applications in which the unit must be powered up and achieve GPS lock for a long period before datalogging commences. Some examples include:

• Roller coasters – the students are planning a visit to a local theme park

• Flight analysis – capturing the angle of attack of a BASE jumper



How the E36312A would help

This power optimisation would benefit hugely from an E36312A power supply, as I am currently using lithium polymer batteries only. The main reasons are:


1. High precision (<20mA) current readback mode. This would allow me to quantify and optimise the standby power consumption. This is not possible with standard bench power supplies as they typically have current precisions of around 10mA.


2. Overvoltage protection (OVP) and overcurrent protection (OCP). Short circuiting or over-charging/discharging of lithium batteries can be dangerous. The safe OCP/OVP modes of the E36312A would enable safer working in my home lab.


3. Programmable. This would permit me to simulate the discharge curve of a battery to understand details such as the total run time of the device and calibrate the battery gauge.


4. Multiple channels. This would accelerate develop



Congratulations Jon.  We are sending you our branding new E36312A!


Don't miss out.  Submit your entries now to win our brand new E36312A Triple Output DC Power Supply!


Go to for more details. #PowerUpYourBench

Our "Power Up Your Bench Contest" Week #5 winner is Rafael Souza. See a video of his story here

Congratulations Rafael.  We are sending you our branding new E36312A!


Don't miss out.  Submit your entries now to win our brand new E36312A Triple Output DC Power Supply!


Go to for more details. #PowerUpYourBench

Our "Power Up Your Bench Contest" Week #4 winner is Martin Glunz from Germany


Here is Martin's story...


1. Purpose of this document Entry to Keysight

"Power Up Your Bench” contest


2. Who am I

My name is Martin Glunz, a professional electronics engineer living in Germany. Not only I’m working for a company that is making industrial electronics as a R&D engineer, I’m doing some engineering at home. One of my goals is to reduce my home’s standby power consumption. I’ve done some research within this topic in the past, one of my projects was a “zero standby power” supply for the doorbell installation, done in a quite unusal way using the standard off-the-shelf doorbell transformer.


3. What do I want to achieve with this power supply

That’s quite easy to say: Testing the power consumption and efficiency of my circuits to save more energy. This power supply has a quite accurate voltage and current readback according to the data sheet, so this saves me the usage of two multimeters in the first place. Using a standard bench supply, there’s always the necessity to use external precision meters if one needs accurate results. My main strategy to save power is to use a intermediate DC bus (running at 19V DC) to provide power to the lot of standby boxes (like internet router, NAS box, ...). Those require various supply voltages from 5V DC to 15V DC. I’m currently using a variety of home made DC/DC converters to convert the 19V DC bus to the required voltages. Since the 19V DC bus must be supplied from the mains power, there’s a centralized power supply unit using redundant sources. To achieve my main goal, the efficiency of the involved power supplies (mains power to 19V DC, 19V DC to whatever is required) is very important to know and to be able to optimize. Your typical wall wart or power brick supply (from mains to 5V / 12V DC) in the low watts range (2W ... 20W) has still a not so good efficiency, even regarding the latest energy star regulations. So it is possible to achieve better total efficiency using the 19V DC bus approach. I’m using comercially available 19V DC supplies with good effiency here, but I’m building my own DC/DC converters to provide the local lower voltages. To achieve better total efficiency than the standard off-line power supply, these must have efficiency better than 95%. Evaluating the efficiency of such a DC/DC converter isn’t a simple task if one needs accurate results. Moreover, variation of input voltage and output load have influence on the efficiency, and the DC/DC circuit must be optimized for the typical load.


4. Where does this supply help me here


4.1. Power consumption of devices All of the devices I plan to optimize fall into the available output power range of the E36312A supply. Typically the range is from 1W at 5V DC to 20W at 12V DC. The accurate voltage and current readback together with the logging and exporting capabilities will be a great help to find out the typical power supply requirements of my devices. The supply promises to provide an easy way to make measurement of consumed DC power over time.


4.2. Efficiency of my converters Second, after knowing the power requirement of a device, I can go for the optimization of the DC/DC converter circuit. Doing so requires a precise measurement of input and output power of the converter, and taking measurements at varying input voltage and load conditions.


4.2.1. Input power measurement done by the supply The E36312A supply eases the task of measuring the input power to the DC/DC by using the precise readback, remote sensing and logging. 


