dry contact, needing power to open or close

[yanert]

Hello,

I wired up a new heating system in a large home the last couple of days. In the system, there are a number of solid-state control boards that switch motorized valves on and off. Also, in the boards is a set of dry contacts. When the zone valve closes, the contacts go from N/O to N/C. The literature calls them dry, but they act a little funny. Normally on a dry set of contacts that I am used to, when the coil closes, you will get continuity on the contacts if they switch to the N/C side. On these particular solid-state boards, the contacts will only close after you have the presence of the control voltage put on one side of the contact. I messed with the boards for about 20 minutes watching the valves open and close and could never get the dry contacts to give me continuity. Only after I put the control voltage on one side of the dry contact did it kick in. Is this common anymore on solid-state control boards? I got to thinking about it, and it doesn?t have a coil or anything to pull in a set of contacts, so I would say yes.
Thanks

 

[hbiss]

Re: dry contact, needing power to open or close

I think you are probably confused here someplace. First, even with solid state controls the dry contacts are actual relay contacts with the associated coils controlled by the circuitry.

You would use an ohm meter, continuity tester or the like directly across the contacts to check them. With the relay coil de-energized the NO contacts should be open (infinite resistance) and the NC contacts should be closed (zero or very low resistance).

Seems like you are using a volt meter to check the contacts and naturally you will have to have a voltage source from someplace for the meter to indicate.

-Hal

 

[yanert]

Re: dry contact, needing power to open or close

Hi Hbiss

Thanks for you reply. I put a continuity tester across the N/O contacts. After the valve would close, the N/O contacts change to N/C. I did this experment a number of times while using a continuity tester. The valve would close and the contact would not change, it would stay open. I thought that the board was not working correctly. Just for kicks I put the control voltage that I was using for the contact point, and all be darn, when the valve would close, the contact would close as well. Wierd, I have a number of these boards, and they are all working fine right now. I just thought I was behind the times and that on some solid state control boards you needed the presence of voltage on on the line side of the contact to make it change position.

 

[hbiss]

Re: dry contact, needing power to open or close

I put a continuity tester across the N/O contacts. After the valve would close, the N/O contacts change to N/C. I did this experment a number of times while using a continuity tester. The valve would close and the contact would not change, it would stay open.

Now I'm confused. I know we are not all literary geniuses here but I think you are trying to say:

When the valve closes the N/O contacts are suppose to close. I put a continuity tester across the N/O contacts. The valve would close but the contact would not change, it would stay open. I did this experiment a number of times.

It would be really dificult to know what is going on without seeing the board and what you are doing.

I can say though that as soon as you introduce a voltage into the equation you can no longer use a continuity tester or ohm meter.

-Hal

 

[yanert]

Re: dry contact, needing power to open or close

yep, that is what I am trying to say, but just not in a precise way. Sorry about that. Yes, when the valve would close, the N/O should change and it would not. I would do this without putting any voltage to one side of the contact. I would put my continuity tester on it, and when the valve would close, I would not get continuity. But.... after I went aheand wired it up, ie inducing voltage on one side of the N/O contact, and then closed the valve, the contact would change and I would have a closed circuit, continuity, or a presence of voltage on the other side.

 

[hbiss]

Re: dry contact, needing power to open or close

continuity, or a presence of voltage on the other side.

And you are seeing this voltage now with a voltmeter correct?

-Hal

 

[tony_psuee]

Re: dry contact, needing power to open or close

Yanert.

I'm probably going to put my foot in my mouth at some point in this post, but what you are explaining is correct for solid state outputs. I think the real confusion is the manufacturer of the boards calling them "dry contacts", which I always relate to an electro-mechanical relay(EMR) with a coil and physical set of mechanical contacts. With transistor or triac outputs you will never measure continuity like you would with an EMR. The actual coil is a low voltage signal on the board that bias' the device so the control voltage and current can flow and complete the circuit. If you would apply the load voltage and insert you meter on milliamps on the "open" terminal you will most likely measure a small amount of current flowing on the output when it is off. Same as if you measure voltage, at least that has been my experience with these types of devices. What has always been a concern is that this leakage voltage and current do not cause a valve to stay on when it should be off.

Tony

 

[hbiss]

Re: dry contact, needing power to open or close

The same thing crossed my mind. A dry contact is a pair of metallic contacts not electrically connected to anything. Referring to a triac or SCR in series with the load as a dry contact is not correct.

I can't tell whether Yanert's test methods are incorrect or the manufacturer is incorrect in stating that these are dry contacts.

If I at least had the board in front of me I would be able to tell by looking if a relay is used or if this is completely solid state and put this to an end.

-Hal

 

[yanert]

Re: dry contact, needing power to open or close

Thanks Tony for your reply.

I am sure this is exactly what I am dealing with. It is just a small solid state board. Very thin and only about 5 inches long. Its nice to know that these controls are around. I do agree that either the definition of a dry contact should be changed or they should label the literature a little different. But anyhow, thanks the information! Learning new stuff every day.