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jrloar, HVAC Technician
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Experience:  Electrician for 12 years, every HVAC certification available
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Good morning all. Have a question regarding the use of a current

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Good morning all. Have a question regarding the use of a current sensing relay. I know how they are supposed to work but am having a problem testing this unit before I install. This is what I understand about the operation:

By running a 110VAC power source thru the top of the sensor I believe I am supposed to read 24VAC thru the 2 wires coming from the other end of the sensor. The 24VAC is coming from a transformer I have hooked up to provide power to the solenoid of a humidifier.

So here is what is happening to me. I have provided the 110VAC at top of sensor and I do indeed get 24VAC from the other 2 wires from the sensor. However, and this is my problem, when I remove the 110VAC from the top of the sensor I continue to read 24VAC across the 2 sensor wires. Isn't the 24VAC supposed to stop once the sensor does not read the 110VAC?

I did one other test as well. I checked continuity across the 2 sensor wires and my meter did move leading me to believe current was moving thru the circuitry of the sensor. If this relay works like an off/on switch should I be reading any sort of continuity between the 2 wires? I would think the sensor should not open or close until it reads the 110VAC. I'm so confused. Heeeeeeeeellllllllllllllppppppppppp. Dave

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Dave, basically all a current sensing relay is is an open set of contacts that close when there is no current going through the loop. If you were to ohm out the contacts when you have 110 volts from some source going through the loop, you would read "0" or continuity. If you take the 110 volts away (going through the loop), you'd read infinity or an open circuit.

Hello, my name is Jim. I am looking forward to helping you solve your problem today. What Tim is telling you is and is not correct. It is correct if it is a set of dry contacts. HOWEVER, if it is solid state (and I am guessing it is) then you will have continuity along with voltage. What you will not have is current. This is why you can not use a solid state relay when using it to turn on and off an LED. It will never turn off when "open" because a solid state relay never truly opens. What model is the relay you are using and this can be given a more precise answer...

This article is also helpful. Even though it refers to an Aprilaire unit, it will be then same. Let me know if you need more help. All you really need to know here is that with current going through the loop, you will have continuity through the contacts. Can I ask what specific problem you are having? Be aware, as in that last article I gave you, if the control current is less than 4 amps, you will need to wrap the wire around the bar a couple of turns.

You may have a bad relay if the contacts stay closed even with the current from the load being sensed is removed.
This is truly an electrical expert type of question. Not that HVAC experts do not know what they are talking about but they are more often than not trained on HVAC equipment and how they work. I personally was a master electrician before I became HVAC certified. I have an associates in Electromechanical technology. Thus am trained extensively in how and why electricity and components work the way they do.
Tim MAY be correct but we need the model of the relay to truly answer your question and be accurate....

Other expert is getting into electronic stuff that really isn't necessary to know here. We're not going to repair this relay. All the customer really needs to know is that with current going through the loop, he will have a closed set of contacts. And if he has 24 volts coming into the wires, he will have 24 coming out if the current is being sensed properly. If he does not, or has 24 volts coming back when the current being sensed is removed, it is a bad relay. As the other expert said, I wouldn't try to ohm out as it is a solid state device.

By the way, I resent the fact that he says HVAC techs don't understand how electrical components work. We have an entire course on just electrical when we attend school. I have helped electricians out that had no clue how to wire an HVAC component. Many electricians have left wiring on HVAC components to us pipefitters because they didn't understand or have knowledge of how to wire it.

Customer: replied 3 years ago.

