Measuring average current when there's high frequency ripple?

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Hello there,


One way to do this is to add a small passive low pass filter network across the current sense resistor. Made from maybe a 1k resistor and a capacitor (0.1uf might be enough or even 0.01uf). You then measure the DC voltage value across the cap. 10k might also work but dont go too high to avoid noise.
Note that if this network is slow enough to average your signal, then it doesnt matter if the meter can measure average or not.
The filter time constant should be about 100 times greater than the fastest period which works out to about:
w=100/RC

so for 1MHz the min RC would be a 1k and 0.1uf cap.




Code: 
  IN
   o
   |
   +---R---+---o  Vout
   |       |
   Rs      C
   |       |
   +-------+---o  Meter Common
   |
   o
  IN

 Rs is the current sense resistor.
 R maybe 1k.
 C maybe 0.01uf to 0.1uf.
 
Last edited:
Put a large inductance in series with the input and a large capacitor next to the circuit (on the circuit side of the inductor). Put the sense resistor between the inductor and the bench power supply and probe across the sense resistor with the scope (watch you dont short anything to ground with the ground clip of the scope). If the voltage is flat, you can then probe the voltage with a voltmeter and get 2 decimal places of accuracy.

Inductors are generally better than resistors as you dont get a large voltage drop with load

The inductor smoothes out the current and the ac current demand for the circuit comes from the capacitor
 


Hello there,

It's actually better without the inductor which adds problems. Large inductors can be troublesome at low currents. The RC circuit is simpler, cheaper, and doesnt have the problems that an inductor could bring into the picture. Not only that, but the inductor is not needed as the RC filters just fine all by itself. If you're crazy for smoothness, add another RC section it doesnt matter, but i doubt you'll find it necessary.
 
simonbramble said:
Put a large inductance in series with the input and a large capacitor next to the circuit (on the circuit side of the inductor).
Click to expand...
For a DVM, probably any combination slower than the ripple frequency will work. But for fast measurements or control loop, you need to choose the combination carfully beacuse large RC will cause slow response to sudden changes.
 
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