General > General Technical Chat
How to determine the voltage rating of an unknown capacitor?
(In)Sanity:
--- Quote from: AndyC_772 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.
--- End quote ---
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.
(In)Sanity:
--- Quote from: ftransform 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.
--- End quote ---
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
TerraHertz:
--- Quote from: plazma on January 23, 2013, 12:27:47 pm ---I have the same problem. I got 3 component drawers for free.
--- End quote ---
<swoon>
ftransform:
--- Quote from: (In)Sanity on January 25, 2013, 02:02:03 pm ---
--- Quote from: ftransform 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.
--- End quote ---
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
--- End quote ---
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.
(In)Sanity:
--- Quote from: ftransform on January 25, 2013, 05:42:04 pm ---I'm just trying to think of the simplest solution with lowest part count and greatest availability for everyone.
--- End quote ---
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
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