4.2.2. Input voltage variation done by the supply An additional useful feature is the sequencing / ramping capability: This will be used to semiautomatically measure input power over input voltage at constant load.


4.2.3. Save a voltmeter In case one output channel provides enough power for the DC/DC converter, a second channel would be used to read back the output voltage of the DC/DC, with current limit set to zero (or a near zero voltage). Now one can read back the output voltage of the DC/DC using one channel of the power supply, only one additional amp meter will be required to read the DC/DC output current and finally calculate the efficiency.


4.2.4. Output load variation done by the supply Imagine now, three of four measurements are done by the bench power supply unit at constant DC/DC output load, and only one measurement left: efficiency over load variation. If one uses a constant current sink or a simple power resistor as the load to the DC/DC converter, and has the DC/DC converters output connected to the bench power supply to read back the output voltage, one could also supply some current (in CC mode) from the bench power supply into the load. This reduces the current drawn from the DC/DC output by the amount supplied from the bench supply. Now it is possible to evaluate (and semi-automate by using the remote control capabilities) the efficiency of my (or anyone elses) DC/DC converters by using just three components: The E36312A bench power suppy, a precise ampmeter and a suitable load resistor / current sink.


Congratulations Martin.  We are sending you our branding new E36312A!


Don't miss out.  Submit your entries now to win our brand new E36312A Triple Output DC Power Supply!


Go to for more details. #PowerUpYourBench

Our "Power Up Your Bench Contest" Week #3 winner is John Hubert.


Here is John's story:

I work for a RF transmitter company called Nautel Ltd., my time is spent doing customer service testing and repairing of circuit boards. I personally feel the image included says a lot but I can say that this power supply would go a long way to improving my bench space. Since I work with RF many of our systems require multiple power supplies of different voltage levels. Most of these boards are very sensitive to voltage noise as they create reference voltages which cause me untold grief with some of these poorly regulated noisy supplies. The data logging features would greatly increase the information available to help me troubleshoot some issues. I also find myself having to simulate fault conditions and inputs; the sequencing and list features would greatly improve my ability to quickly test boards with complex inputs, as manually adjusting voltages usually cause timing problems. I have been trying to get my department head to purchase some of these units and I feel if I could show the company the quality of life and performance improvements built into your device we might be able to justify purchasing new equipment. A while ago we acquired an EXA spectrum analyzer 9010B and it has been a rock solid piece of equipment and a joy to use, it has become the favorite to use by many my fellow co-workers. I thank you for your time and consideration. 


Congratulations John.  We are sending you our branding new E36312A!


Don't miss out.  Submit your entries now to win our brand new E36312A Triple Output DC Power Supply!

There may be times when you need when more current than your DC bench power supply can provide. In these situations, there are traditionally two ways to go. You could add another similar power supply and parallel the two outputs together for higher power. You could also find a totally different power supply thats rated for enough power to satisfy your testing needs. Both methods require the hassle of searching for another power supply unit to meet the requirements of the test. An ideal solution would be a power supply which allows you to get that higher power within the capabilities of the same box.

Which is why you may be interested to meet the new E36312A and E36313A bench power supplies from Keysight Technologies. You can use the built-in auto-parallel mode available in the new E36312A and E36313A bench power supplies to get double the power than you would otherwise get from a single output eliminating the hassle of looking for a different power supply all together.


What enables this is a feature called auto-parallel mode. Auto-parallel mode uses built-in relays to combine channels 2 and 3 into a single higher current DC output channel rated for 25V and 4A. The readback system measures as a single channel too!


You can enable auto-parallel in two easy steps:

Step 1: Press the Output Settings button

Step 2: Press the Mode Parallel button


Auto-parallel front panel output connections are shown on the large graphics display. In figure 1, Channel 2 (highlighted in green) is connected to the load. The display shows that Channel 2 is now rated for 25V and 4A, while the output connectors on Channel 3 (highlighted in blue) are disabled.




Figure 1. Setting up auto-parallel mode.


A 4-wire operation can be easily configured using the Channel 2 rear panel output connector.  Note that 4-wire, or remote sensing, improves the voltage regulation at the load by monitoring the voltage at the load rather than at the output terminals. This automatically compensates for the voltage drop in the load leads, which is especially useful for CV operation with load impedance that varies or has significant lead resistance. Activate 4-wire remote voltage sensing by pressing Source Settings > Sense 4w buttons.


