2 wire AC prox switch tip

[Mendon Systems 22]

I had a requirement today to use 2 wire AC prox switches in series and of course they did not work. I am not 100% sure why but I suspect that the current draw from the 2nd prox in the off state was not sufficient to keep the triac (or whatever) in the first prox turned on. I was able to resolve the problem by installing a 100k ohm 1/4 watt resistor from the output of the first prox to the AC common. The second prox connects to a relay coil. I had a similar problem with a prox connected only to an AC input module and was able to solve it the same way. Yes, I know that a relay connected to each prox would have worked but I did not have the room to install them and the resistors are about 1% of the relay and socket cost. Edited 18 Dec 2010 by Mendon Systems

[paulengr 44]

This is a big no-no. They are not true "two wire" interfaces. The device has to get power from somewhere. It does so by drawing a very small amount of power, enough to cause a voltage drop but not enough for most inputs to trigger. The device expects however for the proper amount of voltage to be present. The general rule is one-device, one output. The way that you've wired it is fairly creative. I wouldn't have tried that at all. As to "cost" and "width" of the relay, when you can get a DIN rail relay this is around 5 mm wide or less (solid state), I'm not sure how in the world you'd ever have a space problem. If you are still using old school "ice cube" style relays or worse yet, "machine tool relays", then yes, you'd have a space problem. Putting two in series means that the second one sees the voltage drop from the first. There's probably not enough voltage drop left to supply the small amount of current necessary to power the device up. If you notice, all DC versions are either "3-wire" or "4-wire" devices. The 3-wire versions share the DC power supply to both the load and the device itself whereas the 4-wire versions split this off separately.