Author Topic: How to determine the voltage rating of an unknown capacitor?  (Read 36328 times)

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Offline (In)Sanity

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How to determine the voltage rating of an unknown capacitor?
« on: January 22, 2013, 07:30:43 pm »
Short of reading the marked values on a capacitor does anyone know of a method to identify the rated voltage of a capacitor?   Let me also add short of blowing them up or catching them on fire.   Does some characteristic of the capacitor change close to it's rated voltage,  such as perhaps the leakage current, etc? 

Thanks,

Jeff
 

Offline houdini

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Re: How to determine the voltage rating of an unknown capacitor?
« Reply #1 on: January 22, 2013, 07:47:31 pm »
You could measure the capacitance and the size and see what caps match up with that and they will also have the same or similar voltage rating.
 

Offline smashedProton

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Re: How to determine the voltage rating of an unknown capacitor?
« Reply #2 on: January 22, 2013, 08:16:02 pm »
If they are scavenged, probe the circuit.  you can also look at surrounding caps.

else, use a really nice equivilant cap
http://www.garrettbaldwin.com/

Invention, my dear friends, is 93% perspiration, 6% electricity, 4% evaporation, and 2% butterscotch ripple.
 

Offline (In)Sanity

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Re: How to determine the voltage rating of an unknown capacitor?
« Reply #3 on: January 22, 2013, 08:21:45 pm »
I actually have two reasons for wanting to figure out how to do this.   First I have hundreds if not thousands of "scavenged" caps as you mention.  Also I have thousands of brand new SMD caps that I have no idea what their voltage rating is.   So short of blowing them up I'm trying to figure out their safe working voltage.

Thanks,

Jeff
 

Online AndyC_772

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Re: How to determine the voltage rating of an unknown capacitor?
« Reply #4 on: January 22, 2013, 09:55:36 pm »
For many ceramic capacitors, the capacitance starts to fall off quite sharply as the dc bias is increased. Perhaps you could devise a circuit which measures capacitance with a gradually increasing dc bias on the cap under test until the capacitance falls by, say, 20%, or you chicken out - whichever happens first!

Offline (In)Sanity

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Re: How to determine the voltage rating of an unknown capacitor?
« Reply #5 on: January 22, 2013, 10:12:34 pm »
For many ceramic capacitors, the capacitance starts to fall off quite sharply as the dc bias is increased. Perhaps you could devise a circuit which measures capacitance with a gradually increasing dc bias on the cap under test until the capacitance falls by, say, 20%, or you chicken out - whichever happens first!

Thanks Andy,  that might be a good idea.  I've already had a couple tantalum's go up like a match from this sort of testing.   Electrolytic's of course normally have their voltage clearly visible.   Most of the tantalum's do as well..but not all.   On another thought I wonder if the dielectric starts to break down which might also be detected via leakage current?   Or perhaps I would have to ask what causes the capacitance to drop at higher voltage  (or the VC as it's known).

Jeff
 

Online AndyC_772

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Re: How to determine the voltage rating of an unknown capacitor?
« Reply #6 on: January 22, 2013, 11:16:54 pm »
In a ceramic, the physics is quite simple. As the voltage increases, electrons build up on the surface of the plates, and as the voltage increases, the additional electrons form a thicker and thicker layer because they're repelled by the electrons already there. The effective plate separation therefore increases with voltage, and so the capacitance goes down. In some dielectrics it can have dropped by 80% by the cap's rated voltage.

Y5V dielectric - "just say no".

Offline (In)Sanity

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Re: How to determine the voltage rating of an unknown capacitor?
« Reply #7 on: January 22, 2013, 11:22:29 pm »
In a ceramic, the physics is quite simple. As the voltage increases, electrons build up on the surface of the plates, and as the voltage increases, the additional electrons form a thicker and thicker layer because they're repelled by the electrons already there. The effective plate separation therefore increases with voltage, and so the capacitance goes down. In some dielectrics it can have dropped by 80% by the cap's rated voltage.

Y5V dielectric - "just say no".

Thanks Andy,  happen to know of any easy way to measure capacitance at higher voltages? My B&K LCR meter isn't going to help me on this one.

Jeff
 

Online AndyC_772

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Re: How to determine the voltage rating of an unknown capacitor?
« Reply #8 on: January 23, 2013, 07:12:29 am »
Time to spend a few of those long winter evenings playing with Spice, IMHO. I'd probably start by building an R-C oscillator and an adjustable negative voltage supply; apply the negative voltage to one plate, and measure the resulting oscillator frequency to calculate C.

It could be quite a fun little project, though it could also blow caps to smithereens (watch out for any tantalums!) or damage them in more subtle ways by exceeding their voltage specs.

