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Old 02-06-2007, 03:19 PM   #1
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Dropped my power cap...

and as it's the first Esc I've had that actually needs one how do I tell if it's still ok as I'd have no idea once running it because I've got nothing to compare it to.

I'd assume once I wire it up and turn the Esc on I can put the voltmeter across the cap to see if I get a reading of any sort?

Thanks in advance
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Old 02-06-2007, 08:34 PM   #2
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Does it look damaged? Dented or something? They are not that sensitive to damage, the can would have to be dented or split to damage the interior. Its not filled with glass or anything. I've taken them apart. They are typically very thin wound up metal foils, separated by an insulating layer of something like paper. Its complicated to test one and know its exact value. They usually either work or they don't. With an old analog multimeter, you could short the cap, then hook it up on ohms. It should swing hard as the cap charges, then go to zero ohms as it should not conduct dc. If it has no charge, it should not read any conduction with a digital multimeter set to say 1 meg ohms range. Your sweaty fingers will conduct at that range.
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Old 02-06-2007, 08:52 PM   #3
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Some DMMs have modes for capacitor testing. These work fairly well to
determine approximate uF rating. However, for most applications, they do
not test at anywhere near the normal working voltage or test for leakage.
However, a VOM or DMM without capacitance ranges can make certain types of tests. For small caps (like .01 uf or less), about all you can really test is for
shorts or leakage. (However, on an analog multimeter on the high ohms scale
you may see a momentary deflection when you touch the probes to the
capacitor or reverse them. A DMM may not provide any indication at all.)
Any capacitor that measures a few ohms or less is bad. Most should test
infinite even on the highest resistance range. For electrolytics in the uF range or above, you should be able to seethe cap charge when you use a high ohms scale with the proper polarity - theresistance will increase until it goes to (nearly) infinity. If the capacitor is shorted, then it will never charge. If it is open, the resistance will be infinite immediately and won't change. If the polarity of the probes is reversed, it will not charge properly either - determine the polarity of your meter and mark it - they are not all the same. Red is usually **negative** with VOMs, for example. Confirm with a marked diode - a low reading across a good diode (VOM on ohms or DMM on diode
test) indicates that the positive lead is on the anode (triangle) and
negative lead is on the cathode (bar). If the resistance never goes very high, the capacitor is leaky. The best way to really test a capacitor is to substitute a known good one. A VOM or DMM will not test the cap under normal operating conditions or at itsfull rated voltage. However, it is a quick way of finding major faults.A simple way of determining the capacitance fairly accurately is to build
a 555 oscillator. Substitute the cap in the circuit and then calculate
the C value from the frequency. With a few resistor values, this will
work over quite a wide range.
Alternatively, using a DC power supply and series resistor, capacitance
can be calculated by measuring the rise time to 63% of the power supply
voltage from T=RC or C=T/R.
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Old 02-07-2007, 01:45 AM   #4
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You need to use special capacitor meter to check if the capacitor is still fine. Any good electronic equipment repair shop should have one (note the emphasis on good). The most common meters are named "Capacitor Wizard" and "Capacitor Genie" or EVB. I prefer the Genie. These 2 meters use AC to measure the capacitor. Almost all other meters use DC. Then you compare the results to a chart that the capacitor manufacturer publishes.
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