The E36300 series bench power supplies have a large, crisp display that lets you see all three output channels at the same time. By toggling the meter key button, you can drill down for more detailed information on each channel. The channels and the display are color coded so you can easily track each channel.


By selecting Channel 2 (highlighted in green) and toggling the meter key button, you will see more detail on Channel 2 settings and measurements.


In figure 2, the power supply was set for 12V, and the current limit default is set to 4A. The power supply read back (measurement system) is displaying, 12.000V, 3.452A, and 41.420 W.  You will notice that Channel 3 (dark blue color) is now blanked out and showing that it is in connected in parallel with Channel 2.

Figure 2. Auto parallel mode example.


You can enter or change the voltage and current limit settings by.

  1. Using the front panel voltage knob. The output voltage will change when the knob is turned. You can easily see this on the display.
  2. Using the numeric entry field in the meter display, if you know the exact voltage value that you want. Use the navigation keys to select the field (it will be highlighted) and use the numeric entry keys and enter the value.
  3. Pressing the Source Settings key and using the navigation keys to highlight the voltage field. Enter the voltage value using the numeric keys. You can use the voltage knob to adjust the value in the voltage field as well.


We hope this helps you get higher current capability on demand, using auto-parallel mode. For other tips, read our application note Speed up Your Test with an Upgraded Bench Power Supply.

Just tell us how a new Keysight power supply will help you “Power Up Your Bench”.
Go to the website: and upload a story, picture or video today!
There will be 22 new power supplies awarded over the next 22 weeks! Each winner will receive a new E36312A with GPIB (valued at $1550 U.S. list price each) from August 28, 2017 through January 26, 2018. A panel of judges will select the best entries based on innovation and creativity, uniqueness of benefits and clarity of presentation. Multiple entries allowed!

Get more from your power source with the surprising capabilities of Keysight’s new E36300 series triple output power supplies
- more confidence with accurate programming/readback and low output ripple/noise
- more convenience with 5.4 inch color LCD, USB/LAN/GPIB and front and rear connections
- more capability with low range current measurement, data logging and auto-series/parallel connections

How can I get more power from my power supplies?

If you need more voltage than one of your power supply outputs can provide, you can put power supply outputs in series to increase the total voltage. If you need more current than one of your power supply outputs can provide, you can put power supply outputs in parallel to increase the total current. However, you do have to take some precautions with series or parallel configurations in a multiple output power supply.


Precautions for series connections for higher voltage:

  • Never exceed the floating voltage rating (output terminal isolation) of any of the outputs
  • Never subject any of the power supply outputs to a reverse voltage
  • Connect in series only outputs that have identical voltage and current ratings

Precautions for parallel connections for higher current:

  • In most applications, one output must operate in constant voltage (CV) mode and the other(s) in constant current (CC) mode
  • In most applications, the load on the output must draw enough current to keep the CC output(s) in CC mode
  • Connect in parallel only outputs that have identical voltage and current ratings

You can use remote sensing with either a series or parallel configuration. Figure 1 shows remote sensing for series outputs and Figure 2 shows remote sensing for parallel outputs.

On our E36300 series multiple output power supplies, configuring series and parallel output configurations is simple. With a single button setup, the E36312A and E36313A can be set to series or parallel mode to double the output voltage (up to 50V) or current (up to 4 A), respectively.  The setting is done through the front-panel display with graphical user interface instructions.


Once set up, you can control the combined channels as a single output and use them to measure as a single channel. You save time by eliminating the need for external wiring between channels for the connection. 


You can find more information about power supply series and parallel configurations in an Agilent power supply document called “Ten Fundamentals You Need to Know About Your DC Power Supply


Refer to tip number 4. This document also covers nine other useful power supply fundamentals.



To learn more, see bench power supply testing

New 80-W and 160-W programmable DC bench power supplies provide best-in-class ease of use, modern connections and multi-channel display

  • Simultaneously displays all three color-coded channels on a 4.3” color LCD 
  •  Provides industry-leading output stability under extreme, dynamic load conditions
  •  Enables data logging plus output sequencing and coupling

Meet the new E36300 Series triple-output programmable DC power supplies from Keysight Technologies. With a large color display, intuitive user interface, modern device connections via LAN (LXI, USB and optional GPIB, the E36300 gives you the performance of system power supplies at an affordable price.

”Today’s complex designs place higher demands on the systems that power them,” said Bill Griffith, application engineer manager at Keysight Technologies. “But these power systems often cause design problems, and basic power supplies may be incapable of uncovering them. With the E36300 Series, engineers can simulate power problems early in the design cycle. And advanced features, such as low-current measurements, auto-series and auto-parallel connections, sequencing and data logging help them detect power problems.”

