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[DPRG] Current Monitoring, boring and with PWM

Subject: [DPRG] Current Monitoring, boring and with PWM
From: Kipton Moravec kmoravec at spectronics.us
Date: Wed Feb 9 14:50:09 CST 2005

If you are doing PWM it is not quite that simple.

First, you want a low ohm resistor, so the resistor does not steal much
power.  As a result the voltage drop will be small and your ADC line may not
have much resolution in that range.

So you will probably have to amplify it with a non-inverting amplifier using

Of course when your PWM is going you want to sample the average current. So
it is better to low pass the data with a simple RC circuit so you are
measuring the average, not the on or off portion.

If you use a coil you will still have to average.


-----Original Message-----
From: dprglist-bounces at dprg.org [mailto:dprglist-bounces at dprg.org]On
Behalf Of Ed Paradis
Sent: Wednesday, February 09, 2005 12:12 PM
To: DPRG List
Subject: [DPRG] Current Monitoring, boring and with PWM

Hi everyone,

I'm looking to do some current monitoring in a device i'm building.
It is sort of a power distribution point that regulates battery
voltage and then provides it (through relays) to 8 different channels.

I'd like monitor the current going through these channels individually
(as a debug measure).  The voltage will remain constant. (or it
better! its Vcc to a bunch of stuff).

Right now i'm thinking about doing the classic 'serial precision
resistor' monitored by an A/D line.  Are there any other ways of doing

Also I need to do something similar with my motor controllers.  I've
been told that using this method with PWM is a pain.  Wouldn't you
just average out the values of the voltage across the resistor?

Thanks for the help.

Ed P
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