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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 22nd Sep 2014, 8:09 pm   #1
Ed_Dinning
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Default Testing capacitor dielectrics

Hi Gents, a work related problem with capacitors.

We use 0.1uF, 250v film caps in a suppression circuit on 28v DC motors. These are epoxied into a small plastics case and are PET dielectric. The data sheet gives an insulation resistance of 30,000 Meg at 300v.
We received a new batch of these from a different supplier and when testing the final motor assembly at 250v were surprised to see readings as low as 15Meg when previously it had barely registered 1000 Meg on our production instrument (this is the largest reading).

I think we may have some counterfeit caps that are possibly ceramic dielectric.

Has anyone any idea how I can test these without specialist equipment to confirm my suspicions?

A Marconi TF2700 shows no difference on "loss balance" and although we have returned some to the distributor I have little faith in them getting back to the manufacturer for his analysis.

Many thanks, Ed
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Old 22nd Sep 2014, 8:21 pm   #2
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Default Re: Testing capacitor dielectrics

I assume that 100nF ceramic would have a pretty permissive temperature coefficient? Would hot vs. cold capacitance tests at least show up a difference between new and old types that would imply differing dielectric and erroneous description perhaps. Scratching around for non-destructive/invasive or X-ray etc. ideas...
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Old 22nd Sep 2014, 9:39 pm   #3
Ed_Dinning
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Default Re: Testing capacitor dielectrics

Hi, both capacitors are nominally the same type but from different manufacturers; only visual difference is the manufacturers logo and cases a different shade of grey.
Temp test and delta C with temp might be an idea.
Loss balance readings on Marconi bridge are similar for both caps.

Ed
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Old 23rd Sep 2014, 12:53 am   #4
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Default Re: Testing capacitor dielectrics

PET, after a stressing process, goes by the trade name of 'Mylar" ! It's unstressed specific resistivity is 10^16 Ohms/centimetre, and dielectric constant about 3.4.
Polythene HDPE is 10^13 to 10^16

So it should be pretty high resistance.

Maybe they used recycled bottles and forgot to remove all the CocaCola?

David
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Old 23rd Sep 2014, 3:44 pm   #5
Ed_Dinning
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Default Re: Testing capacitor dielectrics

Hi David, they are 0.1uF caps so I wonder if I could resonate them on my Advance or Marconi Q meters with about 10uH and get a Q reading for both types?

Ed
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Old 23rd Sep 2014, 4:49 pm   #6
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Default Re: Testing capacitor dielectrics

The Marconi probably goes low enough in frequency, so the prob shifts to making a good enough inductor... but I think you have a Marconi boxful.

You could always string a number in series to bring the C down to something more approachable..

David
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Old 23rd Sep 2014, 7:57 pm   #7
Ed_Dinning
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Default Re: Testing capacitor dielectrics

Hi David, yes I have the TF1245A and the 40KHz oscillator as well as the set of standards.

If I get some time this week I'll give it a go.

Ed
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Old 23rd Sep 2014, 8:09 pm   #8
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Default Re: Testing capacitor dielectrics

Try sawing them in half? Must be quite a difference between a film cap and ceramic?
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Old 24th Sep 2014, 9:43 am   #9
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Default Re: Testing capacitor dielectrics

The fault may be in the plastic used to encapsulate the capacitor innards.

You say that you tested the final motor assembly. It might be the epoxy or the PCB.
Can you clarify if you have tested the insulation resistance of the bare capacitors?
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Old 24th Sep 2014, 9:31 pm   #10
Ed_Dinning
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Default Re: Testing capacitor dielectrics

Hi Folks, assemblies are sleeved with heatshrink, not or PCB's or potted. Caps were also tested in isolation and still showed the low resistance.

Ed
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