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'Current sink & inductive loads'
1996\03\26@110053 by Harrison Cooper

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Well, isn't it wonderfull when you find out a device went obsolete
and no one told you....case in point is a relay I have been using
for years.  Typically, I'll drive it thru a small signal transistor
by putting it in saturation, and sinking the current thru the collector.
So, now (just before I am getting some boards made), I must find a new
relay.  OK, here is one that need 30mA at 5V.  Choices are 1)same old
way using a transistor, or 2) sink the current via the PIC output port.
As I remember some past posts, someone mentioned that you could use two
ports tied together to get a max of 50mA current sink capacity.
However, that makes me a little nervous on making sure that both pins
switch at the same time, such that neither one would be taking a larger
spike of current.  In addition, being this is an inductive load, a larger
current inrush will occur (not sure what this is spec'ed at).  Will the
PIC ports clamp at 25mA for inrush ?  Of course, I will put a diode across
the coil to help with switching noise. Reliability points me to using the
transistor, simplified manufacturing and PC layout push me toward just
using the PIC port.

What say folks...

spam_OUThcooperTakeThisOuTspamcorp.es.com

1996\03\27@175100 by John Payson

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> As I remember some past posts, someone mentioned that you could use two
> ports tied together to get a max of 50mA current sink capacity.
> However, that makes me a little nervous on making sure that both pins
> switch at the same time, such that neither one would be taking a larger
> spike of current.

You certainly should ensure in software that both ports are changed at the
same time (using an instruction like MOVWF, ANDWF, etc.) rather than using
BSF/BCF.  Beyond that, though, there shouldn't be any problems.

>                   In addition, being this is an inductive load, a larger
> current inrush will occur (not sure what this is spec'ed at).  Will the
> PIC ports clamp at 25mA for inrush ?  Of course, I will put a diode across
> the coil to help with switching noise. Reliability points me to using the
> transistor, simplified manufacturing and PC layout push me toward just
> using the PIC port.

Any load inductance will actually REDUCE the current inrush when it's turned
on [but will try to keep current flowing when it's turned off].  Load capaci-
tance will increase inrush currents on power-on, and will require outrush
currents for rapid power-off.  Motors usually behave like moderate-sized
inductors when not moving and like capacitors when they are moving.  Relays
are usually pretty much straight inductive.

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