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Switch Mode Power Supply Bench Testing

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Switch Mode Power Supply Bench Testing

Postby Unidec » Tue Sep 06, 2011 9:38 am

Switch Mode Power Supply Bench Testing

Turn the machine powerr off. Open the machine chassis, locate the DC power supply. Make certain all terminal screws on the power supply are tightened down, the supply is physically mounted, wire insulation and lugs are in good condition and no combustible materials are in close proximity to the power supply. Clean all dust and dirt in and around the supply and its

Only qualified electronics technicians or electrical engineers who have received electrical safety training should attempt to measure voltages on a switch mode power supply. Note that high voltage AC is present, high voltage DC at 320 VDC or higher is also present in the power supply circuits!

Turn on the machine power. Measure the AC input voltage using a DMM. You should either have 110VAC or 220VAC present on the input. Measure the DC output voltages. Depending on the model (see the product label for ratings) you should have a 5 VDC output, +12VDC, -12VDC or +15VDC, -15VDC outputs. Some supplies have +24VDC, -5VDC outputs as well. Make sure all outputs are correct. Any voltages that are too low may also be noisey. If any output is low, the supply most likely needs service (rebuilding, adjustments and full functional test.)

To bench test your supply:
See electrical safety warning above!
Switch mode power supplies in general have a minimal loading requirement to function properly. So, when bench testing a supply make certain that the main output (typically V1) is loaded for 10% of the maximum rating. For example, if the output is 5V rated at 20A max, load this using 2.5 ohms to draw 2A (P=IV = 2*5= 10W so make sure the resistor is rated for at least 10W).

The most important consideration is that all outputs are properly connected to the machine. V2, V3, V4 outputs will not work properly unless V1 is loaded (5A min). Connecting just the
+- 15VDC and 12V without the 5VDC will result in the supply failure or incorrect
operation. A digital voltmeter (DVM) may be used to measure the DC terminal voltages
on the power supply.

All outputs should be at or above (no more than 10%) their rated
outputs. The + and - 15 VDC outputs should be equal within 10% of each other. Using
the AC setting on the DVM will show any AC ripple or unfiltered noise on the supply
outputs, ripple must be less than 5% on all outputs. An AC line isolated oscilloscope
may be used to visually detect any noise on the power supply outputs. Use extreme
caution when using this method to prevent damage or injury. Always use an isolation
transformer or a battery-powered oscilloscope when troubleshooting with an

If the power supply fully function son the bench return it to the machine. Replace all machine covers, safety covers and chassis before attempting to
run the machine.
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