Hi Roman.  The load is not inductive, but the sense resistor has been placed
incircuit so that it is always measuring load current.  Using a pair of RC
filters as Olin has suggested between the amplified sense signal and the
PIC's A/D provides me with a good waveform for averaging because I am not in
fact reading hard on and hard off PWM signals, but the rising and falling
slopes of the exponential waveform segments.

Rgs
Ian.

-----Original Message-----
From: pic microcontroller discussion list
[mailto:PICLIST@MITVMA.MIT.EDU]On Behalf Of Roman Black
Sent: Wednesday, 5 March 2003 6:18 pm
To: PICLIST@MITVMA.MIT.EDU
Subject: Re: [PIC]: PWM output current sensing


Hi Ian, are you measuring PWM current in an inductive
load (that has flyback diode)??

If so you *can't* average your on/off pwm current, you
need to put the sense resistor in circuit with the
inductor+diode so it always measures load current (which
remains constant within a few percent).

OR you can measure the current during PWM on, this will
be close to the load average current and you can maybe
deduct a few percent for flyback period current droop
etc.
-Roman



Ian McLean wrote:
>
> I tried a simple passive RC filter on the A/D input in simulation - this
> changes the square wave to a triangular wave (as I remember it should from
> analog electronics).  I guess this would in itself improve the averages
> because of the rising and falling slopes of the triangular wave being hit
> more times than the low (or high) state.  I only need to sample the
current
> about once a second.

> -----Original Message-----

> > An interesting observation arose for me just now whilst cutting the
> > code on an f877 to do the A/D for current sensing from a device being
> > controlled from a PWM voltage source.
> >
> > My PWM is at 5kHz.  I discovered that the current readings were not
> > averages,

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