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Guest TNT

120VAC outputs

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I have a 120vac output module that is getting feed back voltage. I'm not sure if I can correct this problem. I've attached a drawing that helps explain what I'm trying to do. This is not exactly how this application is wired up, but is in the same manner that will show my problem. The circuit shown is supposed to work like this. If prox 1 or prox 2 is not going to be used then the plc is programmed to turn on the corresponding output to turn on the amber lights. If prox 1 is not going to be used then the plc turns on output 1 and the amber light will light up when the push button is pressed. This shows that prox 1 is simulated. Prox 2 works in the same manner. This all works good until we need to simulate both prox 1 and prox 2. When I say simulate, that is suppposed to mean that the prox is disconnected from the plug. If prox 1 and 2 is supposed to be simulated, then output 1 and 2 will come on. The problem lies in the situation if prox 1 or 2 doesn't actually get disconnected from the system. You can see if prox 1 doesn't get disconnected and is actually turned on then I'm getting 120v through the prox, through output 1 and through output 2 and turning on the second amber light without even pushing the push button. In a nut shell, how can I keep current from coming back into the outputs? I can probably add relays, but there is actually more than just the two outputs that are shown and I don't want to add a bunch of stuff if I don't have two. Thanks to anyone who may help. Edited by TNT

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What sort of output module is it (eg relay, SCR) What sort of lamps are they (wattage & type)

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Relay outputs, the lamps I don't know?

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The problem must be caused by snubber networks across the relay contacts (inside the card), and the low wattage lamps. I guess that even if outputs are off and the button is pressed that the lamps illuminate (with or without px fitted). The problem is that the lamps don't present a big enough load to the card, but you need to keep the lamp current low for the px switches to handle. Options Change the output card for a solid state version Increase the load on the outputs (while remaining within allowed load for px) Remove the snubber circuits from the cards Fit relays and hope they provide enough load

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If the module is disconnected completely then what are the results? I would suspect you have the same problem. If the prox. is series connected with the light then current is flowing at all times. The current is just increased when it is 'on'. Also, if the light is an LED version then this current may be enough to turn it on regardless of the prox on/off state.

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This is most likely caused by the two wire proxes. They have to flow some current to operate and the lamps are most likely LED or neon that will illuminate at those low currrents. A change to incandescent lamps would cure this, or adding a load resistor across the lamps. I've never seen a snubber circuit for a mechanical relay cause this problem, even with neon. Solid state outputs tend to be leaky and will probably make the problem worse. Increasing the load I'm sure will fix it.

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I assumed from this that the PX switches are working OK with the lamps, hence reason for suggesting snubber. I had a PLC output connected to a transformer, which then connected to a strobe, the strobe would flash happily just from the leakage current! I agree about the expectation that the PX supply current (2-wire devices) in the off state is still often enough to illuminate low current lamps. We need some clarification from the OP.

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