Capacitor Insulation Resistance

[willDavidson]

TL;DR Summary

I'm not sure how to read insulation resistance from this datasheet.

Hi,

I am trying to find the insulation resistance of a specific capacitor. I've posted on balancing resistors before and got some pretty helpful information. The previous datasheet gave the information in a different manner though. I'm hoping someone can help me decipher the insulation resistance from this datasheet.

The insulation resistance is given on page 4 as IRxC >= 30.000 seconds at 100 VDC 1 minute @T=+25 C

I don't know I or R and only know C (52.5 uF). Also, would that be read as thirty thousand seconds, or is it 30 seconds? I'm leaning toward 30 thousand seconds but the "period" instead of "comma" is throwing me off.

https://www.mouser.com/datasheet/2/212/1/KEM_F3114_C4AQ-1628369.pdf

 

[berkeman]

Summary:: I'm not sure how to read insulation resistance from this datasheet.

The insulation resistance is given on page 4 as IRxC >= 30.000 seconds at 100 VDC 1 minute @T=+25 C

I don't know I or R and only know C (52.5 uF). Also, would that be read as thirty thousand seconds, or is it 30 seconds? I'm leaning toward 30 thousand seconds but the "period" instead of "comma" is throwing me off.

Can you post a link to the full datasheet? That would be helpful.

And spaces are important...

IR (insulation resistance) x (multiplied by) C (capacitance in Farads) >= 30 seconds

I initially had no idea what IRxC was, and TBH, I still have no idea what the 1 minute test condition means (is that maybe the charge time?). At least R x C has units of time (one RC time constant), so that sort of makes sense.

And for electrolytic capacitors, I'm not sure the "insulation resistance" of the dielectric is linear with applied voltage. I generally look for the Leakage Current spec instead -- how are you wanting to use the "insulation resistance" or "leakage current" specs in your design? Knowing how you want to use them would help us to point you to the right things to look at in the datasheet.

Finally, things like capacitance and leakage current change over time and with temperature for electrolytic capacitors. Do you need to incorporate those changes into your design? (Like do you want your circuit to work within spec for the next 5 years? 10 years? etc.)

 

[Baluncore]

I am trying to find the insulation resistance of a specific capacitor.

The "insulation resistance" may not be an independent fixed parameter. Leakage current may not be voltage dependent.

We do not simply read a data sheet, we analyse, decode and proof read it. By searching for the hidden traps, and questioning everything, you are going the right way about engineering design.

European data sheets may interchange the dot and comma. Look for other examples in the data sheet. Identify the manufacturer and look at other data sheets they produce, in different languages. Since they use commas as thousands separators elsewhere, I think it must be a simple typo. For your specific case, does it matter?

 

[willDavidson]

Sorry about that. I posted the link in the original post but didn't mention that's what it was. I'll post it below again. I just posted the screenshot to make it easier in case that's all that was needed.

These are actually film capacitors. I looked for a leakage current spec or graph and found nothing. It seems like 100,000 hrs is what these are typically rated for so I was assuming that if I kept it below 70 C and below the rated voltage that I should meet at least that lifetime (theoretically).

https://www.mouser.com/datasheet/2/212/1/KEM_F3114_C4AQ-1628369.pdf

 

[berkeman]

https://www.mouser.com/datasheet/2/212/1/KEM_F3114_C4AQ-1628369.pdf

Thanks, that helps. First, it shows that "." is the decimal point and "," is the comma throughout the datasheet. But I still have no clue what they are referring to as Insulation Resistance IR. I haven't been able to find a description of the test that they are referring to, and there is only one non-descriptive reference to Leakage Current in the datasheet. Maybe try doing a more general Google search for IR Insulation Resistance of Capacitors?

 

[berkeman]

I searched the KEMET website, and found this helpful document:

https://sh.kemet.com/Lists/TechnicalArticles/Attachments/6/What is a Capacitor.pdf

There is more information in that document, so please have a look to see if it helps.

 

[willDavidson]

I searched the KEMET website, and found this helpful document:

https://sh.kemet.com/Lists/TechnicalArticles/Attachments/6/What is a Capacitor.pdf

There is more information in that document, so please have a look to see if it helps.

