When the microprocessor turns off the current through the relay, the magnetic fi
eld around the relay coil collapses, and a momentary high voltage appears across
 the relay.  The best cure is to put a silicon diode across the relay, which wil
l allow current to flow from the relay coil through the diode starting at the mo
ment the PIC turns its output off.  The diode should be arranged so that it is b
ackwards for current flow from the pic pin to the voltage source.  If one end of
 the relay is connected to +5 volts, then the cathode of the diode should be con
nected to the +5 volt end.  In other words, the arrow for the diode 'points' to
the +5 volts.   This 'fix' will make the relay take a few extra milliseconds to
'release', but in most applications this is not an issue.
It helps to make sure that the +5 of the relay has a separate wire back to the +
5volt source, instead of sharing the wire or trace that brings +5 Volts to the P
IC.
  Regards,
  Ron Fial


At 10:06 PM 4/6/99 -0700, you wrote:
> Whenever the 5V output from PIC is applied to relay, the microprocessor
>will run unexpectedly. If I put relay in a separated board, it's OK. I think
>the problem is the noise from the inductance of relay. Can anyone tell me
>how to eliminate this problem? Thank you.
>
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