Personally I'd just buy new caps of known quality and spec, but I'd have to admit, it's a less exciting option.

Offline PA4TIM

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Re: How to determine the voltage rating of an unknown capacitor?
« Reply #9 on: January 23, 2013, 08:19:55 am »
Connect the cap to a DC source, limmited in current. For instance 100V through a potentiometer with a currnt limmiting resistor from wiper to cap. Connect your LCR meter to the cap via two large HV MKT caps ( like 4.7 uF or so)

The value you measure will be 1 / ( 1/4u7 + 1/4u7 + 1/Cx ) but the actual value is not important. This way you can see the capacitance change. And then you know upto which voltage they are usable. They drop in capacitance before they brek down.

If they do not decrease they will breakdown and You know the voltage. Mount the testfixture in a box as protection for your eyes and kin.

But your oscillator idea is nice too. The frequency tels you the capacitance. You can test voltage and if frequency goes up and you note frequency you lo know the capacitance, with some luck the oscillator stops befor the cap explodes. I ould use a Wunderhorn oscillator, this uses two transistors, tubes, or fets and it oscillates from a MHz upto some GHz. Make a switch and some inductors for a broad range. Add a microprocessor ( and maybe a FV converter and to meaure frequency. The uProcessor can calculate the capacitance and also meaure leakage current and voltage  if you want.
Use a servo to control the potentiometer and you even can sweep voltage.
The output of the potentiometer to the X of a scope, a voltge proportional to C from the microprocessor to the Y and you can ake graphs.

Nice, good idea, i have the sme problem, thausants of unknow caps. Most pF ceramic. This test eill give a clue sbout the type and rathing.

i think I'm gonna do that one day. But a bit different. I am not a uProcessor guy but I can sweep the voltsge analoeg using my fingers to turn the pot and use my HP 135 X,Y writer for the plots. The X input to the wiper, the Y input to a IV converter i once build to meaure fF ( but it can do nF or uF as easy), choosing the right gain and frequency that gives a voltage reading in direct in capacitance. I measured fF that way. But the value is not important, it is about the change invalue.
www.pa4tim.nl my collection measurement gear and experiments Also lots of info about network analyse
www.schneiderelectronicsrepair.nl  repair of test and calibration equipment
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Offline plazma

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Re: How to determine the voltage rating of an unknown capacitor?
« Reply #10 on: January 23, 2013, 12:27:47 pm »
I have the same problem. I got 3 component drawers for free.

Full of 0805, 0606 and 0402 passives (RLC) and even a lot of semiconductor. Nice and compact drawers. Solves my component archive system :)

The values are marked but no idea about cap dielectrics, inductor current rating or resistor construction.
I'll probably just check digikey what rating the size usually has.

 

Offline (In)Sanity

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Re: How to determine the voltage rating of an unknown capacitor?
« Reply #11 on: January 23, 2013, 01:50:20 pm »
Plazma , Nice collection of caps.  AndyC and PA4TIM, love the ideas.  I need to re-read everything a few times so I understand the concepts fully. 

I had an idea as well,   what if using a micro with a simple 8 bit DAC + op-amp you charge a cap to say 1 volt,  discharge it for a known time interval and then measure the voltage.   Then charge to 2 volts,  discharge for the same time interval and measure it's voltage,  3 volts...discharge for the same time interval and measure.  This process would continue until the percentage of change during the pre determined time interval fell to some level.   Now this approach might work better to determine the voltage coefficient then the max working voltage,  I'm not sure on that one. 

The micro could also do a low voltage run to determine the time interval needed for the entire test.   For example it would require a longer test period for larger capacitance.   It would then use this exact same interval for each voltage range.

Hope this makes sense,

Ohh and by the way a tantalum cap will still catch fire regardless of the current limiting resistor.   It makes for lots of excitement.   

Jeff

Edit:  I forgot to mention that the discharge process might need to use a constant current vs constant resistance. 
« Last Edit: January 23, 2013, 02:03:41 pm by (In)Sanity »
 

Offline PA4TIM

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Re: How to determine the voltage rating of an unknown capacitor?
« Reply #12 on: January 23, 2013, 02:10:07 pm »
Leakage ( for electrolytics most) will polute your reading. Possible also dielectric absorption. But for non electrolyts it could work. Why not use a 16 or 24 bit ADC for the micro. They cost allmost nothing ( i heard, i never do digital stuff but I have a 24 bit here for a auto reformer, leakage tester, VC tester, ESR, C meter project, a friend made it for me on a pcb together with guidelines to make the program on an arduino, a long term project but sub sections are finnished allready, the rest is still in my head and parts on paper, the C measurement will be done with a capacitance to voltage converter IC from AD)
www.pa4tim.nl my collection measurement gear and experiments Also lots of info about network analyse
www.schneiderelectronicsrepair.nl  repair of test and calibration equipment
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Offline (In)Sanity

