Making Sense Out of Sense Lines

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You may have seen bench supplies with two sets of output or probe wires. In supplies, one set is the usual VOUT and GND. Depending on the brand, the other set may be labeled SENSE+ and SENSE- (or +S and -S).

Why are there sense lines? When are they used? These will become clear in the following paragraphs and figures. Say that you have a simple setup like the one shown in Figure 1.

You set your power supply to 3.3V.  A resistor load draws 100mA. Say that this load is situated far away from your supply so you made use of a AWG#28 cable a meter long. This cable has a resistance per unit length of 217mΩ/meter. This gives you 217mΩ one way ;another 217mΩ the other way. You turn on the power and see that the load is not getting the 3.3V that is set on your supply. Simple Ohm's law will tell you that

Now, 3.26V is still quite close to 3.3V so you can let it pass.

But if the load draws much more current than this, say 800mA, then you have your work cut out for you. Doing the math again, Vload is 2.952!

To solve this, you can move the load and supply closer together so the cable can be shortened. Or you can use a cable much bigger in diameter. Both approaches attempt to reduce the wire resistance so less voltage drop appears on the cable. Another way is to adjust the supply to a high enough voltage so that the resulting Vload is 3.3V.

There is another solution which is simple yet elegant. And this is to use a power supply employing SENSE inputs and another cable as sense lines. Let me explain how it works. Refer to Figure 2 below.

The circuit inside the dotted lines is typical of power supplies with SENSE. They include operational amplifiers connected as buffers. Recall that in opamps, the input current is so minute that for all practical purposes, you can approximate it as zero. Also recall that the opamp does its best to adjust its output to make the voltage between its inputs to become nearly zero.

With this in mind, you can inspect the circuit of Figure 2 and verify that the following statements are true.

1.    Because the input currents of the opamps are ~0mA, then there is negligible voltage drops on the resistance of the sense lines, RSW1 and RSW2.

2.    Because of the above and the voltage between the inputs of U1 (vd1) being zero, the voltage at node LOADP is actually VSOURCE.

3.    Because of 1 and  the voltage between the inputs of U2 (vd2) being zero, the voltage at node LOADN is at ground.

4.    Statements 2 and 3 is telling us that VLOAD equals VSOURCE. For the circuit in Figure 2, VLOAD equals 3.3V.

But you ask how can this be when there should be a drop on the wires RW1 and RW2. You are correct, of course. There is a drop on each wire equal to 800mA x 217mΩ, about 174mV. Again I say again that the opamp does its best to adjust its output so that the voltage between its inputs becomes zero. U1 made its output to be 3.474V to compensate for the drop in RSW1. For its part, U2's output adjusted to -174mV.

To wrap up, the use of SENSE lines takes care of the voltage losses on the resistances of the wires that connect the supply to the load, no matter what the load current is. You should employ SENSE lines whenever you require better voltage accuracy at your load.

Mr. Edgar Policarpio works for a Malaysian ODM as a concept development engineer. He is presently engaged in the development of portable amplifiers for cellphones, tyre pressure monitoring, and vehicle tracking system. His career spans 17 years of electronic design work of remarkable diversity. His past works includes:  USB portable storage and network routers with Flex-P (Malaysia), ESD control products with Hugle Technology, device engineering with Analog Devices Inc., ICs with ROHM, wrist instruments circuits with TIMEX, ASIC design with ISSI, and embedded systems with Mitech.

11/21/2006

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Written By:

Edgar Policarpio