NPN output to 12VAC input interface ??

I have an input coming from an optical sensor that has a NPN output, and i need to interface this to a 120VAC input to a GE 90-30 input card. The reason is it is the only input on the machine that isn't 120V. And in don't want to eat up a slot in the rack for a input card for just this one input.
I am thinking of a simple 24VDC relay to provide my input,Maybe even an opto-coupler. However thought i would get some suggections from the experts.
The sensor is monitoring a series of doors on an enclosure that houses several high speed motors and Cutting tools. Therefore it is a safety sensor and i need to treat it as such... Any Ideas.....

milmat1 said:

Therefore it is a safety sensor and i need to treat it as such... Any Ideas.....

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That line is the key, IMHO this should determine how you interface this sensor with your PLC input.

I would use a safety relay as the interface.

There are many on the market, Telemechanique, AB guard master and Pilz are a few manufacturers of dedicated safety relays.

Paul

Thanks for the info everybody !!!!

milmat1 said:

The sensor is monitoring a series of doors on an enclosure that houses several high speed motors and Cutting tools. Therefore it is a safety sensor and i need to treat it as such... Any Ideas.....

Click to expand...

Yes, I have an idea. Use a separate hardwired circuit and safety monitoring relay for the 'safety' section, and use the connection to the PLC ONLY as 'information' that the safety circuit is open. IOW, PLCs are NOT safety devices (except a few that are specially designed for this purpose).



-Eric

Eric Nelson said:

Yes, I have an idea. Use a separate hardwired circuit and safety monitoring relay for the 'safety' section, and use the connection to the PLC ONLY as 'information' that the safety circuit is open. IOW, PLCs are NOT safety devices (except a few that are specially designed for this purpose).



-Eric

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Safety circuits such as this MUST be hardwired with relays and contactors. Solid state devices are only allowed if they are certified for safety, i.e., they fail in a safe condition. A PLC is not used as the main control for a safety circuit.

Wire it so the normal condition is a closed loop circuit drawing current. Any failure in the circuit causes shutdown. Broken wire, failed power supply, etc. all cause a shutdown.
A transformer, contactor, and switches in series is a simple example of this.

When using PLC inputs as secondary safety interlocks, wire them so the input on is the normal mode. Again, a break in the circuit causes the input to turn OFF, causing a safety shutdown.

I doubt that a NPN output device can be used as a safety interlock. The normal failure mode of a transistor is 'shorted', causing a failed device to keep the circuit active.

Use mechanical switches and contactors (not relays) for your safety circuit.

I doubt that a NPN output device can be used as a safety interlock. The normal failure mode of a transistor is 'shorted', causing a failed device to keep the circuit active.

Use mechanical switches and contactors (not relays) for your safety circuit.

Click to expand...

Technically a contactor is a relay and there are numerous solid state (NPN/PNP) devices rated for safety use.

You may want to visit http://www.sti.com if there are safety and machine safeguarding issues involved. These days it is common to use redundant inputs...ie a devices with dual output of NO and NC to verify the integrity of the "interlock" switch. The picture is a simple example of using one of those type switches with a "safety relay".

As was mentioned a risk analysis should be made to determine the safety issues involved, this may be a case where another switch should be used "in series" with the emergency stop circuit or the whole control system may require a more in depth evaluation.

If someone can open the door that switch is on and get hurt by any means then you definitely need to re-evaluate using that switch as you have planned.

Thanks everyone!! I need to safely monitor a section of machine about 160 inches long. Within that distance are several doors that swing out. (Man size doors)

My thoughts are to shoot a beam down the side and watch for any door to open or any people to walk up to the doors. The relay will open my E-Stop circuit, Then i need an input to the PLC so that i know what happened and can splash a alarm screen.
I don't know for sure that this is an exceptable way of doing this. However for this application it seems the best overall !!