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Re: How to determine the voltage rating of an unknown capacitor?
« Reply #13 on: January 23, 2013, 02:27:29 pm »
Leakage ( for electrolytics most) will polute your reading. Possible also dielectric absorption. But for non electrolyts it could work. Why not use a 16 or 24 bit ADC for the micro. They cost allmost nothing ( i heard, i never do digital stuff but I have a 24 bit here for a auto reformer, leakage tester, VC tester, ESR, C meter project, a friend made it for me on a pcb together with guidelines to make the program on an arduino, a long term project but sub sections are finnished allready, the rest is still in my head and parts on paper, the C measurement will be done with a capacitance to voltage converter IC from AD)

Luckily electrolytic's tend to be well marked.  No reason not to use a 16 bit or higher DAC,  perhaps just do a rough run and then narrow in the final value.   If the concept seams sound I may pursue it.   Need to finish up my lab power supply project first.   It's coming together well fortunately.

Jeff
 

Offline jimmc

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Re: How to determine the voltage rating of an unknown capacitor?
« Reply #14 on: January 23, 2013, 03:24:55 pm »
Should mention that not all ceramic capacitors have a significant voltage coefficient.
Those using a Class 1 dielectric (NPO, COG) do not change significantly with applied voltage or temperature .
This dielectric is used for lower value capacitors say less than 1nF give or take a decade.

A quick blast with a hot air gun will show if you have a Class 1 dielectric - if the capacity doesn't change significantly with temperature, it won't change with voltage.

For example see http://www.vishay.com/docs/23140/geninfo.pdf pages 6-8

The only way I can think of testing these is to apply a proving voltage of 3 times the voltage you want to use them at (through a current limiting resistor), if they survive for 60sec then you should be OK.
e.g. If you apply 48v through 47k for 60 sec and no significant current flows then they should be safe at 16v.

Jim
 

Offline G7PSK

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Re: How to determine the voltage rating of an unknown capacitor?
« Reply #15 on: January 23, 2013, 03:53:25 pm »
If you have a quantity of the same capacitors you could just ramp up the voltage until they blow, then run the remaining ones at half or three quarter voltage of that which blows them up.
 

Offline SeanB

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Re: How to determine the voltage rating of an unknown capacitor?
« Reply #16 on: January 23, 2013, 04:04:56 pm »
Very simple, you make a test jig to place the capacitor in, with 2 very good capacitors to provide isolation of the cap under test from the test voltage. Generally i have seen a 10 or 15uF 250V unit used here ( a bloody big capacitor and most likely the largest film unit you can buy) in series with the leads to the DUT, and the power supply is connected via a pair of 100k resistors. You measure the capacitance at )v 9 you are measuring 3 caps in series, so it will measure low) then slowly wind up the voltage applied till you either are approaching the isolation capacitor voltages or the voltage across the capacitor drops due to leakage ( measure via a second set of 100k isolating resistors to the voltmeter) so you know how the capacitance changes with voltage and how leaky they are.

The isolating caps unfortunately will have to be good ones, polypropylene foil, ( polystyrene is best but a 10uF one is the size of a large shoe box), polycarbonate or polyester foil. Not electrolytic or ceramic. 100V, 250V or 400v best. higher voltage bigger size. If you cannot get 10uF use 3 4.7uF in parallel instead, will be easier to find.
 

Offline saturation

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Re: How to determine the voltage rating of an unknown capacitor?
« Reply #17 on: January 23, 2013, 05:18:56 pm »
In theory, the breakdown voltage is characterized :



http://en.wikipedia.org/wiki/Capacitor#Breakdown_voltage

For general dielectric strengths, Eds you can find them here:

http://en.wikipedia.org/wiki/Dielectric_strength

Then guestimate the plate separation distance, d, in meters. 

This is taken from Paschen's law but actual construction and conditions to the capacitor matters, so in the end destroying a few is your best and most simple test.  Using 1/4 or 1/3 of the Vbd as a derating is also just overall good practice for high reliability systems.

http://en.wikipedia.org/wiki/Paschen%27s_law#Basics

Bolded below are 2 techniques I use.   One can get 'grab bag' bulk capacitors in the 100s from someone, even if marked, but could have been QC rejects.  Its good to test those caps are to some spec, including Vbd

 Also, watching them blow is also more fun than measuring capacitance   :-+ so you have incentive to characterize you're salvaged caps  ;)



Should mention that not all ceramic capacitors have a significant voltage coefficient.
Those using a Class 1 dielectric (NPO, COG) do not change significantly with applied voltage or temperature .
This dielectric is used for lower value capacitors say less than 1nF give or take a decade.

