# How does a current sensing relay work?



## forresth (Feb 19, 2010)

I am assuming it not "closses a relay when it senes a strong enough current" If thats the case, I got 3 bad ones in a row.


----------



## Grampa Bud (Apr 6, 2009)

You might try to explain a little further because all electrical relays that I know of are current sensing in one state or another, either to close a set of contacts or to open them.


----------



## forresth (Feb 19, 2010)

a current sensing relay is used to determine if the furnace fan is running, and sends a (low voltage, in my case) signal. It is needed for an Aprilaire whole house humidifyer.

I picked one up on the internet, and one from a local shop. neither passed a continuity test from a multimeter, and the other one the local shop hasd failed too.


----------



## Grampa Bud (Apr 6, 2009)

Thanks for your reply forresth. I worked for Illinois Bell for twenty+ years and the relays They referred to and I knew as "current sensing" had two coils and a set of centering contacts. They were called polar relays for deciphering rotary dial pulses. So when you mentioned current sensing in regard to HVAC I was confused. I think that all you should need here is a 120vac relay tied across the power leads for the furnace blower with a set of dry contacts (No power) wired into one of the two 24vac leads to the Aprilaire. If the blower turns on the relay will operate. If the aprilaire water solenoid valve doesn't operate the 24vac supply could be bad (no power), or the watervalve could be plugged, or the solenoid valve could be plugged or have an open winding. You will need a DVM to check the voltages and/or continuities and you might need to loosen a fitting or two to check for adequate water flow.


----------



## User75130 (Mar 14, 2010)

Those Aprilaire humidistats require a constant 24v power supply so the outdoor sensor will work. They use the current sensing relay to close the 24v circuit and power the solenoid. There is the A50 24v and the A51 120V current sensing relay. Make sure you have the 24V CSR if your using the 24V circuit. Maybe you're trying to use the 120V CSR on the low voltage, and it won't work.
Also if your furnace or air handler has a variable speed blower, there is probably not enough current to close the CSR. If you do have a standard PSC motor, make sure you have the correct blower wire wrapped around the CSR.
Personally I just use standard humidistats and wire them in series with the solenoid and the '"W" and "C" 24V terminals at the furnace. This way your humidifier will only run when there is a call for heat. Some people say you shouldn't "steal" the 24V from the heating circuit, but a flow thru humidifier won't cause a problem. In fact most furnace manufacturers use a 24V HUM terminal that does take the 24V from the systems transformer.


----------



## forresth (Feb 19, 2010)

ya, thats what I picked up. the furnace blower is a 1/3 hp single speed. I kept on adding wraps untill I ran out of room. at about 4 wraps, it seamed like it might have been closing momentarily, but wasn't staying closed. one more wrap did'nt help. This is how both the Aprilaire and the equivalent Generalaire part worked.

Let me add, I never got the humidifyer fully working. I was testing stuff as I went and dont see the point of going on without working components.


----------



## beenthere (Oct 11, 2008)

Its just a magnetic coil/core.

When it has X amount of current passing through it, the contacts are drawn to their closed position.

Do not mount them near the motor itself, or any other magnetic source.


----------



## forresth (Feb 19, 2010)

thats how I was thinking it worked. I just have a hard time believing I got 3 bad ones in a row. I was starting to think they were much more complicated than that and my testing somehow didn't take thier true mode of operation into account. I feal like I building my last computer again or something


----------



## nap (Dec 4, 2007)

forresth said:


> . I was starting to think they were much more complicated than that and my testing somehow didn't take thier true mode of operation into account.


I believe you are testing it incorrectly or at least expecting a different reading than you should be getting.


since this is most likely a relay with N/O contacts, there should be no continuity through the contacts until it is exposed to enough current to activate it. It does not send anything. It controls a voltage source you supply.



It sounds like you are believing this device is a current transformer (CT). A CT does produce a current that is used for some other purpose. You would have a continuity through a ct's terminals at all times as it is simply the secondary winding of a transformer and the wire it is picking up current flow from is the primary.


----------



## forresth (Feb 19, 2010)

no, I was testing continuity or resistance while turning the furnace fan off and on. what was throwing me off is that it wasn't reading OL when doing resistance testing, it was reading a very high resistance. that may have just been the charge passing through my body, I'm not sure. I also know the 24 volt cuircuts are AC and the multimeter is DC. I was thinking there might be something more involved than just a contact closing in the magnetic field produced by the current.


----------



## nap (Dec 4, 2007)

sounds like you need to use a proper meter and use proper methods so you get accurate results. There should be no


> that may have just been the charge passing through my body,


.

If you are checking resistance, you cannot be touching both lead of your tester. You really shouldn't be touching either one.

as to trying to check AC voltage with a DC meter or vice versa: it doesn't work


----------



## beenthere (Oct 11, 2008)

Some current sensors use a bridge. So you will get a very high ohm reading on some of them.

On some, if they didn't use a bridge, you would get a chattering effect when it tried to close under a load on its contacts.

If you want to test it. make a light circuit. Using a 1,5 volt battery and regular flash light bulb. And then test it.


----------



## A2Mich (Mar 22, 2010)

Might I suggest installing an isolation relay with a 24VAC coil between R and W to switch the humidifier voltage?


----------

