Troubleshooting electrical components is a necessary part of keeping equipment functioning. The best tool to use for preliminary troubleshooting in a DC power supply is a digital multimeter (DMM).

Digital multimeters are easy to use and can measure voltage, which is important for pinpointing problems with a DC power supply. Here, we’ll discuss the basics of DMMs and how to check the input and output voltages on a DC power supply when troubleshooting.

Understanding Digital Multimeters

Digital multimeters are the first tool most professionals reach for when it comes to electrical troubleshooting. DMMs provide basic information like voltage, frequency, current, and resistance, which can then be used to narrow down the wide range of possible issues and help you quickly get to the root of the problem.

The information provided by multimeters can help detect common issues like blown fuses, faulty switches, short circuits, faulty components, and other common electrical issues. Multimeters can also be used in the beginning stages of diagnosing power supply issues.

Step-by-Step: Troubleshooting a DC Power Supply

A DC power supply unit (PSU) converts alternating current (AC) to direct current (DC). It’s a critical component for stepping down the AC voltage to the correct DC voltage needed to power a device.

In this case, the "Error" LED on a DC power supply has turned on. Your troubleshooting task is to determine if the problem is the power supply, the input supply voltage, or a load change on the demand side. You can do this by checking the AC input voltage and the DC output voltage.

To do this, you’ll follow these steps:

1.?? Safety Measures

a.???? Make sure your multimeter and test leads are rated for the voltage you’re expecting and for the environment you’ll be working in.

b.???? Double-check that your test leads are in good condition, with no exposed wiring or damage.

c.???? Ohm out your test leads to ensure that the measurement is close to zero before conducting any tests.

2.?? Measuring AC Input Voltage

a.???? Set your multimeter to AC voltage (V~ symbol on the multimeter). If you don’t have an auto-ranging multimeter, select a voltage range that is one degree higher than the voltage you are expecting. If you are unsure which voltage to expect, select the highest voltage setting.

b.???? Insert the black probe into the COM port and the red probe into the VΩ port.

c.???? Identify the AC input terminals on the DC power supply. Typically, these will be labeled as "L" (line) and "N" (neutral).

d.???? With the power supply connected to the AC source but turned off on the output side, touch the multimeter probes to the input terminals.

?????????????????????????????????????????? i.???????? Black probe to the N (neutral).

???????????????????????????????????????? ii.???????? Red probe to the L (line).

e.???? The multimeter should display the input AC voltage (120V or 240V). Verify that it is within the acceptable range for the power supply.

f.????? Remove the red probe first, then the black probe.

If the AC input line voltage is within specification, move on to the next step. If not, the problem is likely with the PSU or input supply voltage.

3.?? Measuring DC Output Voltage

a.???? Set your multimeter to DC voltage (symbolized by V ?). If you don’t have an auto-ranging multimeter, select a voltage range that is one degree higher than the voltage you are expecting.

b.???? Insert the black probe into the COM port and the red probe into the VΩ port.

c.???? Power off or disconnect any devices attached to the DC output if possible. This will help avoid any sudden voltage spikes while measuring the voltage.

d.???? Identify the DC output terminals of the power supply. These are usually marked with a “+” for the positive and a “-” for the negative.

e.???? Touch the black probe to the negative (-) terminal and the red probe to the positive (+) terminal of the DC power supply.

f.????? The multimeter will display the DC output voltage. Check if the reading aligns with the manufacturer’s specified output voltage (12V, 24V, 48V, etc.).

g.???? Remove the red probe first, then the black probe.

If the DC output voltage is not as expected, there is likely an issue with the load change from AC to DC. To get to the root of the problem, you will need to test the PSU with more advanced tools, like an oscilloscope.

Limitations of the Digital Multimeter

A digital multimeter essentially takes a snapshot of the voltage at a point in time, which may sometimes be all the information you need to diagnose a problem with a DC power supply problem. While DMMs can be a helpful tool to begin troubleshooting, they can’t provide enough information to diagnose certain issues that occur intermittently.?

Digital multimeters cannot detect variations like transients, electrical noise, or waveform issues, for example. Since they can’t display changes over time, they also can’t tell you when certain situations happen, such as the voltage suddenly dropping or spiking, unless you happen to take a reading at the exact moment it happens.

Conclusion

While the multimeter provides enough information to get started with electrical troubleshooting, there is often a need for additional tools like an oscilloscope in order to make a comprehensive diagnosis. In a DC power supply, a DMM can provide an initial diagnosis that identifies which part of the PSU is malfunctioning.