Calculate / Measure avg current (PWM)

Discussion in 'General Electronics Chat' started by Dyslexicbloke, Jun 6, 2011.

  1. Dyslexicbloke

    Thread Starter Well-Known Member

    Sep 4, 2010
    Hi folks ..
    I asked a question earlier about smoothing out the current resulting from a resistive load via PWM
    Well I didn’t get any answers and some additional research on the web suggests that the whole subject of high power switching and SMPS’s is somewhat complex which probably explains the lack of any input.
    To be honest I doubt I would have been able to understand and implement the nuances of such a supply anyway, so no harm done.

    That leads me to ask this …..
    Assuming I simply have a low side FET switch in series with a fixed resistor ….
    Feeding that is the supply under test, in this case an alternator driven with a miniature turbine.

    If I measure the voltage pulses across the load resistor can I simply calculate the power being dissipated for each pulse and multiply by the % duty cycle to get average power?

    If that is the case, how would I go about getting an analogue value that approximated the average power, preferably without resorting to a uP?

    As always… any and all comments welcome.
  2. iONic

    AAC Fanatic!

    Nov 16, 2007
    If your duty-cycle is 50% and your load voltage is 10V, then the average voltage will be 5V. This will enable you to calculate the average current and/or power.
  3. THE_RB

    AAC Fanatic!

    Feb 11, 2008
    That might be the way the Moody Blues did it, but I think he would be better off with a squarewave PWM into a resistive load to calculate peak power then average based on duty... ;)

    Minor nitpick; "rials" is actually spelt "riles".
  4. Dyslexicbloke

    Thread Starter Well-Known Member

    Sep 4, 2010
    I did a few tests .. Squarewave into resistance as suggested, and found, somwhat to my supprise, that my DC clamp gets it to within 20mA or so right from 5% up to 100%.

    It seems happiest with a frequency of about 4KhZ.

    I have a couple of cheep field effect current sensors that I bought on a whim so I am going to have a blast with one of them and see if they do as well as my meter, failing that I think a small shunt and a moving iron meter will be the way to go as I cant leave the clamp attached when I put the teat rig into service.

    Thanks for the input