# Calculate / Measure avg current (PWM)

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

1. ### Dyslexicbloke Thread Starter Active Member

Sep 4, 2010
420
19
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.

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.
Thanks
Al

2. ### iONic AAC Fanatic!

Nov 16, 2007
1,422
68
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
5,435
1,305
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 Active Member

Sep 4, 2010
420
19
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
Al