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

"Karl Townsend" wrote in message
anews.com...
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



Karl

It has to do with the output transistor construction. From a practical
point of view, if your load has to be grounded get the PNP, if the load has
to be tied to the power supply, get the NPN.

CarlBoyd

On Sat, 15 Nov 2008 08:21:41 -0500, Carl Boyd 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.
....
It has to do with the output transistor construction. From a practical
point of view, if your load has to be grounded get the PNP, if the load
has to be tied to the power supply, get the NPN.

That's as shown in the data sheet from data sheets page,
http://web1.automationdirect.com/sta..._12mmphoto.pdf
http://web1.automationdirect.com/sta...s/sensors.html

--
jiw

It's like Carl Boyd said but another way to look at it is that when the
output is ON, PNP will give you +12V out (when used with a 12V supply) and
NPN will give you 0V out.

RogerN

"Karl Townsend" wrote in message
anews.com...
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 Sat, 15 Nov 2008 08:21:41 -0500, "Carl Boyd"
wrote:


"Karl Townsend" wrote in message
tanews.com...
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



Karl

It has to do with the output transistor construction. From a practical
point of view, if your load has to be grounded get the PNP, if the load has
to be tied to the power supply, get the NPN.

To clarify a bit: an NPN sensor switches the negative wire; PNP
switches the positive. AN NPN sensor is a current sinking device; a
PNP is a current sourcing device.

Be careful when using the terms "sinking" and "sourcing" in the
context of industrial controls, especially PLCs. Some mfrs used (or
have in the past used) "sinking" to refer to a PLC input that's
compatible with a sinking sensor, even though the input itself is
sourcing current. Other mfrs use the convention that a sinking sensor
connects to a sourcing input. I'm not aware of any ambiguities in the
use of PNP/NPN. NPN devices are generally more flexible in systems
with mixed voltages.

(Fixed font)

PNP:
+V ---(sensor)-----------|
| |
|----(load)----| 0V

NPN:
+V |----(load)----|
| |
|-----------(sensor)--- 0V

--
Ned Simmons

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.

OK, I think I want NPN but I'm easilyconfused. The PLC input is at +12
volt, then a resistor (5.5K), then the switch (photo eye in this case), then
common(0V). Correct?

Karl

Carl Boyd wrote:
It has to do with the output transistor construction. From a practical
point of view, if your load has to be grounded get the PNP, if the load has
to be tied to the power supply, get the NPN.
--Wow that's the simplest explanation of the difference in the two
I've ever read; thanks! :-)

--
"Steamboat Ed" Haas : Never thought I'd live to see
Hacking the Trailing Edge! : our "iron curtain" crumble...
www.nmpproducts.com
---Decks a-wash in a sea of words---

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 --

On Sat, 15 Nov 2008 10:42:02 -0600, "Karl Townsend"
wrote:

OK, I think I want NPN but I'm easilyconfused. The PLC input is at +12
volt, then a resistor (5.5K), then the switch (photo eye in this case), then
common(0V). Correct?


If this is a 12VDC control system, and jumpering the input to 0V turns
the input on, then an NPN is what you want. Unless there's something
unusual about the PLC input or wiring you shouldn't need an external
resistor.

--
Ned Simmons

Bruce L. Bergman wrote:

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.

Generally a 1N40001. I always wondered why many common relays like the Omron octal and 11
pin dc relays didn't have the diode installed already with polarity marked for coil.


Wes
--
"Additionally as a security officer, I carry a gun to protect
government officials but my life isn't worth protecting at home
in their eyes." Dick Anthony Heller

"steamer" wrote in message
...
Carl Boyd wrote:
It has to do with the output transistor construction. From a practical
point of view, if your load has to be grounded get the PNP, if the load
has
to be tied to the power supply, get the NPN.
--Wow that's the simplest explanation of the difference in the two
I've ever read; thanks! :-)

--
"Steamboat Ed" Haas : Never thought I'd live to see
Hacking the Trailing Edge! : our "iron curtain" crumble...
www.nmpproducts.com
---Decks a-wash in a sea of words---

Ed

That statement is specific to the parts Karl was asking about. Do not
interpret that statement as having 100% applicability to everythinng,
although it is the most common approach.

CarlBoyd

On Sat, 15 Nov 2008 20:39:00 -0500, Wes wrote:

Bruce L. Bergman wrote:

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.

Generally a 1N40001. I always wondered why many common relays like the Omron octal and 11
pin dc relays didn't have the diode installed already with polarity marked for coil.

Waitaminit!! The 1N4001 is only 50V - you want to use a 1N4004 or
better (1N4005 or 06) to get above 400V. That spike can easily go
over 50V and the diode won't live forever.

And the dumb things are under three cents each in bulk, so you have
no excuse for cheaping out.

As for pre-installing the Back EMF Snubber on stock products, they
never know what type or voltage relay you're going to use with that
relay socket. But it would be a cool and useful factory option if you
are ordering production quantities of relay sockets.

-- Bruce --

Bruce L. Bergman wrote:

As for pre-installing the Back EMF Snubber on stock products, they
never know what type or voltage relay you're going to use with that
relay socket. But it would be a cool and useful factory option if you
are ordering production quantities of relay sockets.