A quick blast with a hot air gun will show if you have a Class 1 dielectric - if the capacity doesn't change significantly with temperature, it won't change with voltage.

For example see http://www.vishay.com/docs/23140/geninfo.pdf pages 6-8

The only way I can think of testing these is to apply a proving voltage of 3 times the voltage you want to use them at (through a current limiting resistor), if they survive for 60sec then you should be OK.
e.g. If you apply 48v through 47k for 60 sec and no significant current flows then they should be safe at 16v.

Jim
If you have a quantity of the same capacitors you could just ramp up the voltage until they blow, then run the remaining ones at half or three quarter voltage of that which blows them up.
« Last Edit: January 23, 2013, 05:23:58 pm by saturation »
Best Wishes,

 Saturation
 

Offline westfw

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Re: How to determine the voltage rating of an unknown capacitor?
« Reply #18 on: January 23, 2013, 06:35:04 pm »
Quote
Then guestimate the plate separation distance, d, in meters.
Yeah, right.  How do you propose to do that, for a modern ceramic or electrolytic?
 

Offline olsenn

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Re: How to determine the voltage rating of an unknown capacitor?
« Reply #19 on: January 23, 2013, 07:20:50 pm »
The simple answer to this one is, you don't! Throw away your unknown caps and buy a new batch that provide you with a rating.

However, electrolytics usually imprint the voltage rating on the cap sleeve itself, and other caps are usually pretty standardized.
 

Offline smashedProton

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Re: How to determine the voltage rating of an unknown capacitor?
« Reply #20 on: January 23, 2013, 07:50:29 pm »
Blow the ass out of it!   :-+
http://www.garrettbaldwin.com/

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Offline (In)Sanity

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Re: How to determine the voltage rating of an unknown capacitor?
« Reply #21 on: January 23, 2013, 07:52:18 pm »
Blow the ass out of it!   :-+

I would much rather do that then throw them away.
 

Offline Alexei.Polkhanov

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Re: How to determine the voltage rating of an unknown capacitor?
« Reply #22 on: January 23, 2013, 09:48:41 pm »
Hmm, when I am not happy about ESR of an electrolytic capacitor I usually try to replace it with one of same capacity but higher voltage rating. Usually it works so that 6.3V cap will have twice ESR of 16V one and so on. It is true at least for manufacturers that I usually order/use (Panasonic). Higher voltage rated ones are usually (not always) bigger.
Reverting this dependency should give a clue about voltage rating knowing the ESR of similar cap of known voltage rating. Also longer taller caps tend to have lower ESR - I am not sure if there is any such dependency for chip capacitors, but those with higher profile (same size i.e. 3.2x1.6mm) tend to have lower ESR as well.
 

Offline saturation

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Re: How to determine the voltage rating of an unknown capacitor?
« Reply #23 on: January 23, 2013, 10:22:40 pm »
Multilayer ceramics range from 30um to 1 um between plates.  With air as a cheapest dielectric, at 3MV/m then estimated breakdown voltage such caps would be ~ 3V to 90V.  Since dielectrics in reality are rarely air, then most generally purpose caps can be used in 5V applications.  Always best to test it empirically as its is possible to get a ~ < 3V capacitor.

http://my.execpc.com/~endlr/ceramic.html

From the equation, as Vbd ~ d, then physically bigger capacitors, are more likely to have greater Vbd too.  Thus, through hole types are thus likely suitable for 5V applications compared to SMT. 

Older style disc capacitors or electrolytics have larger d, you may be able to see the plate distances if you decapitate one with a dremel then use a micrometer to measure the plate distances. Luckily, larger caps like these still have readable Vbd values printed on them.


Quote
Then guestimate the plate separation distance, d, in meters.
Yeah, right.  How do you propose to do that, for a modern ceramic or electrolytic?

Best Wishes,

 Saturation
 

Offline (In)Sanity

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Re: How to determine the voltage rating of an unknown capacitor?
« Reply #24 on: January 23, 2013, 11:48:17 pm »
Very simple, you make a test jig to place the capacitor in, with 2 very good capacitors to provide isolation of the cap under test from the test voltage. Generally i have seen a 10 or 15uF 250V unit used here ( a bloody big capacitor and most likely the largest film unit you can buy) in series with the leads to the DUT, and the power supply is connected via a pair of 100k resistors. You measure the capacitance at )v 9 you are measuring 3 caps in series, so it will measure low) then slowly wind up the voltage applied till you either are approaching the isolation capacitor voltages or the voltage across the capacitor drops due to leakage ( measure via a second set of 100k isolating resistors to the voltmeter) so you know how the capacitance changes with voltage and how leaky they are.

