Mike Keitz wrote:
>
> On Mon, 14 Sep 1998 09:49:50 +0300 Peter Cousens
> <cousens@her.forthnet.gr> writes:
> >Dave Miga wrote:
>
> >> check out http://www.eds-inc.com/cap.html for an in-circuit cap
> >checker
> >> that really works :-)
>
> There are several devices like this on the market.  I've never used
> one
> but the idea does have some merit.  They are intended to test "large"
> electrolytic capacitors.  In consumer equipment such as TV sets, these
> capacitors cause more problems than even faulty semiconductors.
>
> These capacitors usually fail by drying out (because of overheating,
> usually.  Product designers, keep these guys away from heat, and don't
> undersize them so they have any tendency to heat up by themselves.),
> which causes the capacitance to fall.  Also the "equivalent series
> resistance" increases as if a resistor is connected in series with the
> capacitor.  So a good capacitor will be a low impedance to small AC
> signals.  A bad one won't.  Having replaced a lot of failed
> capacitors,
> I'd say this test will catch most of the faulty ones.  Ones with low
> ESR
> would almost always have their rated capacity as well, so measuring
> the
> capacity wouldn't be necessary.  Often the circuit around the
> capacitor
> has a high impedance, so it is possible to make the test in circuit.
> Those who don't know or care how the circuit works can make a quick
> "shotgun" test of all the electrolytic capacitors and be fairly likely
> to
> identify a defective one.  But of course the ESR tester is more of a
> technician shortcut than an engineer's instrument.
>
> An ESR tester even in an enhanced form as described is still quite
> simple
> to make.  A PIC could be used to good advantage.  The test is
> obviously a
> lot more qualitative than quantitative so a fancy display isn't
> needed.
> The features described seem all useful and relevant to the device's
> function.  One more that I can think of would be testing at more than
> one
> frequency (the unit described does test at two: DC, and one "higher
> than
> others").  A high frequency may lead to a false "good" test if there
> is
> some "good" low ESR capacitance in parallel with the bad capacitor.  A
> frequency that is too low wouldn't test small capacitors (the smallest
> common electrolytics are about 0.47 uF).
>
Not really; most problems are caused by the smaller electros which few
ESR meters can catch. Our selling point is using a very high frequency
along with a shielded wire test probe (the hand-held pomona tweezer,
specially modified), and the simultaneous testing of DC leakage as well.
Sencore liked our design so much they copied it and are trying to patent
it (!). Guess they never heard of "prior art". BTW, the pic is used for
syscon only, the magic is the analog design---cutting edge.
mailto:dave@eds-inc.com.