Get free access to our online edition!

From the Q&A

With TJ Byers


Eat My Volts

Question:

I have a problem with my bench power supply in that it doesn’t hold the voltage constant. It’s a five-volt switching power supply from a reputable company, with specs that say it will provide 25 amps at five volts in the range of 4.75 to 5.25 volts at all temperatures and currents. It doesn’t. It varies anywhere between five volts and 3.5 volts. It seems to be worse at high output currents. Do you think the regulator is defective? Can it be fixed easily?

Johnson


Answer:

I doubt the problem is with the power supply itself, but in the way you are using it. If you’re like me, I’ll attach a couple of clip leads to the power supply’s binding post and route the power to the circuit I have on my bench. This is great for experimental purposes where the current is low. But as current increases, the resistance of the wires going from the power supply to your circuit becomes a significant part of the overall design.

Let’s say that the power supply is on an upper shelf of your test bench and your project is on the work surface. Scattered about the bench are your test instruments and five feet of #16 gauge wire (very common in automobile wiring) connecting the power supply to the project. Between the positive and negative runs, the total length is 10 feet, for a total resistance of 40 milliohms (0.04 ohms). At 10 amps, the voltage drop across the wires is 0.4 volts, 0.6 volts at 15 amps, and a hefty one volt at 25 amps. Moreover, if your circuit has switching elements, the input voltage will look like a roller coaster as the currents switch in and out.

The solution is to move the voltage sensing part of the regulator from inside of the power supply to the outside — commonly called remote sensing. It requires four wires (Figure): two that provide the real power and two that measure the voltage at the load — after the connecting wires. By doing this, the voltage drop across the power leads is nulled out and the voltage across the load remains constant.

But not all commercial power supplies have voltage sense inputs. Of course, you can try cutting into the circuit board to free these inputs, but you have to do it on a case-by-case basis. Fortunately, you can roll your own using a LM317 or 350 voltage regulator — or any adjustable switching buck/boost IC, for that matter. Simply route the voltage sense points to the load, as shown in the Figure. Don’t be tempted to remove the 1N4148 diodes. They prevent the voltage regulator from going postal should the sensor inputs become free floating.


Comments