A safety tip

17 years ago

Awhile back we had a discussion about testing start and run capacitors. many people including me learned a lot of servicing tricks from a manual called the Trouble Shooters Bible, they still are being sold on the internet. Now first let me say I am no way critizing this book, because at the time Bill Doolin published the manual it was a fine source and much of the info in it is still accurate and ok. However, one thing that is definitely wrong and dangerous is the way he reccomends testing capacitors. The books advocates taking a power cord with insulated clips, plugging it in hooking 1 side to a cap terminal and touching the other terminal a few times with other clip to put a charge into the cap, unplug cord, then short a screw driver across cap terminals and observe the resulting spark, if it is fat and heavy makes a fairly loud pop cap is ok, otherwise if it is weak cap is bad. This was fine with the old rugged capacitors made by US firms like Mallory Sprague, GE and others. this is no longer true, most caps are now foreign made their cases are weak and testing in this manner can cause them to explode and spew hot wax and stuff all over. We had an incident at the shop Friday, a tech was using this method to test a cap, it was one of the imports it blew like a hand gernade when he shorted accross it. he escaped serious injury because my son who now is head honcho had put in a policy that everyone including me doing any work in shop wears safety goggles or an eye shield. This guy was wearing a plastic shield got a few cuts on face and hands but no damage to eyes or anything serious. The eye shield had several deep impact marks, in other words had he not been wearing eye protection I hate to imagine the result. So fellas don,t po po shop safety rules it don,t take but once. Iggie

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Comments (5)

baymee
17 years ago
They can be tested with a multi-meter nowadays.
iggie
Original Author
17 years ago
Baymee, true capacitors can be tested with an ohmmeter, but this method can also fool you. I have seen capacitors test ok with the ohmmeter method ,particularly start capacitors and still not have enough kick to do the job. A much better method is to purchase a condenser tester specifically designed to test capacitors, they are not terribly expensive. Now using an ohmmeter it is very hard to locate a partially bad cap, one that has just lost part of it,s capacity, because the capacity of these condensers is so large in relation to the ohmmeter voltage that it will appear to take a charge but still not have enough capacity to start a motor. The best test is by substitution. A capacitor at the upper end of value of most start caps say around 30 mfd with insulated clip leads is fine thing for a tech to have in his tool box. this way it can be used to see if a motor will start and if does the correct size can be installed. I have a big Mallory capacitor that I have been using in this manner for years. I have found quite a few instances where the starting cap has been tested with an ohmmeter, declared good and the motor bad because it would not start. I clip my old mallory on and the motor takes off.
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brickeyee
17 years ago
"capacitors can be tested with an ohmmeter"

Not adequately.

All you can check for are shorts, and by watching them charge up (the resistance appears ton increase over time) see if they hold any charge.

You cannot easily measure their capacitance, or operation at working voltage.

Even a multimeter with a capacitance mode only test at very low voltage using DC.

To verify a capacitor is actually good requires a source of operating voltage AC power, and a way to check for leakage and value.

It is cheaper to swap a new one in.
zl700
17 years ago
The tongue method taught to apprentices works best for testing
iggie
Original Author
17 years ago
The best method I have found for servicing capacitors is substitution, carry a large capacity cap and and a smaller one that you know are good ,these can be substitued in place of questionable caps when needed. Eliminates guessing and needles motor replacement.