Thansk, I read the full document. That helps to answer the question, sort of, I guess. Based on that, (IR) = 30s/C so for a 40 uF capacitor IR >= 750 kOhm @100 V. To me it seems like that's missing a lot of information. Am I wrong in thinking that realistically I should have at least a plot showing leakage current (LC) vs voltage (V) at a certain temperature? Ideally, I would also have one that showed LC vs temperature, but the other at the very minimum. I guess knowing 750 kOhm minimum @100 V is better than nothing...

 

[Baluncore]

The “self-healing property” of metalised polymer film capacitors operates by allowing the metal film to fuse near an insulation failure and so electrically isolate the problem area.

That requires the capacitor be used in a low impedance power circuit, one that is able to deliver sufficient energy to isolate a failure. Following the healing there will still be a very small leakage current.

For that reason insulation resistance (IR) takes a dynamic value, switching from below one ohm to tens of megohms as the self-healing cycles occur.

Self-healing capacitors should not be used in circuits that require low leakage or fixed capacitance.

Where a series chain of capacitors is employed for high voltage, you must assume that one capacitor may be short circuited at the time, and that voltage balancing resistors will share the voltage fairly across the remaining capacitors. The IR during normal operating conditions will be significantly greater than the voltage sharing resistors. Since 10.000 has three trailing zeros, the stop should be interpreted as a thousands separator. Elsewhere in the data sheet trailing zeros are removed after a decimal point.

 

[willDavidson]

The “self-healing property” of metalised polymer film capacitors operates by allowing the metal film to fuse near an insulation failure and so electrically isolate the problem area.

That requires the capacitor be used in a low impedance power circuit, one that is able to deliver sufficient energy to isolate a failure. Following the healing there will still be a very small leakage current.

For that reason insulation resistance (IR) takes a dynamic value, switching from below one ohm to tens of megohms as the self-healing cycles occur.

Self-healing capacitors should not be used in circuits that require low leakage or fixed capacitance.

Where a series chain of capacitors is employed for high voltage, you must assume that one capacitor may be short circuited at the time, and that voltage balancing resistors will share the voltage fairly across the remaining capacitors. The IR during normal operating conditions will be significantly greater than the voltage sharing resistors.Since 10.000 has three trailing zeros, the stop should be interpreted as a thousands separator. Elsewhere in the data sheet trailing zeros are removed after a decimal point.

I will make sure there is enough headroom for one of the capacitors to be shorted without over-voltaging the other capacitors.

That document states that the electrode layers are 1/1000th the diameter of a human hair. With the layers being so thin, it doesn't seem like it would take an unreasonable amount of energy to oxidize the area around a short. If a large current does not flow, that should mean the capacitor has relatively low leakage current and is functioning properly shouldn't it? As far as the fixed capacitance part, what type of circuit wouldn't be better off with a capacitance that decreases slightly after a fault compared to failing completely? From my understanding, the self healing property is a benefit because the percent of capacitance lost is proportional to the area vaporized which is very low considering the many many layers and the small area vaporized.

Also, according to that document, it appears that film capacitors have lower leakage, in general, than ceramic capacitors. DC/DC converters often use film capacitors because of their self healing properties and the low leakage (I thought). I would think a capacitor with high leakage would not be as desirable for a DC-link capacitor actually. So I'm having a hard time understanding the statement that self-healing capacitors should not be used in circuits that require low leakage or fixed capacitance. One of the only places I an see this type of capacitor not being used as often is a resonant converter that requires a set capacitance to maintain resonance. Those values are typically small and wouldn't benefit from a large capacitance value anyway so something like a C0G(NP0) would be used.

That's just my current understanding so hopefully you can tell me where that logic is off?Using the decimal as a thousands separator would make the IR more in the range of what I would expect. 750 MOhm sounds a lot better than 750 kOhm.

 

[Baluncore]

So I'm having a hard time understanding the statement that self-healing capacitors should not be used in circuits that require low leakage or fixed capacitance.

A self-healing capacitor relies on high leakage current to heal the damage.

As sections of the dielectric are isolated, the capacitance falls.

There are more applications for capacitors than are dreamt of in your philosophy.

 

[willDavidson]

A self-healing capacitor relies on high leakage current to heal the damage.

As sections of the dielectric are isolated, the capacitance falls.

