Except that the shunt isn't seeing 50% duty cycle, not if the low ESR input capacitors in the controller are doing their job. It's seeing mostly DC with a sawtooth ripple on top. I would think that the ripple frequency was high enough that the amp meters mechanical latency would average it out enough to be almost correct (within a few percent).
Victor Tikhonov wrote: >John Lussmyer wrote: > >>At 01:11 PM 10/9/2002 -0700, Victor Tikhonov stated: >> >>>How does your analog meter averages battery amps? It is not a DC, >>>it's PWM'ed DC so the error for odd wave form may be as much as 50%. >>> >>The little analog amp meter in my Sparrow is small, and does seem to read >>30-40% low most of the time. I have no idea what kind it is, or anything >>else about it. I do believe it measures battery amps, and not motor amps. >> > >Yes, its battery amps, but the number is not real. > >If you draw PWM 200A pulses 50% duty cycle (like >200A-0A-200A-0A every 100us or so), the average current >is 200*0.5=100A but your analog movement meter may show 60...70A. > >A good way to check is to connect a scope to the shunt and get the >average voltage reading ar some load. Divide it by shunt resistance >and you'll get the current your analog meter should show. > >Victor > > >
