Have you ever set your power supply output voltage to a value and found the voltage at your load was lower than you expected? Many of us have experienced that outcome, and that’s because remote sensing needs to be part of the setup. In this article we are going to share with you three things you need to know about remote sensing to help you get a value you can trust.
1. Use remote sensing to regulate voltage at your load
Remote sensing is a feature on many power supplies that allows the power supply to remotely regulate the voltage right at your load. This is accomplished by using a set of remote sense leads that are in addition to your load leads. The power supply uses the voltage on the remote sense lead terminals to sense the voltage right at the load terminals and regulate the voltage right at the load by adjusting the output terminal voltage.
Figure 1 shows the power supply setup using remote sensing. The remote sense terminals are connected to the load at the points where you want the 5 V setting to be regulated. In this case, the power supply regulates 5 V at the load by adjusting its output voltage to 5.3 V to make up for the drops in the load leads. It does this by using the voltage across the sense leads as part of the feedback loop inside the power supply to adjust the voltage on the output terminals.
The purpose of the power supply is to keep the sense lead voltage constant at the setting; the power supply changes the output terminal voltage based on the sense terminal voltage. The input impedance of the sense terminals is high enough to prevent any significant current flow into the sense terminals – this makes any voltage drop on the sense leads themselves negligible.
2. Use sense leads for overvoltage protection (OVP)
One of our military customers providing DC power to a very expensive device during test asked about the availability of a special option on one of our power supplies. They wanted the option that changed the location of the overvoltage protection (OVP) sensing terminals from the output terminals of the power supply to the sense terminals of the power supply. Since the device under test (DUT) is located quite a distance away from the power supply, they are using remote sensing to regulate the power supply voltage right at the device under test. And since the DUT is very expensive and sensitive to excessive voltage, it’s important to protect the input of the DUT from excessive voltage as measured right at the DUT input terminals.
The power supply used, Keysight N6752A installed in an N6700C mainframe, normally uses the output terminals as the sensing location for the overvoltage protection. OVP is used to prevent excessive voltage from being applied to sensitive devices. If the voltage at the output terminals exceeds the OVP setting, the output of the power supply shuts down.
Since this customer is very interested in preventing excessive voltage from being applied to the expensive DUT, sensing for an overvoltage condition right at the DUT is important. For the N6752A, Keysight offers a special option (J01) that adds the ability to perform OVP sensing with the sense leads. See Figure 2. with the J01 option added to the N6752A, the customer’s DUT is protected against excessive voltage.
You may be wondering why the standard OVP would sense at the output terminals instead of at the sense terminals. Probably the biggest reason for sensing at the output terminals is because that approach provides more reliable protection than sensing at the sense leads even though it is less accurate. The output terminals are the power-producing terminals.
If the sense leads become inadvertently shorted, the voltage at the output terminals would rise uncontrolled beyond the maximum rated output of the power supply. This uncontrolled high voltage could easily damage any device connected to the power supply’s output leads. So, sensing for an overvoltage condition at the output terminals makes sense. It may not be the most accurate way to protect the DUT, but it is the most reliable given all of the things that can go wrong, such as a wiring error or an internal fault in the power supply.
3. Remote sensing can affect load regulation performance
The voltage load effect specification tells you the maximum amount you can expect the output voltage to change when you change the load current. In addition to the voltage load effect specification, some power supplies have an additional statement in the remote sensing capabilities section about changes to the voltage load effect spec when using remote sensing. These changes are sometimes referred to as load regulation degradation.
For example, the Keysight 6642A power supply (20 V, 10 A, 200 W) has a voltage load regulation specification of 2 mV. This means that for any load current change between 0 A and 10 A, the output voltage will change by no more than 2 mV. Also included in the 6642A remote sensing capability spec is a statement about load regulation. It says that for each 1-volt change in the + output lead, you must add 3 mV to the load regulation spec. For example, if you were remote sensing and you had 0.1 ohms of resistance in your + output load lead (this could be due to the total resistance of the wire, connectors, and any relays you may have in series with the + output terminal) and you were running 10 A through the 0.1 ohms, you would have a voltage drop of 10 A x 0.1 ohms = 1 V on the + output lead. This would add 3 mV to the load regulation spec of 2 mV for a total of 5 mV.
When you are choosing a power supply, if you want the output voltage to be well regulated at your load, be sure to consider all the specifications that will affect the voltage. Be aware that as your load current changes, the voltage can change as described by the load effect spec. Additionally, if you use remote sensing, the load effect could be more pronounced as described in the remote sensing capability section (or elsewhere). Be sure to choose a power supply that is fully specified so you are not surprised by these effects when they occur.
Remote sense is used to regulate the set voltage at the DUT, compensating for any loss in your leads. Using remote sense will have an impact on regulation performance, which should be considered along with the benefit of compensating for the voltage drop in your leads. Overvoltage protection at the power supply outputs should be used in conjunction with remote sense to protect the DUT.
You can learn more how to protect your DUT against power-related damage by downloading the Protect Against Power-related DUT Damage application note from Keysight.com