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I'm developing a small testing application for my company that tests various hardware and software functionality.

I do however have a problem developing a simple way to check for short circuits in the system (given a limited knowledge of electronics, I'm mainly a programmer).

The problem I have is that the system has a large capacitor (40 farad) with a fairly low average load (~50mA at 3.8V) where the capacitor may, or may not be previously charged.

Because of the capacitor i cannot simply check for a large current flowing from the power supply because that would probably just be the capacitor being charged.

My idea was to check if the current decreases at an expected rate given the capacitor. I'm afraid I don't know enough about capacitors or electronics in general to be able to accurately predict the capacitor charge and detect such a variation.

I thus would like to ask guidance on how to reliably perform such a check, or if there is any better way to check for a short circuit given a large capacitor. I have a fairly rudimentary knowledge of electronics and don't know much beyond the basics.

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  • \$\begingroup\$ can you brief? i don't understand load draws only 50mA then why such a big (40F) is connected? or it may be like load may have been powered by the cap bank when the charge level at cap bank reduces below the preset level it has to be charged again \$\endgroup\$ – yogece Oct 21 '13 at 9:18
  • \$\begingroup\$ I'm thinking unless the detection has to be especially fast maybe you could use a resettable fuse in series and then check for zero current from the supply when it trips. Do you have the gear to check the maximum inrush when the capacitor is fully discharged? \$\endgroup\$ – PeterJ Oct 21 '13 at 9:26
  • \$\begingroup\$ Have you tried monitoring the voltage across the capacitor? \$\endgroup\$ – Ignacio Vazquez-Abrams Oct 21 '13 at 9:38
  • \$\begingroup\$ Can you give us a schematic of the sort of circuit you are testing, and some examples of the specific sort of short circuits you are looking for? \$\endgroup\$ – Joe Hass Oct 21 '13 at 11:07
  • \$\begingroup\$ Disconnect one side of the Capacitor, and use a Wheatstone bridge. Ohms law should identify a short location. \$\endgroup\$ – Optionparty Oct 21 '13 at 13:30
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Measuring ESR (equivalent series resistance) may give you what you want.

Apply a square wave via a series resistor and monitor the waveform at the capacitor terminals.
The resistive component will cause a step change in voltage at the edges of the square wave, followed by an exponential charging curve. Vstep = I charge x Reffective.

If Rshort is usefully smaller than the capacitor ESR then you will get a lower initial Vstep with a shorted capacitor.

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If you're sure that the part of the circuit you want to survey has some impedance, pass a DC voltage. In case of any existing impedance, the waveform should change by an exponential rate (sometimes which seems to be linear)....but in any case the current should decay or grow depending on whether the impedance is discharging or charging, if the DC input results in a DC output, then the net reactive impedance is zero, which might be either bcoz of a short circuit across your reactive impedance or because the circuit is resonant. So, if you know that your circuit should not be resonant, somebody has shorted your capacitors/inductors.

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