The isolating caps unfortunately will have to be good ones, polypropylene foil, ( polystyrene is best but a 10uF one is the size of a large shoe box), polycarbonate or polyester foil. Not electrolytic or ceramic. 100V, 250V or 400v best. higher voltage bigger size. If you cannot get 10uF use 3 4.7uF in parallel instead, will be easier to find.

Doesn't this just equate to indirectly increasing the voltage until the leakage current starts to increase ?   How does the test isolation capacitors keep for example a tantalum from catching fire ?    Also are you saying the isolation capacitors need to be very high quality to reduce leakage current?    Sorry if I'm being a bit dense on this one.

Thanks,

Jeff
 

Online Jay_Diddy_B

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Re: How to determine the voltage rating of an unknown capacitor?
« Reply #25 on: January 24, 2013, 03:47:43 am »
Hi,

For anybody interested in modelling the voltage coefficient of a ceramic capacitor in LTspice I developed this model.



Instead of specify the capacitor in uF it is specified in terms of charge. in LTspice X is the voltage across a component. The model shows a capacitor that has a value of 1.08 times in nominal value, falling by 40% at the rated voltage.

In the test circuit I compare a linear capacitor with a nonlinear capacitor.

Here is a scope shot showing a real nonlinear capacitor:


Top trace is the voltage across the capacitor. The input step is 6V.

The lower trace is the input current.


Jay_Diddy_B
« Last Edit: January 24, 2013, 03:55:34 am by Jay_Diddy_B »
 

Offline (In)Sanity

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Re: How to determine the voltage rating of an unknown capacitor?
« Reply #26 on: January 25, 2013, 04:14:02 am »
So in the interests of actually being able to use the capacitors I have on hand I've employed the Fred Flintstone method of testing.   It's called a current limited 1000v DC supply.   So what's an acceptable leakage current for an average ceramic SMD cap,  or should I just blow one up and see where it fails ?

I figure if I can leave it run for days at voltages it's going to be seeing in whatever circuit I'll be using it in..then it has a fighting chance.   The VC however might be a big issue.    How about curve tracing with a variable high voltage AC supply ?

Thanks,

Jeff
 

Offline (In)Sanity

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Re: How to determine the voltage rating of an unknown capacitor?
« Reply #27 on: January 25, 2013, 04:43:05 am »
Ok,  so I just tested a 3.3nf cap from the batch of several thousand that I have and it failed @ 930 volts.   Just made a little "tick" sound and the current dropped off.  So I can assume the batch of caps is probably gong to work at a few hundred volts. That pesky VC is what will haunt me.  I had a few others not related to this batch fail @ ~300 volts.   I think I'll try to see if the leakage current is non linear with voltage.

Thanks,

Jeff
 

Online AndyC_772

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Re: How to determine the voltage rating of an unknown capacitor?
« Reply #28 on: January 25, 2013, 07:16:04 am »
The rated voltage of ceramic capacitor is nothing to do with the voltage at which it actually breaks down and fails, it's to do with the voltage at which it retains its capacitance. If you could test the effect on capacitance as the dc bias increases, you'd find that it falls to a fraction of its rated value long before it's actually destroyed.

Offline ftransform

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Re: How to determine the voltage rating of an unknown capacitor?
« Reply #29 on: January 25, 2013, 09:58:15 am »
I had this theory regarding a 555 timer which uses a capacitor to determine frequency or duty cycle. There is a formula for output waveform of the 555 timer. Can the input voltage of a 555 timer be varied (while holding capacitance constant) in order to perform quick and easy capacitance measurements with regards to voltage (at least from 5-15 volts or whatever it is). Or will it exhibit nonlinear behavior because of other circuit properties?

Or of course if any one can suggest a IC that will work for the scenario that I have described it would be appreciated. I think a single chip solution without micro controller would be in everyone's best interest.
« Last Edit: January 25, 2013, 10:09:47 am by ftransform »
 

Offline (In)Sanity

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Re: How to determine the voltage rating of an unknown capacitor?
« Reply #30 on: January 25, 2013, 01:51:36 pm »
The rated voltage of ceramic capacitor is nothing to do with the voltage at which it actually breaks down and fails, it's to do with the voltage at which it retains its capacitance. If you could test the effect on capacitance as the dc bias increases, you'd find that it falls to a fraction of its rated value long before it's actually destroyed.

This part I completely understand.  It's why I mentioned in my post about the VC being the one pesky thing.   My goal was to simply determine a worse case scenario.    If the capacitor physically fails at 300 volts then clearly it's not going to have a working voltage of 500.   Not at all scientific I know,  but it gives me some idea what I'm dealing with.   
 

