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When Circuit Boards are Toast, Part 6

Jul 12, 2011 1:30 PM

A power analyzer can help you stop the circuit board smoking problem. Severe waveform distortion in the incoming AC power supply to the cabinet can produce an unhealthy environment for circuit boards. So, you need to assess that power.

With the power analyzer, view the waveform. Some distortion is OK; power supplies can handle it. But if the distortion is severe and/or the waveform is flat-topped, a high harmonic load is interacting with the supply power (or possibly many harmonic loads on several branch circuits are).

The distortion is related to "source impedance," which is basically the opposition to current flow between this point of use (the cabinet) and the supply transformer.

A common cause of impedance is that the wiring is too small for the length of wire run. This shows up in voltage drop. Voltage drop isn't something you measure — it's something you calculate based on the actual run. Calculate the voltage drop for the run to this cabinet. If it's high, than upsize the conductors or shorten the run.

Once you have the voltage drop issue squared away:

• Use your power analyzer to see where distortion begins.
• Determine if you're losing power through poor connections: Conduct infrared (energized) and low ohm (de-energized) testing on the run.
• Analyze transformer loading. If the transformer can't supply the demanded load, you get flat-topping. Derate or upgrade as needed.

 

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