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PICList Thread
'Pic lock up with motor'
2000\05\09@200827 by Peter McAlpine

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face
Hi All,
I have a little problem that my Pic that is switching on a motor
via a relay is locking up :(
The relay has a diode across it, and if I run it with no motor
there is no problem. Only when the motor runs (12v windscreen
washer motor) for over 2 - 3 seconds things lock up.

Putting a diode across the motor seems to solve it, but have
I only just solved the problem? What else should I look at
doing?

Thanks
Regards
Peter Mcalpine

2000\05\09@202109 by Spehro Pefhany

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At 10:07 AM 5/10/00 +1000, you wrote:
>Hi All,
>I have a little problem that my Pic that is switching on a motor
>via a relay is locking up :(
>The relay has a diode across it, and if I run it with no motor
>there is no problem. Only when the motor runs (12v windscreen
>washer motor) for over 2 - 3 seconds things lock up.
>
>Putting a diode across the motor seems to solve it,

It's a DC motor with a commutator, so it produces a lot of
electrical noise that gets coupled back through the relay.
The diode will kill some of the noise, especially if you
put it right at the motor.

> but have I only just solved the problem?

Determining the margin you have is not easy. It's possible
it could act up with slightly higher voltage or with a worn
commutator.

Maybe you could improve the grounding and relay circuit so
it works *without* the diode, then add the diode.

Operating the relay with an optoisolator is one way to
(potentially) improve things.

Best regards,
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2000\05\09@202743 by Arthur

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face
Hi Peter
If this is a ongoing project you could try a few things separate power
supply for motor.
Filter to the pic and loads of caps low value 1n to 100n as well as 5u to
100u.
A trick I have seen done was to have a large cap 4700u , I think it was this
supplied the controller for the time it was activated.this was a few
seconds.
regards Art

{Original Message removed}

2000\05\09@211402 by l.allen

picon face
> Hi All,
> I have a little problem that my Pic that is switching on a motor
> via a relay is locking up :(
> The relay has a diode across it, and if I run it with no motor
> there is no problem. Only when the motor runs (12v windscreen
> washer motor) for over 2 - 3 seconds things lock up.
>
> Putting a diode across the motor seems to solve it, but have
> I only just solved the problem? What else should I look at
> doing?
>
> Thanks
> Regards
> Peter Mcalpine

Impulsive noise from the motor is coupling to the PIC.
Most likely the supply lines.

You have to either separate or de-couple the power
supplies.

I take it you have a regulator driving the PIC and the
motor is run off the input side to that regulator.
Then a large electrolytic capacitor is a good start say
2200uF or more.
AFAIR wiper motors are very thirsty motors, like 5 to 8
amps at 12 volts. Then there could be a problem the your
common wire developing serious potential across it(since
it is unlikely to be a superconductor), enough to upset
the PIC if the PIC is sharing that wire back to the power
source.
So run one common wire for the motor and another
common to the controller and as much as possible the
same for the +12 volts. More intensive measures would
be a diode before the 5 volt regulator and 2200uF on the
input to the regulator. That stops the motor using the cap.




_____________________________

Lance Allen
Technical Officer
Uni of Auckland
Psych Dept
New Zealand

http://www.psych.auckland.ac.nz

_____________________________

2000\05\09@214902 by Chris Eddy

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Right.  Seen this one before.  The motor represents a somewhat inductive
load.  The relay contacts, when they break, generate a hefty arc across
the contacts.  Of course, this has little to do with your flyback diode
on the coil.  The powerful EMI from the contacts couples through any
means necessary and corrupts your PIC circuit.  I had one unit that
would generate upwards of 60VPP hash surges on the PIC side of the
relay.  The dead giveaway is when you put the scope ground on PIC
ground, and the probe to the same ground point.  (bear with me on this
one).  If you see hash on your scope, then it is EMI events.  The scope
chassis ground cannot keep up with the high frequency events, and you
see a false image on the scope screen.

The easiest solution is to put a 0.1uF capacitor across the contacts to
supress the arc and subsequent EMI/EMC.  If you want to get
sophisticated, you can go further and try to design a snubber of some
fashion that suits your load very closely.

BOL
Chris Eddy

Peter McAlpine wrote:

{Quote hidden}

2000\05\10@011019 by Harold Hallikainen

picon face
On Wed, 10 May 2000 10:07:15 +1000 Peter McAlpine <.....mcalpineKILLspamspam@spam@GME.NET.AU>
writes:
> Hi All,
> I have a little problem that my Pic that is switching on a motor
> via a relay is locking up :(
> The relay has a diode across it, and if I run it with no motor
> there is no problem. Only when the motor runs (12v windscreen
> washer motor) for over 2 - 3 seconds things lock up.
>
> Putting a diode across the motor seems to solve it, but have
> I only just solved the problem? What else should I look at
> doing?
>
>

       For a DC inductive load, I think the diode across it is fine. I recently
had a problem where a PIC was driving a FET driving a 12VDC relay (with a
diode across the coil). All worked fine until the 120VAC vacuum pump the
thing controls was connected. The arcing of the relay contacts crashed
the PIC. A snubber (I used 10 ohms and 10 nF in series) across the
contacts solved the problem.  It's fun trying to get this stuff out of
PIC circuits.
       I had another project a year ago where we were firing up a big xenon
lamp right next to the PIC. Ended up putting a steel plate between the
two to keep the PIC from crashing.

Harold


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