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'Current detection with a PIC?'
1996\08\28@152238 by Norm Cramer

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I need to detect a change in current using a PIC.  I am using the PIC16C73
and was wondering if the A/D would be the way to go.  The application needs
to detect a current pulse from a load that is a minimum of 500usec wide and
a maximum of 5msec wide.  The pulse is the response to a command.  The PIC
only needs to determine if the command was acknowledged or not.  The ack
pulse will be at least 60ma above the normal current draw.  I think normal
current draw is from a few ma to over 500ma.  The drive voltage to the load
is a bipolar wave as follows:


+v     _____
       |    |    |
 0     |    |    |   with a pulse width of ~58usec.
       |    |    |
-v ____|    |____|


I thought about using a resistor in the return line and sensing the voltage
across the resistor, but with a wide range of possible "idle" current levels
it seems kinda hard to get the needed resolution.

Any ideas?

Thanks,

Norm

1996\08\28@182557 by Scott Dattalo

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Norm Cramer wrote:
{Quote hidden}

A couple:

Using your sense resistor you could:

a) use a high pass filter to monitor the voltage across it

or

b) use a peak detector to monitor the voltage across it.

Since it appears you only are trying to see if the ACK was received
or not the A/D is overkill. A comparator is more appropriate. (OTH,
if it's already there ...) Even though I've been recently reamed by
the PIC16C622's comparators, I can still recommend them for this
application.


Scott

Aside... A complaint not worthy of its on message:

The PIC16C622 comparators have one mode that brings the comparator
outputs off of the chip. Typically, you would want to use these outputs
as part of a hysteresis circuit. Unfortunately the comparators have
been designed (mis-designed) in such a way that it is nearly impossible
to use them this way. While I don't totally agree with everything in
Martin's PIC wish list, I would think it would be nice if you could
program the comparator's output polarity....

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