On 22 Jun 2015 at 14:19, David C Brown wrote:

> I need an overcurrent detector for a small DC motor.  I thought of using
> low side sensing with the ADC of a PIC measuring the voltage across the
> sense resistor and then doing the integration and peak suppression in
> software.  I can tolerate half a volt or more voltage drop in normal
> operation.
>=20
> This seems to such an obvious idea that I am surprised that I can't find
> any reference to it in a search for "overcurrent detection using PIC".

Yes, it's a really common thing, perhaps typically appears along side motor=
 control=20
so less often referred to directly. Simplest is just a current sensingf res=
istor, like you=20
say done on the low side (everything referenced to ground) makes some thing=
s=20
easier.

A decent sized silicon diode in parallel with the shunt is a good trick to =
limit the peak=20
shunt voltage/power dissipation during startup/overload/motor stall conditi=
ons. Just=20
need to be aware diode messes up current to voltage conversion as it starts=
 to=20
conduct.... though typically you might be measuring motor current somewhat=
=20
accurately at the lower end of the scale (a working range of say 0-400mV or=
 so) and=20
anything higher you'd just flag as an overload condition.

Shunt resistor voltage to an ADC input, or with Op Amp gain first for bette=
r=20
reolution, should always have over-voltage protection and some filtering.

Also a common alternative to using an ADC input is to feed shunt voltage in=
to base=20
of an NPN transistor... basically a cheap ~0.7V voltage detector. Emitter t=
o ground,=20
pullup resistor on Collector, pulls down I/O line or interrrupt input when =
overcurrent=20
situation exists (whenever voltage drop accross shunt resistor is >=3D ~0.7=
V).



--=20
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