The E36300 Series’ low, normal mode noise specifications mean that you get quality power for precision circuitry applications, so you can power your designs with confidence. These bench power supplies are acoustically quiet too. Each model provides excellent line/load regulation of 0.01%, fast transient response time of <50 us, low current measurement down to 80 uA and over-voltage, over-current and over-temperature protection to prevent damage to the device under test.

Additionally, you can control these bench power supplies to set parameters and status alerts, visualize power output, and log changing voltage and current over time with Keysight BenchVUe software. The included Test Flow capabilities let you quickly automate power-supply setups and measurements into test sequences.

In addition to design engineers, the E36300 Series is ideal for new users and students learning to use power supplies. With a full-menu, front-panel interface, the E36300 Series is easy and intuitive to use. The units provide individual knobs for voltage and current, individual On/Off on each channel, and a keypad and softkeys to easily configure the power supply, simplify operation and improve productivity. Even new users can quickly control and measure the unit to deliver results.

The E36300 Series is available now, with prices starting at $1,100 USD.

Learn more about these new bench power supplies and see how upgrading can save you testing time.



Product number






Max. Voltage



Max. Current




80 W

6V, +25V, -25V

5A, 1A, 1A


80 W

6V, 25V, 25V

5A, 1A, 1A


160 W

6V, 25V, 25V

10A, 2A, 2A

Today I want to talk about what causes an overvoltage condition. An overvoltage condition is a condition that causes the power supply output voltage to exceed its setting. Let’s take a look at some of the things that can cause this to happen.


Causes of power supply output voltage exceeding its setting:
User-caused miswires
These miswires should be found and corrected during test setup verification before a device under test (DUT) is connected to the power supply. Possible miswires and their effect on the power supply output voltage are:

  • Shorted sense leads – the output voltage will rapidly rise above the setting. Keysight power supplies will prevent the output from rising above the overvoltage protection (OVP) setting.
  • Reversed sense leads – on most power supplies, the output voltage will rapidly rise above the setting and on Keysight supplies, it will be stopped by the OVP circuit. On our N6900/N7900 Advanced Power System (APS) power supplies, this condition is caught sooner: OV- is triggered when the output reaches about 10% of the rated voltage, so the output does not have to rise to the setting and above.
  • Open sense leads – If your power supply does not have protection for open sense leads, it is possible for your output to rapidly rise above the setting if one or both sense leads are open. Keysight power supplies have built-in sense protect resistors which limit the output voltage rise to about 1% above the setting. The voltage will continue to be regulated there. In addition to limiting the output to about 1% above the setting with an open sense lead, Keysight N6900/N7900 APS power supplies have a feature called open sense lead detection. When enabled, open sense lead detection will cause a sense fault (SF) status about 50 us after open sense leads are detected. This status does not turn off the output, but it can be configured to turn off the output using the advanced signal routing capability.
  • Special note about N7900 power supplies (not N6900): these models have output disconnect relays that open upon a protection fault. These mechanical relays take about 20 ms to open. Before they open, the output downprogrammer circuit is activated for about 2 ms and draws about 10% of rated output current to reduce the output voltage. The N7976A and N7977A (both higher voltage models) also have solid state relays in series with the mechanical relays. Upon a protection fault on these 2 models, the downprogrammer activates for 2 ms followed immediately by the solid state relays opening and then the mechanical relays open about 20 ms later.


Download the free "4 Ways to Build Your Power Supply Skill Set" eBook.


Inadvertent wiring failure
  • Sense leads inadvertently become shorted – power supply response is the same as mentioned above under shorted sense leads
  • Sense leads inadvertently become open – power supply response is the same as mentioned above under open sense leads
  • Sense leads should never become inadvertently reversed, nevertheless, the power supply response is the same as mentioned above under reversed sense leads

