How to measure capacitor ripple current
How to measure capacitor ripple current
(OP)
Is it as simple as measuring the ripple voltage across the cap and dividing by the ESR?
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How to measure capacitor ripple current
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RE: How to measure capacitor ripple current
You could lift one leg of the capacitor and put a current meter in series. I remember seeing another thread on EngTips where someone had a small current sense loop that fit around the legs of SMD components, but I cannot find that thread now.
RE: How to measure capacitor ripple current
Anything in series with the cap will greatly reduce the actual ripple current messing up the measurement.
You would need something like a clamp-on current probe.
Ripple heats capacitors. It limits the system's ultimate running temperature. If you can run your system steady-state until the temperature is stable and determine the cap temp, you can assess whether or not your ripple level is going to be an issue. If the temp rise is too great, you will need more capacitors to share the ripple.
Keith Cress
kcress - http://www.flaminsystems.com
RE: How to measure capacitor ripple current
we are used to hearing how academic calculations are useless.
but not in this case.
U dont say topology so i assume boost.
U know your duty cycle by scoping fet gate etc.
u work out the boost diode trapezoid current shape.
then either use integration
..or go to google and search for "rms of smps waveforms" or similar.
"smps waveform calculator" etc etc........
...put in peak and pedestal current levels of the trapezoid, and duty cycle and it will give you the rms, the dc and the AC.
the AC is what your caps take.
of course if you want to se what each of the paralleled caps take then it will be in proportion to their faradic values.
so you can try this. it definetly give u the tops value.
RE: How to measure capacitor ripple current
There may be easier ways if you have advanced knowledge of the circuit/waveform, but this should work in general.
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RE: How to measure capacitor ripple current
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RE: How to measure capacitor ripple current
RE: How to measure capacitor ripple current
Makes a lot more sense than what I suggested.
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RE: How to measure capacitor ripple current
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: How to measure capacitor ripple current
I think the best method would be to place a low value series resistor or ammeter shunt in series with the capacitor then measure the voltage with a scope. Of course your series resistor will have some small effect on the current but then so would almost any other form of ammeter.
BTW the poster is not very specific, does he want average, peak or something else?
Roy
RE: How to measure capacitor ripple current
That's why the Rogowski coil is such a useful device: no insertion loss to note, tremendous bandwidth, and immune to magnetic saturation. The negatives are that they can't detect a DC level, and they need signal conditioning which requires a power supply. The problem with adding a resistor is that a shunt of a size large enough to be useful is likely to be equal or larger than the ESR of the cap under test, and will shift the circuit operating point so far that the readings are potentially meaningless.
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If we learn from our mistakes I'm getting a great education!
RE: How to measure capacitor ripple current
RE: How to measure capacitor ripple current
Draw out the equivalent circuit of the capacitor including the parasitics and insert it in your equation i = C dv/dt.
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If we learn from our mistakes I'm getting a great education!
RE: How to measure capacitor ripple current
I didn't think a shunt resistor would add that much, you could probably get a decent signal with < 10mV and it meets KISS rules eh!
A clip-on ammeter ammeter would likely add as much impedance and have the effect of damping out any spikes due to the iron. Perhaps some sort of Hall Effect transducer (not ammeter) reading the waveform with a scope.
Is a Rojowski coil something readily available?
Cheers
Roy
RE: How to measure capacitor ripple current
250V DC, 6800uF, 43.5A @ 40°C, ESR = 20mΩ @100Hz, ESR = 12mΩ @100kHz, ESL = 16nH
I suppose you might get away with using a part of the interconnecting wiring or PCB trace as a shunt. A PCB trace should be of a known thickness so it should be very easy to determine a convenient length to act as shunt.
Rogowski coils are readily available, but it's simple to wind your own too. Any bit of flexible tubing will do as a former providing it can be wound with fine magnet wire. The inner bore provides a return path so both ends of the coil are conveniently placed together for connection to the outside world. The governing equations are readily available - I ain't fighting TGML to post them here, ha-ha - but the trick (if there is one) is usually the integrator design, making sure that it is behaving correctly at the frequencies of interest. For precision work I'm told it is possible to add compensating networks to the coil to get a response accurate down into the handful of PPM level - the guy who owns Ro-Coil has done some pretty specialised stuff and is very knowledgable about the subject, and he's been kind enough to improve my understanding of these transducers.
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If we learn from our mistakes I'm getting a great education!
RE: How to measure capacitor ripple current
It all hearkens back to the capacitor not actually caring about waveform or magnitude. No particular ripple current will hurt the capacitor. It is the temperature that hurts the capacitor. Run your circuit and measure the cap temperature as that is the only thing you should ultimately care about. Why go to painstaking, expensive, effort if the result is a result that then gets inferred to yet something else, (temperature). Measure the actual end result directly.
Keith Cress
kcress - http://www.flaminsystems.com