Checking capacitors

Might be bad soakage--micas are horrible for that. If you stick a 1-Hz square wave into it and look at the voltage across a the 1-M input impedance of your scope, you might see something interesting.

Scoping what the meters are doing to the cap would be interesting too.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
ElectroOptical Innovations LLC / Hobbs ElectroOptics 
Optics, Electro-optics, Photonics, Analog Electronics 
Briarcliff Manor NY 10510 

http://electrooptical.net 
http://hobbs-eo.com

The first two are wound, and the third is multi layer. Winding adds some inductance, if the ends are not fully bonded their entire length. That add ed ESL will change the reading. Silver Mica are designed for low ESR in RF applications. .01 and .06 uF are more use at lower frequencies.

I have a decent test bench. Not lab quality,but not too bad.

I hooked a function generator set as square waves to 2 1000 ohm resistors for isolation. From each resistor I went to a capacitor. One was the Sprague and the other was the poly something capacitor. Then back to the ground side of the generator.

A dual track Hanteck 200 mhz scope with 10:1 probes were hooked across the capacitors. I started out at .1 Hz and went up to around 10,000 Hz. At all times the traces were almost identical. They started out as almost perfect square waves as expected . At a couple of hundred cycles the leading edge started to show a rounding off near the rise of the cycle (top of the trace) and same for the negative part of the cycle. As I increased the frequency they started resembling sine waves. About like I expected. At no time was there any measurable difference in the waveformes.

I measured them again on the capacitor checkers and same results. The poly capacitor was very close on the meters and the Spraque was showing .1 instead of .06 or close to .06 on hte Fluke and .08 on another tester. Had it shown .07 or .05 I would have called it meter tollorance but not almost double.

I don't have a capacitor tester that I can put any high voltage on them like 500 or so volts.

When I have more time I may try scoping the meters and see what they are putting across the capacitors.

My suggestion was to look at the current through the capacitors going into a high impedance. That way you don't load down the generator.

Hmm, dunno. The meters may be using different schemes.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
ElectroOptical Innovations LLC / Hobbs ElectroOptics 
Optics, Electro-optics, Photonics, Analog Electronics 
Briarcliff Manor NY 10510 

http://electrooptical.net 
http://hobbs-eo.com

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** That does *NOT* happen !!!!

With square wave input, a simple RC filer converts the wave to a TRIANGLE shape if the frequency is high.

Your ancient Sprague ( Black Beauty ?) is almost certainly leaky due to moisture ingress.

A DMM on ohms should show you that.

Forget Terrell's nonsense about wound and non wound cops - he is just blowing it out his arse as usual.

.... Phil

e

. s

shape if the frequency is high.

oisture ingress.

owing it out his arse as usual.

Yawn. You are clueless about ESL, SRF and other REAL factors once you ar e above audio. NO component is perfect, and there are valid reasons. Differ ent construction methods have different effects on their performance as wel l as heir failure mode. SM fails due to Silver Migration which causes them to short. Film capacitors pinhole, and short. Electrolytics have many failu re modes, but they are the cheapest to manufacture. SM is one of the most e xpensive, but they can handle higher RF current that ceramics of the same r atings. I'll listen to you, when you have decades of high power RF work to your sorry name. All capacitors will fail, in time. Even Vacuum capacitors fail. The problem is choosing the right class for a design.

When I said resembled sine waves, I knew they were not really sine,but did not crank the sweep far enough to see what they actually were. It has been a while back that I did any thing with what circuit converts what, but did remember that LC low pass filters converted to the sine wave.

The Sprague is not the Black Beauty lable. I think it is the next generation, but not sure. I don't recall the value, but at 500 volts dc it had less than .1 ma leakage. It did surprise me that it did not have a lot of leakage.

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** Yes, cos they filter with a -12dB/octave slope wiping out all the harmionic of a square wave. ** Equates to 5M ohms so would fully self discharge in about 1 second.

( 5exp6 times 0.06exp-6 = 0.3 seconds )

Easily enough to upset many capacitance meters.

A good film cap of such value has a R value in the Gohm range and leakage under 1uA.

.... Phil

..... Phil