Power Supply Input and Output Troubleshooting

Troubleshooting is a process that so often seems to point us back to the simplest problems. When many parts of a system suddenly go dead, whether it be the sensors or an entire PLC cabinet, it might indicate a faulty power supply.

Testing power supplies is usually a fairly quick process, but without the right information, there are some results that can be misleading. Different types of power supplies also provide differing abilities to be repaired. In many cases, once a faulty supply is identified, it’s much more efficient to simply replace the supply.

Step One: Testing with a Voltmeter

The first step is to test the power supply terminals with a voltmeter, but is it more logical to test the input or the output terminals first? The answer may depend on the symptoms. 

If we observe that every single device receiving power from the supply is turned ‘off,’ then the supply is the primary suspect. Here we have one key symptom that can help diagnose the issue. Most power supplies have a 'DC OK' or 'Power' LED that lights up if the power supply is functioning. If the LED is on, then we can move down to the second step with load condition testing.

If that LED is off, then the power supply is not working, and it would be reasonable to check the input terminals first. If the input is properly receiving power, we can eliminate any problems from upstream. Most power supplies are provided with AC voltages, either 1-phase or 3-phase, so meter should be set to AC voltage mode. If no voltage is detected, check any upstream fuses, circuit breakers, or branch disconnect switches.

Figure 1. Most industrial power supplies use 1- or 3-phase input power, indicated with L/N or L1/L2/L3 terminal labels.

If the input measures a voltage within the acceptable rated range, the output has now become the main factor that determines whether the devices will receive proper power. Testing the output terminals should yield something very close to the rated output voltage. Use the DC voltage mode on the meter for this test.

Voltage Adjustment

Industrial power supplies can sometimes be adjusted within a small range to fine-tune the output, but this is designed to compensate for loading effects, not failures over time. You should plan to adjust this at the very beginning of the installation when the system is switched on and all devices are running. It should not be used to increase the voltage if it suddenly drops one day after years in service.

Figure 2. Voltage adjustment terminals are present on most power supplies.

Breakout Board Testing

Nearly all control cabinets use terminal blocks to distribute power and control signals. The + and - of the power supply usually have wires that carry the voltage to these distribution points. If the power supply is difficult to reach, locate these terminal blocks to perform the output voltage test.

Sometimes the output is simply too small to test, like on a phone charger USB cable. Other times, the terminals are extremely difficult to access, hidden behind cable runs. If testing is very important, breakout boards can be purchased. These boards turn the tiny or inaccessible cable into a larger board with terminals that can be easily reached. I try to keep a couple of these types of boards around in my shop all the time.

Figure 3. Breakout boards allow testing when the power supply terminals are not easily accessible.

Step Two: Load Testing

If we find that the power supply input is correct, but the output is OFF, it doesn't necessarily mean that the power supply has failed. In the previous step, the testing was measured without respect to whether the load was active or not. Should the load be connected when the test is performed or not? The correct answer is both.

No-Load Output Test

First, test the output terminals when the load is disconnected (open-load voltage). If the output voltage is correct, it means that the problem is probably located in an over-load, perhaps a short-circuit, downstream. If the open-load voltage has suddenly become too low or has turned off entirely, the power supply is probably the faulty component. 

Again, be sure to test the input voltage before this step. If there is no input, then there should also be no expected output - the power supply is not the fault.

Load Test

Once the open-load output is verified correct, then the load test should be completed. This is the real-world test since the open-load voltage isn’t actually what we are trying to determine. We really need to know if the power supply can still function when the devices are connected.

Reconnect the load wiring and check to see what happens. It should be one of two scenarios:

The output voltage should reduce slightly. This is normal. If the current is being drawn through the load, the source voltage will always drop. However, the drop should not be significant, but it’s hard to tell how much this should be. All that can be said is “If the drop is too much, the power supply is failing and cannot keep up”. Reduce the load or replace the supply.

The output voltage may drop to zero. This is often seen in supplies with an overload protection function, as many industrial supplies include. When too many load devices are used, they can pull a bit too much power. Miswired or failed load wiring can create short circuits and pull WAY too much power. In either case, the load current is above the rated maximum, and the output voltage will drop to zero. As soon as the load is removed, the voltage is restored. Find the source of the overload and remove it.

Step Three: Component Testing

Linear and switching power supplies both provide output voltages, but the method is quite different.

When a switching supply is plugged in, many components inside are connected to the main line voltage, even those on the output side of the transformer. It is not advised to test or repair inside a switching supply unless you have the right tools and training.

Linear supplies provide more power and are often used for motion devices. They tend to be more expensive, which means that repair might be more cost-efficient. Even still, hours of testing and repair may be more costly than a whole new supply. But the structure of a linear supply is fairly simple, so testing isn’t too bad.

First, a transformer (usually a large one) will have an input primary coil side with the supply AC voltage across it. The secondary coil voltage should be stated somewhere on the datasheet or the part sticker, and can also be measured as AC voltage. There is usually a fuse in-line or even built into the transformer. This may be the failed component.

Next, a bridge rectifier consists of two diodes always in series with each other at any time. All following components, such as the smoothing capacitor, will now show DC voltage. Be careful when testing capacitors; they can often store a dangerous charge, but the high impedance of a voltmeter creates a safe test for these relatively low voltage supplies. The section between the rectifier and the regulator can really only be correctly measured with an oscilloscope.

Figure 4. This linear power supply has a fuse (1) in-line with the transformer (2) that has primary and secondary terminals, the bridge rectifier (3), and a capacitor (4). This type of supply would be quite expensive, and the ease of component replacement might make a favorable repair.

A voltage regulator is the final component before the load terminals. The output of the regulator should be a smooth, constant DC output, and can once again be tested with a DC voltmeter. Sometimes the output is an adjustable linear supply, like with a common benchtop double or triple-output supply. These regulators can also be tested, but there needs to be some method of calculating the correct output voltage or the measurement will be meaningless.

Summary

A consistent, multi-step process can isolate power supply failures. First, test the input voltage. Then, verify both the open-load output and the output with the load connected, and the source of the problem should be traceable. Finally, the fault can be corrected or the power supply replaced as required. Testing power supplies usually does not include testing the internal components, but in a good linear supply, it may prove useful. For switching supplies, component-level testing is almost never attempted.

All images used courtesy of the author. Original article published Sept, 2020.