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We have a device which can work with dry contact input and a device which gives open collector output. Will it inter-operate?

Is there any difference between dry contact mode output and open collector output?

If a device works with dry-contact input, does it work straight away with open collector input?

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If by "dry contact" you mean "floating", then yes. Something designed to work with a floating input will work with a ground (or other) referenced input, as long as the common mode voltage range is not exceeded. You need to define what you think "dry contact" means. – Olin Lathrop Aug 22 '12 at 19:44
@Olin - I've only seen it used a few times (mostly to refer to an isolated relay output) but I think this is referring to the control of an input, and meaning you don't need to apply any voltage to activate it, just short it together. I may be mistaken though, have a look at the link in my answer and see what you think. – Oli Glaser Aug 22 '12 at 19:54
Another link on them here. – Oli Glaser Aug 22 '12 at 19:57
@OliGlaser: My point was we shouldn't have to guess, as there is another totally different usage of "dry contact". The OP shouldn't be using ambiguous terms like this without defining them. – Olin Lathrop Aug 22 '12 at 21:11
@Olin - Also knowing exactly whether it's input or output (the terms are both used with dry contact in the question, e.g. the title mentions "dry contact mode output") would be nice. – Oli Glaser Aug 23 '12 at 1:09

2 Answers

up vote 4 down vote accepted

"Dry contact input" means it just needs to be shorted together to activate (intended to be used with a dry contact like a switch/thermostat/relay/open collector, etc).
"Open collector" means a transistor with collector connected to the output - it can only be used to pull something to ground (i.e short output wire to ground wire)
Since that is all we need to do here, then it should work (as long as the ground wire of the open collector is connected to the negative side of the input)

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Thanks a lot for your answer. – Sunny Aug 23 '12 at 4:59
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A "dry contact input" generally expects that its two contacts will be shorted to each other to activate it -- and NOT shorted to anything else.

An "open collector output" is one that is open-circuit when inactive, and shorted to ground when active. This type of output is usually assumed to be an NPN transistor (or its equivalent), and is intended to sink current to ground only.

In general, it is NOT OK to connect an open-collector output to a dry contact input, unless you know by other means that a) the dry contact circuit is DC and b) it's safe to connect one of the dry contact inputs permanently to the open-collector circuit's ground.

Just as an example, consider an ordinary HVAC thermostat. The heater control circuit usually runs on 24 VAC, and the dry contact in the thermostat can safely activate it. But the AC voltage would probably damage the circuitry of an open collector output. Or, if the latter includes a diode for reverse-voltage protection, it would either be half-on all the time (shunt connection) or only half-on when active (series connection).

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