Power supply fault (circuit failure)
Note that Keysight’s overall power supply failure rate is very low. Since the below mentioned failures are a subset of all failures, they are very rare. This means that failures that cause the output to go to a higher-than-desired value are a small percent of a small percent, and while not impossible, they are extremely unlikely events.
  • Power element fails (shorts)
    • Series regulator – when a series regulator power element shorts, the output very quickly rises above the rated voltage of the power supply. The only way to limit this is to trip OVP and either fire an SCR across the output to bring the voltage back down or open output relays. For example, the Keysight N678xA models use a series regulator. When OVP trips on N678xA models, output relays are opened to protect the DUT. Solid state relays very quickly open first followed by mechanical relays about 6 ms later.
    • Switching regulator – when a Keysight switching regulator power element shorts, the output will go toward zero volts instead of rising since Keysight switching regulators use power transformers and no power can be transferred through the transformer without the switching elements turning on and off. For example, all N6700 and N6900/N7900 series models use switching regulators except the N678xA models (series regulators).
    • Note that if a power element fails open using either power regulation scheme, the output voltage will fall, not rise, so this condition is not a concern when looking at excessive output voltage possibilities.
  • Regulation circuit failure (bias supply, DAC, amplifier, digital comparison processor, etc.)
    • There are various circuits that could fail and cause the output voltage to rise in an uncontrolled manner. Keysight power supplies have OVP designed to respond to these failures. In series regulators, an SCR across the output can fire to reduce the voltage or output relays can open. In switching regulators, the pulse width modulator is turned off to prevent power from flowing to the output, downprogrammers are activated to pull any excessive voltage down, and output relays are opened (when present) to disconnect the output from the DUT.
    • Multiple parallel failures – if both a regulating circuit fails that causes the output to rise AND the OVP circuit fails, there would be nothing to prevent the output voltage from rising above the setting. While this is possible, it requires just the right combination of multiple circuit failures and is therefore extremely unlikely.
Output response to load current transients
  • It is possible for the output voltage to temporarily rise above the setting for short transients in response to fast load current changes (especially unloading). If the voltage excursion is high enough and long enough, it is possible that the OVP will activate and respond as outlined above.
External power source
  • It is possible for an external source of power (such as a battery, charged capacitor, inductor with changing current, or another power supply) to cause the voltage to go above the setting. The OVP will respond to this condition as outlined above. If the external power source can provide more current than the rating of the power supply and an SCR circuit is used in the power supply, it is prudent to put a fuse in series with the external source of power to prevent damage to the power supply SCR and/or output circuit from excessive current.
So you can see that there are a number of ways in which the output voltage can rise above the setting. Luckily, Keysight design engineers are aware of these possibilities and have lots of experience adding protection circuits to prevent damage to your DUT!

Occasionally, one of our power supply users contacts us with a question about voltages measured from one of the power supply output terminals to earth ground (same as chassis ground). All of our power supply outputs are floating with respect to earth ground. See my previous post about this here. In that post, I stated that neither output terminal is connected to earth ground. To be more specific, no output terminal is connected directly to earth ground. We do have internal components, mainly resistors and capacitors, connected from each output terminal to earth ground. These components, especially the caps to ground, help mitigate issues with RFI (radio-frequency interference) and ESD (electrostatic discharge). They help prevent our power supplies from being susceptible to externally generated RFI and ESD, and also help to reduce or eliminate any internally generated RFI from being conducted to wires connected to the output terminals thereby reducing RFI emissions.

So even though our outputs are considered floating with respect to earth ground, there frequently is a DC path from at least one of our output terminals to earth ground. It is typically a very high value resistor, such as several megohms, but could be as low as 0.5 MΩ. This resistor acts as a bleed resistor to discharge any RFI or ESD caps to earth ground that could be charged to a high float voltage.

As an example of a power supply with a resistor to earth ground, the Keysight N6743A has 511 kΩ (~0.5 M) from the minus output terminal to earth ground. This resistor was responsible for the voltage measurements to earth ground observed and questioned by one of our power supply users. He was using this power supply in the configuration shown in Figure 1 and measured 9.7 Vdc from his common reference point to earth ground (again, same as chassis ground).

He understandably did not expect to measure any stable voltage between these points given that the output terminals are floating from earth ground. But once we explained the high impedance DC path from the minus output terminal to earth ground inside each power supply (see Figure 2), and the 10 MΩ input impedance of his DMM, the measurement made sense. The input impedance of the voltmeter (DMM) must be considered to accurately calculate the measured voltage. This is especially true when high impedance resistors are in the circuit to be measured.

Figure 3 shows the equivalent circuit which is just a resistor divider accounting for the 9.7 V measurement. (The exact calculation results in 9.751 V.) Notice that the voltage of the 28 V power supply does not impact this particular voltage measurement (but its resistor to ground does). If the user had measured the voltage from the plus output of the 28 V power supply to earth ground, both the 28 V supply and 20 V supply would have contributed to his measurement which calculates out to be 37.05 V (if you check this yourself, don’t forget to move the 10 MΩ resistor accounting for the different placement of the DMM impedance).

