Dielectric Absorption

Where we discuss new analog design ideas for Pro Audio and modern spins on vintage ones.
emrr
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Re: Dielectric Absorption

Post by emrr » Tue Mar 27, 2018 8:53 am

mediatechnology wrote:
Tue Mar 27, 2018 8:24 am
I need a Duracell 504 15V and found that Exell were making a lot of vintage battery replacements.
I don't suppose there is enough of a demand for those but Exell is a good resource. http://www.exellbattery.com/
Last I looked a few years ago there wasn't a source I could find for those discontinued types. Cost still puts me off, but nice to see someone is doing it. I'm thinking (4) 45V batteries at $120 isn't in my future.

I still have that Mallory cell somewhere, I'll test and photograph when I find it again. It was in a box of parts in an unheated garage! Part of an estate.


On DA: I had a Sytek preamp go squirrelly after 20 years, symptom being gain that would zipper up and down by a few dB, almost like an oscillation. Seems like a possible cause. Not all channels, 2 of 4.
Best,

Doug Williams
Electromagnetic Radiation Recorders

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JR.
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Re: Dielectric Absorption

Post by JR. » Tue Mar 27, 2018 10:37 am

mediatechnology wrote:
Tue Mar 27, 2018 7:15 am
This morning's voltage read was 1.546V.

Higher than yesterday and could be measurement error. I only leave the meter connected for about a second since I don't want its 10MΩ load to discharge it too much.
You're no fun.... :lol: I would expect the current to stabilize at some low but nearly constant rate for a while....

Likewise if you charge the cap from a stable DC it would keep drawing a tiny current for a similar while after nearly full.

You have an interesting combination of low leakage (preventing self-discharge), and high distributed capacitance (like many RCs in parallel.)

JR
Doug: Do you suppose that the increased series R of the vintage capacitors might be due to a combination of heat, voltage/current and epoxy/Bakelite reacting to form an oxide or crud on the plates? Similar to cathode crud? Maybe lead attachment to plates junction?

Gold
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Re: Dielectric Absorption

Post by Gold » Tue Mar 27, 2018 2:40 pm

Semi OT: I'd like to put together a jig to test DC leakage current in Al caps. I don't have a high voltage DC bench supply. +/- 20VDC is all I've got. I have a variable auto former and was thinking I could make a full wave rectifier from diodes. This of course would be dirty DC without any filtering. Would the raw DC be okay for testing DC leakage current?

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mediatechnology
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Re: Dielectric Absorption

Post by mediatechnology » Tue Mar 27, 2018 2:56 pm

How high a voltage do you need?

The auto-transformer won't provide any line isolation which you really want from a safety viewpoint.
You could use an autotransformer adjusted to a low voltage and a 120:24 transformer in reverse (24:120) to obtain a poor-man's isolation transformer.
Then rectify the secondary...

Doug - I think you mentioned that they battery was physically clipped to the cathode terminal. Was it electrically connected from the cathode to ground where we would normally see a resistor?
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Gold
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Re: Dielectric Absorption

Post by Gold » Tue Mar 27, 2018 3:54 pm

Thanks. 100VDC is probably the highest voltage cap I use on a regular basis. The variac into a reversed 120:24 transformer is a good idea and I have some on hand.

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JR.
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Re: Dielectric Absorption

Post by JR. » Wed Mar 28, 2018 10:02 am

You might be able to get a sense of good/bad parts with phantom power (48V DC), and a sensitive ammeter (VOM).

JR

[edit] I returned it already but I borrowed a dedicated insulation tester when I was designing my outlet tester... it would provide up to 500V DC for testing up to 100s of Megohms Image (I was measuring that 550M across a mosfet input).[/edit

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Re: Dielectric Absorption

Post by Gold » Wed Mar 28, 2018 11:20 am

I have a few different power transformers. I’ll throw a few in a box with a bridge rectifier and some binding posts to patch in what I need. I mostly want it to test big computer caps in power supplies. Those caps can be upwards of $40 each sonits worth taking the time to test them. It’s not worth the time to test a $0.25 cap.

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JR.
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Re: Dielectric Absorption

Post by JR. » Wed Mar 28, 2018 1:18 pm

Before I borrowed a proper 500V tester I rolled my own high voltage supply with some common transformers I had laying around.

Connect 120V to the primary of one, and connect all the secondaries together in parallel. Then string the floating energized primaries in series. I stacked up 4x28V transformers to make 480V... Of course be careful... I think I put a 1/10A fuse in series with first primary but it could still make hazardous currents at deadly voltage. :o

JR
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mediatechnology
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Re: Dielectric Absorption

Post by mediatechnology » Mon Apr 02, 2018 7:05 am

I didn't want to post this yesterday on April Fools Day.

The peak voltage this morning was about 1.56V.
After leaving the meter on for a few seconds it "discharges" and actually goes negative a bit.
Removing the meter and allowing a few seconds for recovery and re-measuring shows about 1.56V again.
Dielectric absorption is sort of like stubbornness.

This particular capacitor was the one I used to check the Meanwell RS-25-48: viewtopic.php?f=7&t=943#p11175
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JR.
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Re: Dielectric Absorption

Post by JR. » Mon Apr 02, 2018 12:11 pm

The failure to self discharge seems unusual... Either extremely high impedance for internal distributed capacitance, or perhaps a non-linear self discharge characteristic.

If it repeatedly returns to a nominal 1V+ you could calculate an internal impedance from equilibrium current.

JR

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