Good point on the 1N4XXX. Should not have trusted memory. As far as the prewired diode,
I wanted it in the relay itself. AB makes some dc coil control relays with an internal
snubbing diode.

Wes

Wes wrote:

Bruce L. Bergman wrote:


As for pre-installing the Back EMF Snubber on stock products, they
never know what type or voltage relay you're going to use with that
relay socket. But it would be a cool and useful factory option if you
are ordering production quantities of relay sockets.


Good point on the 1N4XXX. Should not have trusted memory. As far as the prewired diode,
I wanted it in the relay itself. AB makes some dc coil control relays with an internal
snubbing diode.

Wes
The diode is reverse biassed when power is applied. It conducts ald
allows the coil current to recirculate decaying slowly when power is
removed. It *never* sees the full back EMF spike as it shunts it before
it builds up. You'll never see a problem from the back EMF if the diode
is rated at twice the supply voltage so an 1N4001 is fine for a 24V
relay. The only 'gotyas' are that the diode *must* be rated to carry
the coil current and as the flux will decay a lot more slowly, the relay
*will* be slightly slower opening.

"IanM" wrote in message
...
Wes wrote:

Bruce L. Bergman wrote:


As for pre-installing the Back EMF Snubber on stock products, they
never know what type or voltage relay you're going to use with that
relay socket. But it would be a cool and useful factory option if you
are ordering production quantities of relay sockets.


Good point on the 1N4XXX. Should not have trusted memory. As far as the
prewired diode,
I wanted it in the relay itself. AB makes some dc coil control relays
with an internal
snubbing diode. Wes
The diode is reverse biassed when power is applied. It conducts ald allows
the coil current to recirculate decaying slowly when power is removed. It
*never* sees the full back EMF spike as it shunts it before it builds up.
You'll never see a problem from the back EMF if the diode is rated at
twice the supply voltage so an 1N4001 is fine for a 24V relay. The only
'gotyas' are that the diode *must* be rated to carry the coil current and
as the flux will decay a lot more slowly, the relay *will* be slightly
slower opening.

That's what I've wondered about electronic ignition. Do they use real high
voltage ignition coil switching transistors or would the magnetic field
still collapse fast enough to spark a spark plug with a diode? Or perhaps
they use a condenser (or equivalent) on the solid state components like they
do on ignition points.

RogerN

RogerN wrote:

"IanM" wrote in message
...

Wes wrote:


Bruce L. Bergman wrote:



As for pre-installing the Back EMF Snubber on stock products, they
never know what type or voltage relay you're going to use with that
relay socket. But it would be a cool and useful factory option if you
are ordering production quantities of relay sockets.


Good point on the 1N4XXX. Should not have trusted memory. As far as the
prewired diode,
I wanted it in the relay itself. AB makes some dc coil control relays
with an internal
snubbing diode. Wes

The diode is reverse biassed when power is applied. It conducts ald allows
the coil current to recirculate decaying slowly when power is removed. It
*never* sees the full back EMF spike as it shunts it before it builds up.
You'll never see a problem from the back EMF if the diode is rated at
twice the supply voltage so an 1N4001 is fine for a 24V relay. The only
'gotyas' are that the diode *must* be rated to carry the coil current and
as the flux will decay a lot more slowly, the relay *will* be slightly
slower opening.


That's what I've wondered about electronic ignition. Do they use real high
voltage ignition coil switching transistors or would the magnetic field
still collapse fast enough to spark a spark plug with a diode? Or perhaps
they use a condenser (or equivalent) on the solid state components like they
do on ignition points.

RogerN


The usual setup for switched mode power supplies is to use a transistor
rated for a very high voltage for a very short time. Even so they have
to use a snubber consisting of a very fast high voltage diode in series
with a small capacitor. Instead of clamping the spike to the supply
rail, this alows the series combination of the coil and capacitor to
'ring' for half a cycle, ending up with all the remaining energy in the
coil primary that didn't get coupled into the secondary transferred to
the capacitor which is typically chosen so it will reach 1 to 2 times
the supply voltage. There also is a high value bleed resistor (usually
a high power one) that is chosen to effectively discharge the capacitor
for the next pulse.

If you want the transformer to 'ring' for more than 1/2 cycle then the
snubber can be just a capacitor, often with a low value resistor in
series with it so you dont blow the transistor at switch on.

if you fitted a normal anti-parallel diode, the primary current would
rise and fall smoothly in a sawtooth and the output voltage would be
insufficient for a spark plug.

However, ignition circuits frequently used a thyristor and that a whole
other ball game . . .

Unfortunately I cant find my old Heathkit electronic ignition manual to
see what approach they used, but I did find a site with some links to
circuit diagrams that might be of interest :

http://www.molla.org/DIY-CDI/Others/Others.htm

Pete C. wrote:
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.
--OBTW a pal showed me a trick to limit current drain from the PIC
that you're driving the transistor with: put an LED on the input side. Dunno
the specifics but it works a treat and also gives you a visual reference
when testing.

--
"Steamboat Ed" Haas : Never thought I'd live to see
Hacking the Trailing Edge! : our "iron curtain" crumble...
www.nmpproducts.com
---Decks a-wash in a sea of words---