2-wire ac NC proximity sensor

WrongWay

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May 2011
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These are not used very often I know. I do not have a PLC on this piece of equipemnt that I am attempting to install a monitoring/alarm circuit in. My intent is to use this prox to monitor for weld wire presence and to switch the load to a timer relay when no wire is detected. When the relay times out at 11 seconds it will latch a relay that sounds/lights an alarm condition that requires a key reset to unlatch. I installed the prox using a micro qd with #3 suppling load(120VAC) and #2 to the timer relay. Sometimes the prox doesn't light at all and other times after cycling power I have gotten a flash on/off from the red LED indicating a short circuit. Also, the micro qd cable I have the red/white to load and the red/black to the relay
How far off the track am I?
 
Is your "micro qd" some type of smart relay or electronic relay?

You have to be careful about what type and size of load you use with a prox switch. Not all prox switches will successfully switch all different loads (relays). DC relays can be difficult, as are some electronic relays.
 
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If it's a two wire AC prox, then you should be able to wire it either way with the same results. I have seen some that also include a 3rd connection for ground.

If the prox is directly feeding the coil of a relay, then its indicator should always function unless it is damaged. Perhaps the prox was shorted or the load of the relay coil is too much for it?

Can you post a link to the part(s) you're using?
 
If it's a two wire AC prox, then you should be able to wire it either way with the same results. I have seen some that also include a 3rd connection for ground.

If the prox is directly feeding the coil of a relay, then its indicator should always function unless it is damaged. Perhaps the prox was shorted or the load of the relay coil is too much for it?

Can you post a link to the part(s) you're using?
Ive seen both.
I agree with Paul.
Chances are that the load of the relay is too much for the prox. The prox is primarily used as a signal/feedback device and it is not capable of load switching. Check the prox specs.
 
A 2 wire prox needs a good size load to power it's internal electronics. Check specs. I believe it's minimum 20 mA or so. Give it a load like an incandescent lamp.
 
Is it an electronic Timer? It may have too high of a resistance (minimum current to power the prox) Try putting a coil relay in between, or us a resistor to draw a few mA.
 
Thank you for the replies, both parts are AB.
The prox is a 871TM-B4C12-R3,switches up to 200ma. The timing relay is a 700-HRP42TA17. I grabbed the timer I had on the shelf and did not qualify it fully, it appears that it requires quite a load to energize it. As for the load to the prox, I put a 250ma AGC fuse inline before the prox.
 
The prox needs a minimum load of 5 mA. There isn't a load spec on the timer datasheet. I suspect it might be small. Looked at similar Fuji timers from AD. They are only 10 VA. You should use a relay that draws at least 5 mA in between.
 
I threw in a omron MY2 between the prox and timing relay. I was trying to minimalize the amount of components in the circuit by landing directly on the timer. Of course, no I can just use a NO prox. I guess what I learn here is that is a certain item is rarely used, its probably that there is a better, more mainstream way of doing it.
thanks again
 
I would put a 10K ohm 2W resistor across the coil of the timer. AC switches need a minimum load to stay "on", this is common when using solid state timers to turn on a load.
tommy23t
 
That's a common value, good for most applications. The problem is it will run a little hot.
For this application, 22K will give us a little over 5 mA, enough to meet the prox spec. Watts drops down to .6 Watts.
 
I was able to test this today with no problems making the latch. The prox energizes the coil on a omron MY2 relay which closes a circuit to the timer. I'm probably missing something, but isn't there plenty of amperage to power that coil?
 

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