Dry Contact

[RSin]

"what is the definition of a dry contact in a home automation scenario?"

This is something I've struggled to get my head around and thought this may help newbs like myself. Now I'm purposefully opening myself up to ridicule and correction to get a definitive answer :)

My understanding:

A motor, typically blind or rooflight, requires 240v to operate. A small signal (12v or 24v) is sent down a 'dry contact' cable that switches a relay and enables the 240v to flow to the motor so it can turn. The term dry contact suggests that no current flows down this cable, the contacts are not 'wetted' by any voltage, but it must otherwise how does it know to switch the power to the motor. What I've found is that there appears to be a loose definition, that it is a negligibly small current ~10mA or less.

Because this is so low it means you can use practically any cable as the voltage drop over any distance will be negligible. e.g. 0.01A @ 12v over 100m with a 3% drop only requires 0.1mm.sq cable cross-sectional area. 

I presume the loxone retractive switch wiring falls into this category as no current is written anywhere but all literature states to use CAT cable.

Does this all sound about right?

Thanks.

[mosseltje]

RSIN, What is your question?

Is it why is a dry contact called that?
Or are you asking a cable thickness question.

For me personally its not clear what you are asking.

[RSin]

My question is "what is the definition of a dry contact in a home automation scenario?"

I've stated my understanding from what I've found but no-one is explicit. Just want to clear it up for me and perhaps others...leave nothing to assumption!

thanks,

Richard

[Duncan]

a dry contact is where a devices output closes a pair of contacts with no connection to any internal circuit or power - eg a PIR sensor can be dry contact with its power supply of 240v or low voltage, but we connect 24v to one of the dry contacts and its switched to the other contact when it detects movement.

PIrs without dry contacts switch their own power supply to a single output pin on detection, meaning that we need either a 24v pir, or to use a coupling relay with a 240v pir to switch 24v to a loxone digital input as the 240v output would kill the miniserver and be dangerous.

in another thread there is a discussion about voluto blind motors being 'dry contact'  - in this case we need to use a dry contact i.e. relay to short together 2 wires to make the motor do something. we dont know the internal electronics so cant substitute a low voltage, only use a relay triggered by loxone to short together the required wires. We do know these wires are low voltage and are safe to handle, but we dont know which voltage and relative to what 0v signal so cant just send a 5/12/24v pulse directly from loxone.

if we dissected one of these motors we might be able to work out the cicruitry and possibly use low voltage to trigger these blinds instead of having to use relays, but its not something ive done yet.

 

[RSin]

Thanks Duncan, I've read it about 10 times and still getting my head around it!

I understand the PIR example but just exploring the motor example further (see pic, is this understanding correct?). So we need a circuit with a relay to switch the 'dry contact cable' because of unknowns. Assuming that you have these relays DIN mounted in the cabinet with the rest of the kit and the blind/Rooflight is some distance away how do you determine the cable CSA required? e.g. you have you blind 20m away and its supplied with 2m cable so you need to extend it. Another thing is datasheets tend to have the 240v circuit current but not the dry contact circuit other than 12 or 24v so how do you determine the current to determine the CSA? Would you just slap a multimeter on it? How did you work out that a CAT cable was sufficient?

[Duncan]

the blinds motors im referring to are the decorquip voluto motors -they have a 240v fixed supply, and a 4-wire dry contact cable.

a suitable 240v supply has to be provided locally to each blind motor that meets the ieee requirements

you refer to 12v and 24v in your diagram but there is no 12v, or 24v involved in the blind motor wiring at all

the 'dry contact' cable is very low voltage low current and connected to some unknow electronics inside, so there are no specific requirements for cable long runs as they probably pass micro or milli amps when briefly contacted together. Ive used cat5 cable from the central cabinet to each blind location, which gives me enough cores for 2 blinds per cable (4 per blind)

back at loxone, you simply need a relay that connects the 'common' to 'up' briefly to make the motor go up, 'common' to 'down' to make the motor go down, and 'common' to 'stop' to stop the motor before it reaches the end-point set in the blind motor itself.

because you have to actively stop the motor, then you need 3 relays and a push switch at the output of the blinds block to fire off the stop function

the push switch has its trigger connected to both the up and down outputs of the blinds block, has a output time of around 0.5 secs and triggers the stop function of the blind motor when either the up or down motion stops.

[RSin]

I understand the how and whys of the relays but the crux is this bit:

"the 'dry contact' cable is very low voltage low current and connected to some unknow electronics inside, so there are no specific requirements for cable long runs as they probably pass micro or milli amps" 

How do we know (rather than assume) the low current? How do you know there isn't a large current being passed down the additional cable when you connect two of the wires?

(FYI the 12v came from the datasheet on the Voluto. My diagram was just taking one loop e.g. the 'up' loop. The 24v was just an assumption of the voltage required to switch the relay in the cabinet)

Thanks again.

[Duncan]

well you can ask decorquip, or just take my word for it - ive installed 14 of these so far using cat5 as the control cable without any issues.

i did try to measure any voltage differences between the inputs but didnt come up with anything meaningful, as i was hoping to find say a 0v and have a +v input to trigger events but couldnt work out what was going on without destroying a motor.

a typical circuit for this kind of thing would be like the inputs to the pokeys - theres an analog or digital switch inside with a pullup resistor passing a few microamps, and shorting the input to the common 0v causes the input to change state.

like the 24v input of the miniserver, they have a high input resistance meaning they pass almost no practical current when on, and therefore you can use long runs of very thin cable without issues.

[RSin]

Yeah I was chasing them to find out from the motor manufacturer. I just thought I was missing something about electronics theory and wanted to learn i.e. those 'in the know' know that dry contact = mA therefore you know you can use thin cables. I guess its not as black'n'white as I was hoping for!

response from them is its actually 5v (not 12v as the pic in the catalogue) and there is "no Amp data, just a weak amp" which ties in with your findings and the original definition I posted/found that dry contacts operate <10mA.

Cheers again Duncan.