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Old June 13th, 2011, 02:34 PM   #1
WrongWay
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2-wire ac NC proximity sensor

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?
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Old June 13th, 2011, 02:44 PM   #2
Lancie1
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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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Old June 13th, 2011, 02:45 PM   #3
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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?
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Old June 13th, 2011, 02:51 PM   #4
bkottaras
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Quote:
Originally Posted by OkiePC View Post
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.
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Old June 13th, 2011, 02:52 PM   #5
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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.
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Old June 13th, 2011, 02:53 PM   #6
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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.
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Old June 13th, 2011, 03:57 PM   #7
WrongWay
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micro qd is a type of "quick disconnet" that has 3 terminals and 2 keyways to assure proper mating
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Old June 13th, 2011, 04:20 PM   #8
WrongWay
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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.
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Old June 13th, 2011, 04:47 PM   #9
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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.
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Old June 14th, 2011, 06:49 AM   #10
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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
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Old June 14th, 2011, 11:20 AM   #11
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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.
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Old June 14th, 2011, 11:28 AM   #12
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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.
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Old June 14th, 2011, 03:39 PM   #13
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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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Old June 14th, 2011, 03:42 PM   #14
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Coil draws 9.2 mA at 120V. Well within your range of 5-250 mA for the prox.
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