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Power Supply Troubleshooting: Test Equipment

Power Supply Reference: Consumption, Savings, And More
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Using Digital Multimeters

One simple test you can perform on a power supply is to check the output voltages. This shows whether a power supply is operating correctly and whether the output voltages are within the correct tolerance range. Note that you must measure all voltages with the power supply connected to a proper load, which usually means testing while the power supply is still installed in the system and connected to the motherboard and peripheral devices.

Selecting a Meter

You need a simple digital multimeter (DMM) or digital volt-ohm meter (DVOM) to perform voltage and resistance checks on electronic circuits (see below). Only use a DMM instead of the older needle-type multimeters because the older meters work by injecting 9 V into the circuit when measuring resistance, which damages most computer circuits.

A typical DMM.A typical DMM.

A DMM uses a much lower voltage (usually 1.5 V) when making resistance measurements, which is safe for electronic equipment. You can get a good DMM with many features from several sources. I prefer the small, pocket-size meters for computer work because they are easy to carry around.

Some features to look for in a good DMM are as follows:

  • Pocket size—This is self-explanatory, but small meters that have many, if not all, of the features of larger ones are available. The elaborate features found on some of the larger meters are not really necessary for computer work.
  •  Overload protection—If you plug the meter into a voltage or current beyond the meter’s capability to measure, the meter protects itself from damage. Cheaper meters lack this protection and can be easily damaged by reading current or voltage values that are too high.
  • Autoranging—The meter automatically selects the proper voltage or resistance range when making measurements. This is preferable to the manual range selection; however, really good meters offer both autoranging capability and a manual range override.
  • Detachable probe leads—The leads can be damaged easily, and sometimes a variety of differently shaped probes are required for different tests. Cheaper meters have the leads permanently attached, which means you can’t easily replace them. Look for a meter with detachable leads that plug into the meter.
  • Audible continuity test—Although you can use the ohm scale for testing continuity (0 ohms indicates continuity), a continuity test function causes the meter to produce a beep noise when continuity exists between the meter test leads. By using the sound, you quickly can test cable assemblies and other items for continuity. After you use this feature, you will never want to use the ohms display for this purpose again.
  • Automatic power-off—These meters run on batteries, and the batteries can easily be worn down if the meter is accidentally left on. Good meters have an automatic shutoff that turns off the unit when it senses no readings for a predetermined period of time.
  • Automatic display hold—This feature enables you to hold the last stable reading on the display even after the reading is taken. This is especially useful if you are trying to work in a difficult-to-reach area single-handedly.
  • Minimum and maximum trap—This feature enables the meter to trap the lowest and highest readings in memory and hold them for later display, which is especially useful if you have readings that are fluctuating too quickly to see on the display.


Although you can get a basic pocket DMM for as little as $20, one with all these features is priced closer to $100, and some can be much higher. RadioShack carries some nice inexpensive units, and you can purchase the high-end models from electronics supply houses, such as Newark or Digi-Key.

Measuring Voltage

To measure voltages on a system that is operating, you must use a technique called back probing on the connectors. You can’t disconnect any of the connectors while the system is running, so you must measure with everything connected. Nearly all the connectors you need to probe have openings in the back where the wires enter the connector. The meter probes are narrow enough to fit into the connector alongside the wire and make contact with the metal terminal inside. The technique is called back probing because you are probing the connector from the back. You must use this back-probing technique to perform virtually all the following measurements.

Back probing the power supply connectors.Back probing the power supply connectors.

To test a power supply for proper output, check the voltage at the Power_Good pin (P8-1 on AT, Baby-AT, and LPX supplies; pin eight on the ATX-type connector) for +3 V to +6 V of power. If the measurement is not within this range, the system never sees the Power_Good signal and therefore does not start or run properly. In most cases, the power supply is bad and must be replaced.

Continue by measuring the voltage ranges of the pins on the motherboard and drive power connectors. If you are measuring voltages for testing purposes, any reading within 10% of the specified voltage is considered acceptable, although most manufacturers of high-quality power supplies specify a tighter 5% tolerance. For ATX power supplies, the specification requires that voltages must be within 5% of the rating, except for the 3.3 V current, which must be within 4%. The table below shows the voltage ranges within these tolerances.

Voltage Ranges

Loose ToleranceTight Tolerance
Desired VoltageMin. –10% Max. (+8%)Min. (–5%)Max. (+5%)
+3.3 V
2.97 V3.63 V
3.135 V3.465 V
+/–5.0 V4.5 V5.4 V4.75 V5.25 V
+/–12.0 V10.8 V12.9 V11.4 V12.6 V


The Power_Good signal has tolerances that are different from the other voltages, although it is nominally +5 V in most systems. The trigger point for Power_Good is about +2.4 V, but most systems require the signal voltage to be within the tolerances listed here.

Signal  Minimum
Maximum
Power_Good (+5 V)3.0 V 6.0 V


Replace the power supply if the voltages you measure are out of these ranges. Again, it is worth noting that any and all power-supply tests and measurements must be made with the power supply properly loaded, which usually means it must be installed in a system and the system must be running.

Specialized Test Equipment

You can use several types of specialized test gear to test power supplies more effectively. Because the power supply is one of the most failure-prone items in PCs today, you should have these specialized items if you service many PC systems.

Digital Infrared Thermometer

One of the greatest additions to my toolbox is a digital infrared thermometer. This is also are called a noncontact thermometer because it measures by sensing infrared energy without having to touch the item it is reading. This enables me to make instant spot checks of the temperature of a chip, a board, or the system chassis. They are available from companies such as Raytek (www.raytek.com) for less than $100. To use these handheld items, you point at an object and then pull the trigger. Within seconds, the display shows a temperature readout accurate to +/–3°F (2°C). These devices are invaluable in checking to ensure the components in your system are adequately cooled.

Variable Voltage Transformer

When you’re testing power supplies, it is sometimes desirable to simulate different AC voltage conditions at the wall socket to observe how the supply reacts. A variable voltage transformer is a useful test device for checking power supplies because it enables you to exercise control over the AC line voltage used as input for the power supply. This device consists of a large transformer mounted in a housing with a dial indicator that controls the output voltage. You plug the line cord from the transformer into the wall socket and plug the PC power cord into the socket provided on the transformer. The knob on the transformer can be used to adjust the AC line voltage the PC receives.

A variable voltage transformer.A variable voltage transformer.

Most variable transformers can adjust their AC outputs from 0 V to 140 V no matter what the AC input (wall socket) voltage is. Some can cover a range from 0 V to 280 V as well. You can use the transformer to simulate brownout conditions, enabling you to observe the PC’s response. Thus, you can check a power supply for proper Power_Good signal operation, among other things.

By running the PC and dropping the voltage until the PC shuts down, you can see how much reserve is in the power supply for handling a brownout or other voltage fluctuations. If your transformer can output voltages in the 200 V range, you can test the capability of the power supply to run on foreign voltage levels. A properly functioning supply should operate between 90 V and 135 V but should shut down cleanly if the voltage is outside that range.

One indication of a problem is seeing parity check-type error messages when you drop the voltage to 80 V. This indicates that the Power_Good signal is not being withdrawn before the power supply output to the PC fails. The PC should simply stop operating as the Power_Good signal is withdrawn, causing the system to enter a continuous reset loop.

Variable voltage transformers are sold by a number of electronic parts supply houses, such as Newark and Digi-Key.

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