Dry Contacts

[Bob O 8]

I may feel pretty stupid for asking this but what is a dry contact? I have heard it used before but never need one until now I guess. Is it just an isolated set of contacts that when closed will provide continuity just like the other contacts but it is separate, meaning not sharing the same common? Someone is coming in and stated they need a set of dry contacts from my ML1500 for a demo. Thanks, Bob

[panic mode 246]

dry contact is "potential free contact" or "isolated contact" this is what you need when interfacing pieces of equipment that is not referenced to same potential. most people understand dry contact to be a relay contact and the "dry stuff" is the insulation between relay coil (one circuit) and relay contacts (second circuit). this is ok but not 100% true. transistor output can be dry contact as well (as long as there is proper isolation like through optocoupler). warning: do not confuse this with standard PLC transistor outputs. they have separate common (or two, or more...) so that proper branching is possible (applying power to outputs from different sources, but they are (normally) NOT isolated. unless this is explicitly stated in the manual, they are still connected to PLC backplane through driver logic and so referenced to same potential (or ground) as other cards on that same PLC. for example you have plc with few 16-point input and output cards. one of the output cards and inputs are wired locally. one of the output cards need to be connected to same voltage (such as 24V) but different circuit. just because both of them are 24V, doesn't mean that there is no tension between them. if this tension is significant, it will destroy the PLC and whatever is connected to it. to prevent that, people usually specify so many dry contact (isolated) I/O points as interface. not many people are familiar with electronics and datasheets are not always clear so - when in doubt go with interposing relay anyway. might not be cheapest solution and it may distort and delay signal but it will not fry anything.

[Bob O 8]

Ok, if I understand this then I could use an output to energize an ice cube relay that has their wires attached to the contacts. Or if I have a group of outputs not in use on my ML1500 I could use one of those as long as the commons are not wired into it. I am off to .NET class so I will check back in later. Thanks for the help, Bob

[DanW 41]

For what it's worth, I think your client is expecting relay contacts as the outputs. I run into the term "dry contacts" weekly and the people using it mean either relay contacts as outputs or microswitch contacts as inputs, like those on a pressure switch, capillary bulb temperature switch, or flow switch. Dan

[P Daniil 0]

I remember reading in an IEEE mag some time ago that the terms "wet-" and "dry-" contact originated in the old power switchgear days (like "busbar" and "bus") and that in practice "dry contact" refers to a low current, low voltage, potential free set of relay contacts. However, see also this definition. As these definitons can easily get misunderstood, it may be a good idea to ask/specify a minimum/maximum switching current and voltage specification from/to your client.

[forqnc 2]

I always understood it as your definition. A 'Wet' contact can switch all voltages 5V and up. A 'Dry' contact switches to 0V or neutral.

[panic mode 246]

statement on Sick website is correct although it refers to relays only. contact doesn't necessarily have to be a relay contact (can be relay, contactor, circuit breaker, switch, sensor, solid state relay, optocoupler etc.). interpretation from previous post however is very wrong... dry contact is isolated contact, period. has nothing to do with voltage level in the circuit or sink/sorce. it only means that two parts of the interposing device (let's name them "driver" and "contact") are isolated from each other and so can be in completely different circuits (AC/DC, AC/AC, DC/DC, different voltage, polarity, whatever...). an example of "driver" is coil or solenoid (relay, contactor) or LED (optocoupler, solid state relay). 3-wire devices such as DC proxy sensors and photoeyes for example are not isolated (they are sharing one wire for both "driver" and "contact"). therm "common" is something to be watched carefully as well - it does not always means 0V or neutral (it could be but it doesn't have to).

[Bob O 8]

Thanks Guys. It turns out i didn't need one for the demo but this has been a good thread. Thanks Again, Bob

[P Daniil 0]

Apologies for not being more clear. My reference to contact current and voltage was for switching capability (for example, a set of 1 A, 250 VAC contacts can switch a 1 A, 250 VAC load). I absolutely agree that the defining characteristic is isolation from the controlling circuit. As no isolation and switching capabilities are implied/given with this generic definition it is good practice to include them as additional/clarifying specs (say "1500 VAC isolating, 1A/250 VAC switching" in the above example). Opto-isolators (output device being transistor, triac etc) also satisfy the dry-contact definition but may not work in a specific application if a maximum output off-state leakage current and/or a maximum on-state voltage drop is specified. For example, an optotransistor may be marginal when driving low a TTL stage (TTL low-state input threshold being anywhere from 0.8 to 1.2 V). This is a good thread Bob O. Thanks for starting it.

[forqnc 2]

Thank you for the correction Panic

[panic mode 246]

that's what forum is for - nothing like some good discussion. TTLs will accept anything below 0.8V as low. bipolar transistor will have much lower Vce voltage drop when saturated (tipically <=0.1V) so they can be used to drive TTL inputs directly. usually those will even have darlington output and this is a sure sign that device is meant for digital (on/off) applications. one just has to be careful in optocoupler selection because there are many different types. for example there are analog optocouplers as well. their characteristic is quite different from digital versions. digital optocouplers will be happy even with low mA signal and output will be turned on and able to sink current much bigger current than that in the input (many devices will easily allow output current such as 100-200mA with just 2mA input for example). analogs on the other hand need currents in the same order as output current (their range for both input and output is tipically about 5..60mA). transistor outputs are not suitable for AC circuts either but there are devices that have triac as output element. these outputs are meant for AC circuits, in fact they depend on zero crossing to turn off. then there are more integrated versions that have gates as output (ready to drive TTL/CMOS inputs).