So you can see that even with power supply output terminals that are considered floating, there can still be a DC path to earth ground inside the supply that will cause you to measure voltages from the floating terminals to ground. As one of my colleagues always said, “There are no mysteries in electronics!”

Previously I posted about hurricane Irene and inverters. In that post (click here to read), I talked about the power ratings for inverters and just skimmed the surface about the differences between ratings in watts (W) and volt-amperes (VA). In this post, I want to go further into detail about these differences. Both watts and VA are units of measure for power (in this case, electrical). Watts refer to “real power” while VA refer to “apparent power”.

Inverters take DC power in (like from a car battery) and convert it to AC power out (like from your wall sockets) so you can power your electrical devices that run off of AC (like refrigerators, TVs, hair dryers, light bulbs, etc.) from a DC source during a blackout or when away from home (like when you are camping). Note that this power discussion is centered on AC electrical power and is a relatively short discussion about W, VA, and inverters. Look for a future post with more details about the differences between W and VA.

Watts: real power (W)
Watts do work (like run a motor) or generate heat or light. The watt ratings of inverters and of the electronic devices you want to power from your inverter will help you choose a properly sized inverter. Watt ratings are also useful for you to know if you have to get rid of the heat that is generated by your device that is consuming the watts or if you want to know how much you will pay your utility company to use your device when it is plugged in a wall socket since you pay for kilowatt-hours (power used for a period of time).

The circuitry inside all electronic devices (TVs, laptops, cell phones, light bulbs, etc.) consumes real power in watts and typically dissipates it as heat. To properly power these devices from an inverter, you must know the amount of power (number of watts, abbreviated W) each device will consume. Each device should show a power rating in W on it somewhere (390 W in the picture below) and you can just add the W ratings of each device together to get the total expected power that will be consumed. Most inverters are rated to provide a maximum amount of power also shown in watts (W) – they can provide any number of watts less than or equal to the rating. So, choose an inverter that has a W rating that is larger than the total number of watts expected to be consumed by all of your devices that will be powered by the inverter.

Volt-Amperes: apparent power (VA)
VA ratings are useful to get the amount of current that your device will draw. Knowing the current helps you properly size wires and circuit breakers or fuses that supply electricity to your device. A VA rating can also be used to infer information about a W rating if the W rating is not shown on a device, which can help size an inverter. Volt-amperes (abbreviated VA) are calculated simply by multiplying the AC voltage by the AC current (technically, the rms voltage and rms current). Since VA = Vac x Aac, you can divide the VA rating by your AC voltage (usually a known, fixed number, like 120 Vac in the United States, or 230 Vac in Europe) to get the AC current the device will draw. To combine the apparent power (or current) of multiple devices, there is no straightforward way to get an exact total because the currents for each device are not necessarily in phase with each other, so they don’t add linearly. But if you do simply add the individual VA ratings (or currents) together, the total will be a conservative estimate to use since this VA (or current) total will be greater than or equal to the actual total.

What if your device does not show a W rating?
Some electrical devices will show a VA rating and not a W rating. The number of watts (W) that a device will consume is always less than or equal to the number of volt-amperes (VA) it will consume. So if you need to size an inverter based on a VA rating when no W rating is shown, you will always be safe if you assume the W rating is equal to the VA rating. For example, assume 300 W for the 300 VA device shown in the picture above. This assumption may cause you to choose an oversized inverter, but it is better to have an inverter will too much capacity than one with too little capacity. An inverter with too little capacity will make it necessary for you to unplug some of your devices; otherwise, the inverter will simply turn itself off to protect its own circuitry each time you try to start it up, so it won’t work at all if you try to pull too many watts from it.

Some electrical devices will show a current rating (shown in amps, or A) and not a VA rating or W rating. Usually, this current rating is a maximum expected current. Maximum current usually occurs at the lowest input voltage, so calculate the VA by multiplying the current rating (A) times the lowest voltage shown on the device. Then, assume the device consumes an equal number of W as mentioned in the previous paragraph. For example, the picture below shows an input voltage range of 100 to 240 V and 2 A (all are AC). The VA would be the current, 2 A, times the lowest voltage, 100, which yields 200 VA. You could then assume this device consumes 200 W.