dogears

Low leakage
« on: May 27, 2020, 10:07:36 AM »
Forgive the basic question, but I've looked and failed at finding this myself. When talking about a low leakage capacitor (for example, for use in a timing circuit) is a film capacitor considered low leakage? How about an MLCC? Where on a datasheet would this kind of information be?


JohnRoberts

Re: Low leakage
« Reply #1 on: May 27, 2020, 10:32:19 AM »
Leakage should be specified on data sheet. (Usually in terms of C and applied voltage).

Yes film caps are better. This is well studied for use in sample and hold circuits. Besides leakage, dielectric absorption (soakage) can also be a factor.  IMO DA is overstated as audible mechanism in audio path DC blocking capacitors, but in timing circuits, especially when charged and discharged using different time constants it matters (IMO).

My favorite low leakage film cap is polystyrene, but they are not very robust for use in mass production. Teflon dielectric is well regarded for S/H circuits but expensive. Polypropylene is good, and even Mylar (polyester) is decent.
======
For today's TMI back in the 80's when I made a kit CX record NR decoder, I observed that the encoder (designed by Urie) used a tantalum cap for the primary time constants. Tantalum is notorious for high DA, and in my judgement it could matter for the Urie encoder. In a case of obvious over design I used a tantalum capacitor for my playback decoder kit time constant. To reduce cost, I used a 1uF tantalum cap and scaled resistors to realize the same time constants (Urie used a 10uF in their encoder). 

JR
Don't only half-ass tune your drums. Visit https://circularscience.com to hear what properly "cleared" drums sound like.

EmRR

Re: Low leakage
« Reply #2 on: May 27, 2020, 10:53:43 AM »
I put this elsewhere, but it should probably go here too.

Several recent tangential observations about old caps. 

After looking at many many amps of the 70-80 year old age group which lived in radio service for decades, it is normal for ESR to be very high in all caps.  A new cap may have ESR of an ohm or fraction of an ohm, with the old one being 50Ω or more.  The piece may still sound fine, but would probably sound better with new caps.  There are those that don't want anything touched, which leads to quandaries.    Comparing against NOS equivalent era caps, the NOS will be much much lower, but still higher than a modern cap.    Like 2.5Ω on the NOS versus 0.5Ω on a new polypropylene Orange Drop.  No one can say what the original ESR would have been when new, but certainly a newer type would be lower in virtually all cases.

This paper is interesting:

https://www.illinoiscapacitor.com/pdf/Papers/impendance_dissipation_factor_ESR.pdf

Quote
An important observation is the Fr parameter. Fr is the self-resonant frequency. Defined as the frequency where Xl and Xc are equal.

At this frequency the impedance is equal to the ESR.
  Below self-resonance the Xc component is dominant and the capacitor behaves like a capacitor. Above the self-resonant frequency the inductive component is dominant and the capacitor behaves more like an inductor.


I just went through a couple of late 1930's RCA line amps.    ESR on the coupling caps was almost 80Ω.   The capacitance value of 0.5mfd  looked fine at 120Hz and 1kHz.    It's use caused a large treble rolloff compared to a new cap, no difference in lows.   As much as I wanted to keep using the groovy old hermetically sealed paper in oil film cap, it was past the expiration date.   This is the only time I've ever noted this behavior, but then, how many 80 year old amps that are unmodified do any of us ever get to restore?

 In this case the amp was on-air daily for 50 years, and then sat in an abandoned unconditioned space for 22 more years. 


I also recapped a 1960ish Collins 212Z SS remote mixer for someone, which had already seen a recap sometime in the '70's.  Most of the recap was wet slug tantalums.   They all still had very good ESR readings and proper capacitance readings.  What I don't recall is if the wet slug types also fail shorted like the typical tantalums, I suppose that might be a case of DECREASING ESR with age?      The standard electrolytics that were there were all pretty high, 12-16 ohm readings on 300-500 mfd caps.  Still working, but.....


Carry on.....
Best,

Doug Williams
Electromagnetic Radiation Recorders

"I think this can be better. Some kind of control that's intuitive, not complicated like a single knob" - Crusty

"Back when everything sounde

Whoops

Re: Low leakage
« Reply #3 on: June 06, 2020, 05:28:16 PM »
Forgive the basic question, but I've looked and failed at finding this myself. When talking about a low leakage capacitor (for example, for use in a timing circuit) is a film capacitor considered low leakage? How about an MLCC? Where on a datasheet would this kind of information be?

Check this video also,
really good explanations here:

https://www.youtube.com/watch?v=67M7fsbLUIU


Re: Low leakage
« Reply #5 on: June 09, 2020, 04:28:41 AM »
I put this elsewhere, but it should probably go here too.

Several recent tangential observations about old caps. 

After looking at many many amps of the 70-80 year old age group which lived in radio service for decades, it is normal for ESR to be very high in all caps.  A new cap may have ESR of an ohm or fraction of an ohm, with the old one being 50Ω or more.  The piece may still sound fine, but would probably sound better with new caps.  There are those that don't want anything touched, which leads to quandaries.    Comparing against NOS equivalent era caps, the NOS will be much much lower, but still higher than a modern cap.    Like 2.5Ω on the NOS versus 0.5Ω on a new polypropylene Orange Drop.  No one can say what the original ESR would have been when new, but certainly a newer type would be lower in virtually all cases.

This paper is interesting:

https://www.illinoiscapacitor.com/pdf/Papers/impendance_dissipation_factor_ESR.pdf


I just went through a couple of late 1930's RCA line amps.    ESR on the coupling caps was almost 80Ω.   The capacitance value of 0.5mfd  looked fine at 120Hz and 1kHz.    It's use caused a large treble rolloff compared to a new cap, no difference in lows.   As much as I wanted to keep using the groovy old hermetically sealed paper in oil film cap, it was past the expiration date.   This is the only time I've ever noted this behavior, but then, how many 80 year old amps that are unmodified do any of us ever get to restore?

 In this case the amp was on-air daily for 50 years, and then sat in an abandoned unconditioned space for 22 more years. 


I also recapped a 1960ish Collins 212Z SS remote mixer for someone, which had already seen a recap sometime in the '70's.  Most of the recap was wet slug tantalums.   They all still had very good ESR readings and proper capacitance readings.  What I don't recall is if the wet slug types also fail shorted like the typical tantalums, I suppose that might be a case of DECREASING ESR with age?      The standard electrolytics that were there were all pretty high, 12-16 ohm readings on 300-500 mfd caps.  Still working, but.....


Carry on.....

ESR readings on old capacitors tell you very little, unless it is way off. The best for old audio equipment is to measure capacitance and the leakage current at the rated voltage, both should be within certain limits depending on capacitor size, rated voltage and type of dielectric used. I own a couple of capacitor analyzers, I have a Sencore LC77 capacitor/inductor tester, and I also have an old EICO capacitor tester with an "eye" tube to indicate if the capacitor is leaky or not, the Sencore is more precise but the EICO is great for a quick go-no-go test. Both instruments can test electrolytics and film caps, however, I usually don't bother with electrolytics, I just replace them,  but sometimes it is good if you have a fault to check if an electrolytic is the one causing it.


 

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