Offline (In)Sanity

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Re: How to determine the voltage rating of an unknown capacitor?
« Reply #31 on: January 25, 2013, 02:02:03 pm »
I had this theory regarding a 555 timer which uses a capacitor to determine frequency or duty cycle. There is a formula for output waveform of the 555 timer. Can the input voltage of a 555 timer be varied (while holding capacitance constant) in order to perform quick and easy capacitance measurements with regards to voltage (at least from 5-15 volts or whatever it is). Or will it exhibit nonlinear behavior because of other circuit properties?

Or of course if any one can suggest a IC that will work for the scenario that I have described it would be appreciated. I think a single chip solution without micro controller would be in everyone's best interest.

In one of my earlier posts I had mentioned a way to use a micro with a DAC, op-amp and ADC to determine the VC at least.   It might need to be a boot-strapped op-amp with the higher voltages.   A PWM controlled switch mode supply might be easier.   My 1000v supply is just a PWM controlled high voltage transformer from some LCD back lighting with a high voltage bridge and high voltage caps.   It does the job.   

The theory was to charge to a fixed voltage and measure the amount lost at a constant current for an exact duraciton,   then charge to the next higher voltage and repeat the process.   As the capacitance falls off do to the VC the percentage of loss will increase.   My other post might clarify this more.

https://www.eevblog.com/forum/chat/how-to-determine-the-voltage-rating-of-an-unknown-capacitor/msg184583/#msg184583
 

Offline TerraHertz

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Re: How to determine the voltage rating of an unknown capacitor?
« Reply #32 on: January 25, 2013, 03:06:58 pm »
I have the same problem. I got 3 component drawers for free.

<swoon>
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Offline ftransform

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Re: How to determine the voltage rating of an unknown capacitor?
« Reply #33 on: January 25, 2013, 05:42:04 pm »
I had this theory regarding a 555 timer which uses a capacitor to determine frequency or duty cycle. There is a formula for output waveform of the 555 timer. Can the input voltage of a 555 timer be varied (while holding capacitance constant) in order to perform quick and easy capacitance measurements with regards to voltage (at least from 5-15 volts or whatever it is). Or will it exhibit nonlinear behavior because of other circuit properties?

Or of course if any one can suggest a IC that will work for the scenario that I have described it would be appreciated. I think a single chip solution without micro controller would be in everyone's best interest.

In one of my earlier posts I had mentioned a way to use a micro with a DAC, op-amp and ADC to determine the VC at least.   It might need to be a boot-strapped op-amp with the higher voltages.   A PWM controlled switch mode supply might be easier.   My 1000v supply is just a PWM controlled high voltage transformer from some LCD back lighting with a high voltage bridge and high voltage caps.   It does the job.   

The theory was to charge to a fixed voltage and measure the amount lost at a constant current for an exact duraciton,   then charge to the next higher voltage and repeat the process.   As the capacitance falls off do to the VC the percentage of loss will increase.   My other post might clarify this more.

https://www.eevblog.com/forum/chat/how-to-determine-the-voltage-rating-of-an-unknown-capacitor/msg184583/#msg184583

That method looks good but a single chip solution would be nicer. Especially if it could be done with a 555 timer. I think a duty cycle change would be the best because then you would not need a oscilloscope and it could be measured with the lowliest of multimeter, perhaps if some math is done? I think some multimeters even measure frequency?

I'm just trying to think of the simplest solution with lowest part count and greatest availability for everyone.
« Last Edit: January 25, 2013, 05:46:23 pm by ftransform »
 

Offline (In)Sanity

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Re: How to determine the voltage rating of an unknown capacitor?
« Reply #34 on: January 25, 2013, 06:21:07 pm »
I'm just trying to think of the simplest solution with lowest part count and greatest availability for everyone.

I had mentioned even perhaps just old fashioned curve tracing?   I've not tried this concept yet,  but might be useful.   It should show the drop in capacitance with increased AC voltage.   

Jeff
 

Online AndyC_772

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Re: How to determine the voltage rating of an unknown capacitor?
« Reply #35 on: January 25, 2013, 06:50:21 pm »
I think it's worth revisiting what the voltage rating of a capacitor actually means.

With a ceramic, as we've discussed, it's primarily to do with the fall-off of capacitance with voltage. It's a limit imposed by the useful functional performance of the device. Exceed it and the part just doesn't do its job properly - and that's easily defined and measured.

With other types it's not so clear cut, though. Tantalums catch fire and go bang, so I'd be very wary indeed of trying to measure the rated voltage of one at all.

Other types may simply suffer a reduced lifetime at higher voltage, so you might find that a 10V cap works perfectly well at 20V for a while, but dies prematurely. A test instrument won't tell you that, unless you specifically set out to perform long-term reliability testing.

