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PICList Thread
'PIC driving a relay'
1998\10\26@200041 by Harold Hallikainen

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       I've got an application where I need to drive a relay with the
PIC.  To keep the parts count down, I'd like to do it with no additional
parts.  The relays available have a 5V, 30mA coil or a 12V, 13mA coil.
The 30 mA looks like more than can be driven with a single output pin.
If i go to the 12 V relay, the current is ok.  What is the maximum
voltage on the RA4 open drain pin?  Can it go that high (or higher)?  Of
course, I'd have to add a clamp diode to protect the chip when the relay
is shut off.  I cannot find the maximum voltage rating on the RA4 output
in the Microchip documentation (this is on a 16c74a).
       Another possibility is to just use two I/O lines in parallel and
turn them both on at the same time.  Seems like this could drive the 5V
relay.  Also, the internal clamp diodes could probably protect the chip
from the relay, right?

Comments, please!

Thanks!

Harold



Harold Hallikainen
spam_OUTharoldTakeThisOuTspamhallikainen.com
Hallikainen & Friends, Inc.
See the FCC Rules at http://hallikainen.com/FccRules and comments filed
in LPFM proceeding at http://hallikainen.com/lpfm

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1998\10\27@064247 by Mark Willis

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face
Open thy File DS30390E.pdf, to page 201, line 8, it saith:

Voltage on RA4 with respect to Vss
...............................................................................
....................................
0 to +14V

 And Harold readeth, and rejoiceth <VBEG>

 Mark, .....mwillisKILLspamspam@spam@nwlink.com, who suggesteth adding extra clamp diodes,
lest evil things befall ye {and who should go to sleep before he gets
too weird <G>}

Harold Hallikainen wrote:
{Quote hidden}

1998\10\27@065242 by Dr. Imre Bartfai

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face
Hi,

either use a transistor, or look for the new HV signed PIC 16HV540.

Imre

1998\10\27@094648 by lilel

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face
Mark wrote, in the Ancient Toungue:
>   Mark, .....mwillisKILLspamspam.....nwlink.com, who suggesteth adding extra clamp
>   diodes,
> lest evil things befall ye {and who should go to sleep before he
> gets too weird <G>}

Which made me ROLF.


Harold Hallikainen wrote:
> >
> >         I've got an application where I need to drive a relay with the
> > PIC.  To keep the parts count down, I'd like to do it with no additional
> > parts.

Harold, I do this all the time in very cheap cost sensitive
applications using a $0.03  NPN transistor and a 2K resistor from the
PIC port to the base.  I usually use a 24 volt relay, because that
results in minimum current and relay cost.  This allows me to use a
cheap unregulated power supply, consisting of a diode, 1 watt
resistor and a 63 volt  22 Uf Cap.  BEWARE the trap of trying to
minimize parts count and then requiring extra expense in your power
supply.  You can trade off a couple of parts for a much cheaper
overall circuit.  BTW I'm putting these things in $15 steam irons,
they must be very cheap.
-- Lawrence Lile

"Nyquist was an optimist."

=> Median Filter Source Code
=> AutoCad blocks for electrical drafting

at:  http://home1.gte.net/llile/index.htm

1998\10\27@114330 by Peter L. Peres

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On Mon, 26 Oct 1998, Harold Hallikainen wrote:

>         I've got an application where I need to drive a relay with the
> PIC.  To keep the parts count down, I'd like to do it with no additional
> parts.  The relays available have a 5V, 30mA coil or a 12V, 13mA coil.
> The 30 mA looks like more than can be driven with a single output pin.
> If i go to the 12 V relay, the current is ok.  What is the maximum
> voltage on the RA4 open drain pin?  Can it go that high (or higher)?  Of
> course, I'd have to add a clamp diode to protect the chip when the relay
> is shut off.  I cannot find the maximum voltage rating on the RA4 output
> in the Microchip documentation (this is on a 16c74a).

12 V according to many, and in any case, 0.6 V less than Vppmin. This
means, that the kickback suppressor must be a Shottky and 12V is to be
12.0V. BEWARE: I haven't tried this with your part !

>         Another possibility is to just use two I/O lines in parallel and
> turn them both on at the same time.  Seems like this could drive the 5V
> relay.  Also, the internal clamp diodes could probably protect the chip
> from the relay, right?

Parallel: right. Diodes: maybe. An external 1N4148 or Shottky would make
me sleep better. Maybe it can be omitted from placement during
production, if it turns out to be ok.

Peter

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