On Sat, 15 Nov 2008 09:39:10 -0600, "Pete C."
wrote:
Karl Townsend wrote:

I'm about to purchase a photo eye. http://tinyurl.com/6bjmeb

What's the difference between PNP and NPN? I just want an eye that's NC
powered by 12VDC that uses very little power.

Karl

On top of what everyone else said, pay close attention to the output
current rating as these are not contact closure outputs and have very
low current ratings. You need to add a relay if you need to switch
anything of significance.

And on top on top on top on top on top... ;-)

If you add a relay to a transistor drive circuit and you aren't
absolutely sure how well the device was engineered, put a back-EMF
diode or a neon lamp or other snubber across the coil leads.

Even PC mount reed relays can generate enough of a back EMF pulse
when the magnetic field collapses to fry the unprotected transistor
driving it, and larger items like 40A definite purpose contactor coils
certainly will.

You /really/ want to absorb that spike before it gets back to the
transistor. And while they usually internally protect photo-sensor
modules like that, this is a good place for a 'belt and suspenders'
diode of your own.

Oh, and the pull-up current surge is a lot larger than the hold
current on the relay coil. If you have a 50ma limit on the drive
transistor, a coil that draws 50ma hold may fry it on the pull-up
surge. Sometimes they rate them conservatively, sometimes not.

Just like a light dimmer at home - if it's rated 600W /NEVER/ load
it to 600W, because when a lamp burns out the current spike will push
it over the edge. Only load it to 500W max and leave a cushion.

That's the time to cascade with a pilot relay, the module pulls up a
tiny reed relay that pulls up the motor starter coil.

-- Bruce --