Personally I still think the cheapest, easiest way to get a capacitor of known value and rating for a project is to buy a new one from a reputable supplier - but I applaud the academic exercise nonethelsss.

Offline ftransform

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Re: How to determine the voltage rating of an unknown capacitor?
« Reply #36 on: January 25, 2013, 10:58:55 pm »
I think it's worth revisiting what the voltage rating of a capacitor actually means.

With a ceramic, as we've discussed, it's primarily to do with the fall-off of capacitance with voltage. It's a limit imposed by the useful functional performance of the device. Exceed it and the part just doesn't do its job properly - and that's easily defined and measured.

With other types it's not so clear cut, though. Tantalums catch fire and go bang, so I'd be very wary indeed of trying to measure the rated voltage of one at all.

Other types may simply suffer a reduced lifetime at higher voltage, so you might find that a 10V cap works perfectly well at 20V for a while, but dies prematurely. A test instrument won't tell you that, unless you specifically set out to perform long-term reliability testing.

Personally I still think the cheapest, easiest way to get a capacitor of known value and rating for a project is to buy a new one from a reputable supplier - but I applaud the academic exercise nonethelsss.

me and insanity are chinese quality control. :-DD

I think I am going to do some experiments in order to measure these guys tonight.
 

Offline robrenz

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Re: How to determine the voltage rating of an unknown capacitor?
« Reply #37 on: January 25, 2013, 10:59:28 pm »
Don't forget the dielectric is an insulator and has a breakdown voltage where it will puncture. A higher voltage capacitor is going to have a thicker dielectric.

Online Fraser

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Re: How to determine the voltage rating of an unknown capacitor?
« Reply #38 on: January 26, 2013, 12:07:17 am »
I bought a Duoyi DY294 Semiconductor analyser a couple of years ago that also claims to provide the breakdown voltage of capacitors. I have never used it for such but suspect it works on the leakage across the dielectric as the voltage is ramped up to 200V  :o I don't like causing a breakdown in a dielectric....it just seems the wrong thing to do somehow. I suppose if the capacitor is sacrificial and not for use it is of little consequence.

I have attached the DY294 manual

A review is here:

http://www.jestineyong.com/review-of-the-dy294-digital-transistor-tester-part-1/

http://www.jestineyong.com/review-of-the-dy294-digital-transistor-tester-part-2/

http://www.jestineyong.com/review-of-the-dy294-digital-transistor-tester-part-3/

For GBP30 delivered, it wasn't a bad buy.

Regards

Fraser


« Last Edit: January 26, 2013, 12:30:59 am by Aurora »
 

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Re: How to determine the voltage rating of an unknown capacitor?
« Reply #39 on: January 26, 2013, 07:58:22 pm »
Well one thing I can say about breaking down the dielectric on a capacitor.   If you thought it was rated at say 200 volts and it goes up in smoke at 150 volts then clearly you were mistaken.   If on the other hand it survives to 930 volts like one of mine did,  then it might be anything from 1 volt to 500 volt rated :)   Yep I know..so helpful isn't it. 

Jeff
 

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Re: How to determine the voltage rating of an unknown capacitor?
« Reply #40 on: January 26, 2013, 09:32:01 pm »
That's a really interesting device.  Rather than just a transistor or any other semiconductor tester, its really a 1kV or 200V current limited supply that you could use for testing anything for its reverse breakdown or voltage limits and a handheld at that.  I don't think anyone else makes such a thing in such a form or cost.

Its very helpful for salvaging parts or overstock and characterizing them.

I bought a Duoyi DY294 Semiconductor analyser a couple of years ago that also claims to provide the breakdown voltage of capacitors. I have never used it for such but suspect it works on the leakage across the dielectric as the voltage is ramped up to 200V  :o I don't like causing a breakdown in a dielectric....it just seems the wrong thing to do somehow. I suppose if the capacitor is sacrificial and not for use it is of little consequence.

I have attached the DY294 manual

A review is here:

http://www.jestineyong.com/review-of-the-dy294-digital-transistor-tester-part-1/

http://www.jestineyong.com/review-of-the-dy294-digital-transistor-tester-part-2/

http://www.jestineyong.com/review-of-the-dy294-digital-transistor-tester-part-3/

For GBP30 delivered, it wasn't a bad buy.

Regards

Fraser

Best Wishes,

 Saturation
 

Offline JackOfVA

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Re: How to determine the voltage rating of an unknown capacitor?
« Reply #41 on: January 26, 2013, 10:15:29 pm »
The high K ceramic dielectric capacitors have complex C versus V behavior, to say the least.

The first plot below demonstrates two behavior characteristics.

First, hysteresis - the capacitance at a particular voltage depends upon the history of the voltage. This is the electric field analog of B-H hysteresis seen in magnetic core inductors. Note that the starting point, just like a B-H curve, does not fall on the hysteresis loop.

