Checking capacitors

Do you have a question? Post it now! No Registration Necessary

Translate This Thread From English to

Threaded View


Today I received one of the roughly  $ 20 component testers.  Checking  
it out and comparing capacitors I notices a big difference in a couple  
of them

I was using a Fluke 87,  a LCR meter from China, an older component  
tester and the new component tester.

The first capacitor was a Sprague .06 uF 600V.  Two China testers showed  
near the value.  Within the 10%  tollorence,    The LCR tester showed it  
to be .08 and the Fluke as .1 uF.
This is a new,but very old capacitor.

Next capacitor was a 20 year old no name of .068 of 50 V made with the  
Poly something dielectric.  All meters were with in less than 10 %.  Ok  
here.
Same results with a newer one of .01 uF .  

Next came a Silver mica.  It is .01 at 600 V. Fluke shows up at .0150,  
LCR at  .0120.  Two component testers were close and in spec.


What gives with some capacitors checking like they should and some being  
way off, not just 10 % or so ?  I ran the tests several times on each  
capacitor to see if maybe the leads were not making good contact and any  
other similar thing I may have missed like having my fingers across the  
leads.

Ralph ku4pt

Re: Checking capacitors
On 2020-05-28 15:54, Ralph Mowery wrote:
Quoted text here. Click to load it

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
We've slightly trimmed the long signature. Click to see the full one.
Re: Checking capacitors
snipped-for-privacy@electrooptical.net says...
Quoted text here. Click to load it

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.


Re: Checking capacitors
On 2020-05-28 17:48, Ralph Mowery wrote:
Quoted text here. Click to load it

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.
Quoted text here. Click to load it

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

Cheers

Phil Hobbs

--  
Dr Philip C D Hobbs
Principal Consultant
We've slightly trimmed the long signature. Click to see the full one.
Re: Checking capacitors
Ralph Mowery wrote:

================
Quoted text here. Click to load it
** 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  


Re: Checking capacitors
On Friday, May 29, 2020 at 12:05:38 AM UTC-4, Phil Allison wrote:
Quoted text here. Click to load it
e  
Quoted text here. Click to load it
  
Quoted text here. Click to load it
  
Quoted text here. Click to load it
.  
Quoted text here. Click to load it
s  
Quoted text here. Click to load it
  
Quoted text here. Click to load it
 shape if the frequency is high.  
Quoted text here. Click to load it
oisture ingress.  
Quoted text here. Click to load it
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.

Re: Checking capacitors
snipped-for-privacy@gmail.com says...
Quoted text here. Click to load it

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.


Re: Checking capacitors
Ralph Mowery wrote:

=====================

Quoted text here. Click to load it

** Yes, cos they filter with a -12dB/octave slope wiping out all the harmionic of a square wave.  

Quoted text here. Click to load it

** 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  

Re: Checking capacitors
On Thursday, May 28, 2020 at 3:54:41 PM UTC-4, Ralph Mowery wrote:
Quoted text here. Click to load it
  
Quoted text here. Click to load it
  
Quoted text here. Click to load it
  
Quoted text here. Click to load it
  
Quoted text here. Click to load it
  
Quoted text here. Click to load it
  
Quoted text here. Click to load it


   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.

Site Timeline