titbin

Remote Sensing is Important for Your Power Supply

Blog Post created by titbin Employee on Jan 25, 2018

Do you have issues where the voltage at your load is lower than what you’ve set at your power supply? Do you always need to guess the amount of voltage to increase (to compensate for lead losses) just to get the right amount of voltage to appear at your load? If you have these issues, remote sensing can help! Remote sensing is a life saver, especially when you are setting test stations on your manufacturing floor or performing part qualifications.

 

Remote sensing allows you to have your desired voltage appear at your load. It works by sensing the voltage that appears at your load instead of the voltage that appears at the output terminals of your power supply. This is accomplished by connecting the load directly to your power supply’s sense terminals using two separate wires. By measuring voltage across the load, the power supply will adjust the output voltage until the voltage across the load reaches the desired voltage. No need to manually compensate for voltage drop across your load leads.

 

How Does Remote Sensing Work?

You’ve probably seen that sometimes the voltage at your load is different than the voltage at your supply. What’s causing this accuracy issue, and why does remote sensing help? To answer this, let’s use an example. In Figure 1, we have a power supply set for 5 V output. If your load is located at the output of the power supply, you’ll get almost 5 V at your load. Now, imagine that the load is 6 feet away from your power supply. You’re now transferring power to your load using a pair of 6-foot wires. If you’re using 14WAG wire for your connection, each wire will have a resistance of about 0.015 Ω.

The resistance of your copper wire doubles for every 3-gauge increase in wire size

Now, when you have 10 A flowing to your load, each wire will cause a voltage drop of 0.15 V (10 A x 0.015 Ω). You now have a total drop of 0.3 V on the wires. Instead of 5 V, you now have only 4.7 V (5 V – 0.3 V) across your load.

 

Figure 1 shows sense lead tied to output terminals

Figure 1. Sense lead tied to output terminals

 

The thinner the wire, the less voltage you have across your load. In the table below, you can see that wire resistance increases as wire size decreases. As a general rule, the resistance of your copper wire doubles for every 3-gauge increase in wire size.

 

AWG wire sizeResistance in mΩ/ft (at 20°C)
2216.1
2010.2
186.39
164.02
142.53
121.59
100.999

Table 1. Wire size vs. wire resistance

 

Let’s use the same setup, but now with remote sensing. To set up remote sensing, connect the sense terminals directly to the load. Wire size doesn’t matter for remote sensing ─ more on that shortly. When using remote sensing, the power supply will regulate the voltage across the load so that 5 V appears across the load. In this case, the power supply will increase the voltage at the output of the power supply to 5.3 V to offset the 0.3 V drop across the load wires. This will give you 5 V across your load. This is all done automatically by the power supply. No need for manual adjustments and calculations.

 

Figure 2 shows sense lead being connected directly to load.

Figure 2. Sense lead connected directly to load

 

The sense terminals on the power supply function like a voltmeter and have high input impedance. This means current flowing into the sense terminals is negligible and wire size does not significantly affect accuracy. You can use thinner wires for sense, but make sure these wires are properly shielded to reduce noise.


As you can see, remote sensing works pretty much like 4-wire resistance measurements. Instead of a small source current used in resistance measurement, we now have large current following through the leads and load. Remote sensing is especially useful if you have to connect to your load through long wires, complex relay topologies, or connectors.

 

Best Practices for Connecting Sense Leads

We just learned that remote sensing can significantly improve the accuracy of your output voltage at load. However, connecting your sense leads incorrectly can do more harm than good. To avoid this, let’s talk about best practices for connecting your sense leads to get the best results.

 

1. Use Two-Wire Twisted, Shielded Cables

Whenever possible, use two-wire twisted and shielded cables for your sense leads. A twisted pair, shielded cable protects your sense leads from noisy environments. You want to make sure the sense terminals are getting the cleanest possible measurements from your load. Noisy sense measurements will lead to fluctuations of your output voltage.

 

2. Make the Right Ground Connections to Avoid Ground Loops

If you are using a shielded cable for your sense leads, make sure to connect the shield to ground at only one point. Connecting your shield to ground at multiple points may look like a good idea because you are making more solid connections to ground, but it creates ground loops.

Ground loop current can cause noise to appear on your sense leads

How is that possible? Well, not all grounds are at the same potential to each other, especially grounds located far apart. When you connect these grounds together through your cable’s shield, current will flow between these points. This is called ground loop current. Ground loop current can cause noise to appear on your sense leads.

 

Figure 3 shows ground loop current flowing between to ground points.

Figure 3. Ground loop current flowing between to ground points

 

Figure 4 shows how in a correct ground connection, the shield is only connected to ground at a single point.

Figure 4. In a correct ground connection, the shield is only connected to ground at a single point

 

Figure 5 shows the physical connection on a typical DC power supply using a 2-core twisted and shielded cable

Figure 5. Physical connection on a typical DC power supply using a 2-core twisted and shielded cable

 

3. Keep the Sense Leads and Load Leads Separate

Do not twist or bundle your sense leads together with the load leads. Crosstalk will occur between the sense leads and load leads, causing inaccurate measurements on the sense leads.

 

4. Connect Your Sense Leads Properly

It may seem obvious, but you should have a solid connection between your sense terminals and load. An open connection at the sense terminal may cause the power supply to quickly increase output voltage because the sense terminal detects no voltage. This can be disastrous for your load!

 

Fortunately, Keysight power supplies use internal sense protect resistors. These resistors prevent the output voltage from rising too high if there’s an open connection at the sense leads.


Conclusion

Using remote sensing significantly improves your power supply’s accuracy with little investment. I encourage you to take advantage of this feature. Most modern power supplies come equipped with remote sensing. Use the best practices we discussed above to get better accuracy from your power supply.

 

I’d love to hear your questions and feedback in the comments section below!

 

Download the 10 Practical Tips You Need to Know About Your Power Products application note for more ways to improve your power supply operation and measurement capabilities.

Outcomes