How to Troubleshoot a DC Power Supply

By Dan Keen

All electronic devices, from TV sets, to steros, to computers, have numerous voltage and current requirements needed to supply to their circuits. However, the power delivered to our homes' wall outlets is 120 volts AC, and this is not in a form needed by most of these appliances. It is the job of power supplies in each appliance to change this power into voltages needed by the device.

Remove power from the unit (unplug it from the AC outlet).

In electronic repair, technicians learn to use their senses. Use your nose to smell for any burnt components or carbonized traces on the circuit board. Look for any components that are discolored, a sign that they may have been overheated. Overheating in a resistor, for example, may indicate that too much current has gone through it, no doubt the result of a short somewhere beyond it.

Carefully touch components to see if they are too hot, again, indicating a short circuit. Use caution when touching integrated circuits, which can get very hot if shorted.

Basic schematic for a power supply

Using an ohm meter, check fuses to be sure they have continuity. Remove them from their holder and test them while they are out of the circuit to avoid false readings. There may be fuses both in the AC side of any transformers, and also on the B+ side that supplies DC voltage to the system. If you discover that a fuse is blown, replace it with the exact value. Never replace a fuse with one that allows more amperage to flow before blowing, otherwise the purpose of the fuse as a safety device is lost. Also, never replace a fast blow fuse with a "slo-blow" fuse, even if the amperage rating is the same.

Replace the fuse with a new one, plug the device back into the electric wall outlet, and turn the device on. Watch the fuse and the other components in the power supply. If the fuse blows instantly, then there is a short circuit that you must troubleshoot. Even if the unit appears to be operating fine, watch it for a few minutes, looking for any signs of smoke (in case there is a component that is overheating or a carbon arc between traces on a circuit board).

Check diodes for short circuits. A common failure in power supplies are the diodes that convert AC to DC. In circuit configurations such as a bridge rectifier, where four diodes comprise a full-wave rectifier circuit, you may have to disconnect one end of each diode before you can check it with an ohm meter. Trying to check a diode in-circuit may result in a false reading, because current from the ohm meter can flow through other parts of the circuit. By clipping one end with a wire cutter, the diode is isolated from the rest of the circuit, and it can be tested. The diode should check almost a dead short in one direction, and an open circuit in the other. Use a soldering iron to connect the diode back into the circuit after checking it.

Other components in a power supply often include capacitors and coils, which are used to smooth the DC voltage after it has rectified. Coils can be checked for continuity with an ohm meter. Clip one end with a wire cutter to isolate the coil from the circuit, and check it with an ohm meter. It should show continuity if it is good. Capacitors can be checked for shorts by clipping one end to remove it from the circuit while testing. Small capacitors should read infinity on the ohm meter, but as to their capacitance value, this is hard to check without a special piece of test equipment designed to measure capacitance. Typically, if a technician suspects a capacitor is defective, he simply replaces it with a new one. Capacitors are inexpensive.

When testing large electrolytic capacitors with an ohm meter, the meter needle may initially indicate a short, and then slowly move toward infinity. This is caused by voltage from the ohm meter charging the capacitor. Ohm meter tests of diodes, capacitors and coils must be performed while the power supply is disconnected from the AC source.

Visually check the step-down transformer for any signs of overheating (you can use your nose, too). With power supplied to the unit, check to see if 120 volts AC is getting to the transformer's primary, and that the proper voltage is present on it's secondary. The coils in transformers can either open or short.

Integrated circuits are often used in modern power supplies, and these are difficult to test. The best method is substitution. Replacing a suspected defective integrated circuit is the best way to either fix the problem or at least eliminate it as a possible cause of the problem.

Before replacing an integrated circuit, check for any obvious shorts, such as diodes, capacitors, and burnt traces on circuit boards. You want to be sure there are no short circuits that could cause the new IC to blow as soon as it is replaced.

Obtain a schematic diagram of the appliance you are working on. This will make troubleshooting a power supply much easier, since often voltages at different points in the circuit will be given, and these can be checked with a voltmeter to help isolate the problem. A schematic will definitely be important if you have checked all of the above mentioned components and done all the preliminary troubleshooting steps, yet the unit is still not operating correctly.

About the Author

Dan Keen is the publisher and editor of a county newspaper in New Jersey. For over 30 years he has written books and magazine articles for such publishers as McGraw-Hill. Keen holds a degree in electronics, was chief engineer for two radio stations and taught computer science at Stockton State College.