pnp /npn sensors

guys

i am trying to learn sinking and sourcing concepts and need some clarification . the way i understand is pnp sensors switch + side of dc power and npn sensors switch - side of dc power and that when using a pnp sensor you need to connect it to a sinking input card and an npn sensor to a sourcing input card . do i have it right or am i still confused ?

you got it right ...

thanks . i have been reading tutorials and trying to understand this all day . seems to me a lot of tutorials that try to explain it way over complicate it

ttt

XIO said:

ttt

Click to expand...

Just add to your ignore list.

plc noob said:

guys

i am trying to learn sinking and sourcing concepts and need some clarification . the way i understand is pnp sensors switch + side of dc power and npn sensors switch - side of dc power and that when using a pnp sensor you need to connect it to a sinking input card and an npn sensor to a sourcing input card . do i have it right or am i still confused ?

Click to expand...

I'm printing this and handing it out to my maintenance guys. Good simple explanation.

There is no easy way to explain the difference between pnp and npn because the issue gets convoluted when connected to a plc.

The problem stems from the fact that you can swap the devices and they still work but work "backwards".

http://www.patchn.com/npnpnp.htm
One of these days gonna try to translate that into even simpler terms, until then look at AutomationDirect manuals, they offer a decent explanation.

I don't get it

"The problem stems from the fact that you can swap the devices and they still work but work "backwards".

Did you mean I can use a npn conected to a sink input card and it will work but backwards?

I recently ran into a problem I told my boss I would use two sensors off the shelf and never even looked at the diagram, hence they only provided a npn output and I only had a sinking input card.

I ended up using a relay and powering the coil and using the npn output for the nuetral. worked ok but not very fast. I ended up scrounging around the supply room for some diffrent sensors.

Clint

CJones said:

I don't get it

"The problem stems from the fact that you can swap the devices and they still work but work "backwards".

Did you mean I can use a npn conected to a sink input card and it will work but backwards?

I recently ran into a problem I told my boss I would use two sensors off the shelf and never even looked at the diagram, hence they only provided a npn output and I only had a sinking input card.

I ended up using a relay and powering the coil and using the npn output for the nuetral. worked ok but not very fast. I ended up scrounging around the supply room for some diffrent sensors.

Clint

Click to expand...

YES

I'll try it

thanks

clint

Let's have a little warning here regarding NPN devices wired to a sinking input.

At some point, something must be supplying a flow to plus into the sinking input to pull it high. This is typically a reistor of some type. The NPN device is connected to the input which sinks enough current to pull the voltage at the input low. This give a LOW input when the sensing device is ON thus reversing the normal meaning of the input.

You have to look at a variety of specifications to pull this off.

1. What is the amount of CURRENT on the input does it take to turn it on. What is the minimum VOLTAGE on the input does it take to turn it on. Using OHM's Law this translates into an effective resistance of the input. Subtracting the minimum ON voltage from the supplied voltage gives you the maximum voltage which can be dropped across the 'pull-up' resistor. Again using OHM's Law you can calculate the maximum resistance and needed power dissipation of the pull-up resistor.

2. Now comes the fun part. The NPN device must pull down sinking enough current to drop the voltage at the input below the maximum OFF voltage. The voltage across the pull-up resistor is calculated by the supplied voltage minus the maximum OFF voltage of the input. Using OHM's Law with the resistance gives the current the NPN device must sink. Is it up to the task?

These calculations give the maximum value for the pull-up resistor. Usually you will want to supply a little more current/voltage than just the minimum ON specifications. This lowers the value of the resitor.

You would probably also want to pull the input voltage a little below the maximum OFF voltage. This increases the current the NPN device must be able to supply.

If all this is giving you a headache, stick to the interposing relay with mismatched devices. (AB has some really thin terminal sized relays.) Only use the resistor pull-up (or pull-down if mis-matching in the oopposite sense) if an emergency exists.

there a bunch of companies out there that make low cost PNP/NPN converters to solve just these sorts of issues. I have used the units from Woertz ( www.woertz-usa.com) but there are plenty of others. Can be easier than dealing for relays and keeps everything transistorized if you need high speeds.

Thanks Bernie,

The relay worked (after the second one) the first one would only do 1800 transitions per hour, and I dug up some that would do two or three a second, I needed to do about 6000

Clint

Try solid state relays (SSR) www.crydom.com