Second, the C versus V relationship depends upon the interval between voltage steps. See Bob Pease on "dielectric soakage" for a discussion of this phenomena. A short version of it is that the dipole charge elements in the dielectric do not all orient and reorient at the same speed with respect to voltage changes.  This is modeled by Pease as multiple RC elements in parallel.

The data is taken with an HP4192A impedance meter using the internal bias voltage, with the HP 4192A controlled over the IEEE-488 interface by a program I wrote. Data analyzed and plotted with Origin software.  I intentionally used a Z5U part for these measurements to illustrate how far their behavior diverges from the perfect capacitor we often think of.

To illustrate the dielectric soakage further, I ran the same 0u1 Z5U capacitor through a series of alternating +35V/-35V cycles. The first plot shows the full data run and the second shows an expanded view of a couple of reversal periods.

To show how badly the non-linear capacitor can be in real life, I looked at current through a Y5U at 1000 Hz and compared it with a high quality film cap. Voltage is the upper trace, current through the capacitor is the lower trace. The distortion is obvious just looking at the current waveform.

Finally, it's not just capacitance that can change with applied voltage; so can the dissipation factor. The D versus V plot is older data, taken with a General Radio bridge.



 

Offline robrenz

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Re: How to determine the voltage rating of an unknown capacitor?
« Reply #42 on: January 26, 2013, 10:22:01 pm »
Nice work JackOfVA :-+

Online Fraser

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Re: How to determine the voltage rating of an unknown capacitor?
« Reply #43 on: January 26, 2013, 10:54:28 pm »
Saturation,

I bought the DY294 because it looked useful for all manner of tests and appeared well made for the money. I must be honest and say that I have not used it in anger. It does seem to be a lot of test capability for not a lot of money. I know of no other kit in my lab that will voltage test a capacitor or VDR so easily.

The user manual is pretty appalling and no new versions beyond V3 seem to be available. The ergonomics of the unit are not optimum either....but for GBP30 delivered....its not bad.

Worth having in the test kit inventory ? I think so.

Fraser
« Last Edit: January 27, 2013, 08:08:26 pm by Aurora »
 

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Re: How to determine the voltage rating of an unknown capacitor?
« Reply #44 on: January 27, 2013, 06:32:30 pm »
Well the graphs provided by JackOfVA certainly add more complexity to this puzzle.   It's starting to sound like their is no simple way to perform these tests.   That's not to say I've given up.   

Thanks,

Jeff
 

Offline JackOfVA

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Re: How to determine the voltage rating of an unknown capacitor?
« Reply #45 on: January 28, 2013, 12:58:03 am »
To return this to the original question, if you have sufficient number of capacitors of a particular type to test to destruction, apply a current limited DC voltage to a representative sample of the lot, increasing the voltage until the dielectric ruptures, calculating the max, min, mean and standard deviation. 

Apply a reasonable safety factor to the result, perhaps 3:1 or 4:1 from the mean.

That strategy won't be useful if you have 5 or 10 parts of each particular type, but if you have 100 or 200, sacrificing 5 or 10 for a destructive breakdown test isn't too bad a price to pay.

With a knowledge of a reasonable maximum recommended operating voltage, you can then look at C versus V over the rated voltage to get an idea of the dielectric class. NP0/C0G ceramic will have next to no change in capacitance with applied voltage. X7R will have a lot and Z5U even more. (I assume these are ceramic capacitors.)

If you want, I'll can run a destructive test on a few parts for you - at least up to 1KV, don't think I can easily go beyond that point without kludging up a HV transformer and a Variac and I don't have time for that. But, I can run up to 1 KV with a standard lab HV supply. Contact me directly if you are interested. I can't run hundreds of different parts, but be glad to look at a dozen or so samples of four or five different values and/or capacitor types.

With respect to surface mount ceramic caps, most every C0G/NP0 part I purchase is  light gray color whilst the X7R types are brown or tan. That's not universal and I don't suggest it as a definitive test for dielectric class, but I can say that I've never seen a light gray ceramic surface mount cap that was not a C0G/NP0 type and that I've never seen an X7R of light gray color. I have seen a few C0G/NP0 parts that are brownish, particularly in values of a 100 pF or less. 

 
 

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Re: How to determine the voltage rating of an unknown capacitor?
« Reply #46 on: January 28, 2013, 03:20:44 am »
I have no problems with 1000V AC or DC.   I've only done DC destruction tests so far.  I can try AC of various frequencies.   I'll see what I get.  My focus has shifted to trying to fix a 60 inch TV with back-lighting that acts up after 10 hours.   But that's another subject.

Jeff
 
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