General > General Technical Chat
How to determine the voltage rating of an unknown capacitor?
Jay_Diddy_B:
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
(In)Sanity:
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
(In)Sanity:
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
AndyC_772:
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.
ftransform:
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.
Navigation
[0] Message Index
[#] Next page
[*] Previous page
Go to full version