There are more applications for capacitors than are dreamt of in your philosophy.

Understood, I was only thinking about power converters. I'm sure I'll be introduced to more applications like you're meaning with time and experience.

Thanks for the help on this topic.

 

[Tom.G]

I, and the manufacturer TDK, agree with @Baluncore that '30.000' should be read as '30 000'.

See pg.11 of:
https://www.tdk-electronics.tdk.com/inf/20/20/db/fc_2009/X2_B32921_928.pdf

Based on that, if you charge a sample in the morning it should self-discharge to 37% of voltage at the end of your work shift (8.33Hrs)! (but you better make sure the case is clean, the atmosphere is dry, and the leads don't touch anything! )

Cheers,
Tom

 

[willDavidson]

I, and the manufacturer TDK, agree with @Baluncore that '30.000' should be read as '30 000'.

See pg.11 of:
https://www.tdk-electronics.tdk.com/inf/20/20/db/fc_2009/X2_B32921_928.pdf

Based on that, if you charge a sample in the morning it should self-discharge to 37% of voltage at the end of your work shift (8.33Hrs)! (but you better make sure the case is clean, the atmosphere is dry, and the leads don't touch anything! )

Cheers,
Tom

Thanks for the info. Looking at page 11, they state "Special care must be taken in case of use several capacitors in a parallel configuration." Do you know why and what the special care is? I was under the impression that the parallel configuration was the easiest/safest and series required more attention. As long as the maximum current rating isn't exceeded for each parallel capacitor, what else is there to worry about? Of course there's voltage but but that doesn't seem like special attention.

 

[Tom.G]

"Special care must be taken in case of use several capacitors in a parallel configuration." Do you know why and what the special care is?

Nop, no idea why. I would just take them at their word, or possibly try to contact one of their Application Engineers for an answer... other than curiosity, it's probably not worth the effort though.

Cheers,
Tom

After thought: Perhaps they don't want a bunch of parallel caps discharging thru a failed one, could be spectacular.

(edit: spelling)

 

[willDavidson]

Nop, no idea why. I would just take them at their word, or possibly try to contact one of their Application Engineers for an answer... other than curiosity, it's probably not worth the effort though.

Cheers,
Tom

After thought: Perhaps they don't want a bunch of parallel caps discharging thru a failed one, could be spactacular.

Good point on the discharging. Just out of curiosity, I think I'll contact them. Thanks again.

 

[Tom.G]

If you do find out, please satisfy our curiosity too!

 

[willDavidson]

Ok will do. I just submitted a request so I'll let you know when I find out.

 

[Baluncore]

After thought: Perhaps they don't want a bunch of parallel caps discharging thru a failed one, could be spectacular.

That is correct.

The repair cycle must self terminate before it becomes major damage. Termination may not happen if too much stored energy is available and the dielectric becomes pyrolysed.

Some monoblock multilayer ceramic capacitors, such as skycaps, have a similar self-healing mechanism. They fuse the connection to faulty internal layers. They should not be used in high impedance signal circuits as they become leaky when the fuse does not blow.

 

[willDavidson]

That is correct.

The repair cycle must self terminate before it becomes major damage. Termination may not happen if too much stored energy is available and the dielectric becomes pyrolysed.

Some monoblock multilayer ceramic capacitors, such as skycaps, have a similar self-healing mechanism. They fuse the connection to faulty internal layers. They should not be used in high impedance signal circuits as they become leaky when the fuse does not blow.

I was thinking it would be to limit the discharge current to the rest of the circuit. Placing multiple capacitors in parallel doesn't increase the charge each capacitor can/will store, so I don't understand how placing 1000 capacitors in parallel will effect the operation of one of the individual capacitors compared to only having 2 in parallel.

PS, I haven't received a response yet. Just an update on that part.

 

[Baluncore]

https://www.ecicaps.com/tech-tools/technical-papers/self-healing-affect-metallized-capacitors/

https://passive-components.eu/the-self-healing-characteristics-of-metallized-film-capacitors/

https://www.researchgate.net/publication/338029268_Energy_characteristics_of_self-healing_process_in_metallized_film_capacitors/fulltext/5dfae2374585159aa487e6bc/Energy-characteristics-of-self-healing-process-in-metallized-film-capacitors.pdf?origin=publication_detail