Hey Tim and I guess a few others. I was a little surprised by the varying responses but if we can all come to a consensus on my problem everyone wins. What I have is an Electronic Relay #50 made by Research Products out of Madison, Wisconsin. I know that under certain circumstances you do have to wrap the wire more than once around the top of the relay. Maybe the bench test I did was incorrect. Here is what I did to test the relay. I have a 24VAC transformer and the relay. I simply ran a wire from an outlet to simulate the needed 110VAC at the top of the relay. Now I have 2 wires from the transformer and 2 wires from the relay. I hooked up one side of the transformer to one side of the relay. I then hooked up the remaining wire from the other side of the relay to the remaining wire on the transformer. My thought was that if I energized the 110VAC I should read 24VAC across the 2 relay wires which in fact it did. However, when I removed the 110VAC from the top of the relay, I continues to read 24VAC across the 2 relay wires. My thought was that if I removed the 110VAC it would close/open the relay and stop the 24VAC at the relay. I hope this makes sense. By the way, I have 3 of these relays and they all react the same way. I must be doing something wrong. Shouldn't the relay just act like a switch and shut down when the 110VAC is taken away from it? This is really bumming me out because logically this should be working unless all 3 relays are bad. I appreciate everyones input. Regards, Dave

Customer: replied 3 years ago.

One last piece of information. I just Ohmed out each of the relays with no 110VAC input at top of sensor. My thought was that I should not have any reading whatsoever on my meter. However, each of the 3 units made the needle swing like current was making it thru the relay when in fact it should not be allowing any current thru because there was no 110VAC involved in this test. I'm starting to think I have 3 bad relays.

Since the other expert seems to suggest he is more knowledgdable about these, I'll opt out and him answer.
Customer: replied 3 years ago.

How does it happen that more than 1 expert jumped on this question? In the past, the first person who responded to me was the only person I worked with. This is the first time I have ever seen multiple responses. I thought the way it worked was that the first person who answered the question stayed with me until issue was resolved or we mutually agreed to open up the question to someone else. When you answered first I thought you were my guy until the end and as far as I'm concerned you are still the expert for this question. If you really don't want to help me I guess I'll repost the question. Just let me know so I can close this particular chat and rate the service I have received up to this time.

That's the way it is supposed to work and most of us are respectful of other experts on here. For example, if he thought I should know something about current sensing relays, he could have done a neutral "assist" which wouldn't have interfered with the question in any way, but would have sent me a private email.
There is one possibility that I thought of here. I'd have to check with the manufacturer about this particular relay and whether or not it can be ohmed out to check it. But the thing that occurred to me (with 3 possible "bad" relays), is do you think it's possible that you exceeded the load rating for these relays and damaged them?
Customer: replied 3 years ago.

Hey Tim. Thanks for continuing with me. All three of these relays were pulled off old furnaces and were used to open the solenoid on Aprilaire humidifiers. They were all working at that time. They have been sitting on the shelf for some time so I guess they could just be bad. Normally I use the 24VAC off the draft inducer pressure switch to power the solenoid but it is causing a problem with the new furnace. I don't know why but when I use that 24VAC source it screws up the start up cycle by locking out the draft inducer pressure switch. I remove the piggyback wire and the furnace goes thru a perfect start up. Absolutely no problem with furnace until I try and use that 24VAC power source. The furnace mfg says it's OK to use this source but it's not working as stated in the instructions. So that is why I am going back to the old school way of hooking up the Aprilaire by providing a separate transformer and current sensing relay. At this point I think I will go to my HVAC supplier and pick up a new relay. They might also be able to test my current relays. If you can think of anything else I might be missing just let me know. I appreciate your input. Hope you are having a nice evening. Dave

I would place the sensing part of the relay (the loop) and place it around the low speed lead of the blower motor. Then whenever heat is called for (and low speed blower comes on), the relay will power the humidifier.
Tim H., HVAC Professional
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Did you have additional questions, Dave, or should we close this one out?
Customer: replied 3 years ago.

Nope. Let's close it. Thanks again.

Thanks, XXXXX XXXXX I was able to help. Have a great evening!

Just so you both know I was not trying to take a question. Here is disadvantages of a solid state relay like you have.

  • Voltage/current characteristic of semiconductor rather than mechanical contacts:
    • When closed, higher resistance (generating heat), and increased electrical noise
    • When open, lower resistance, and reverse leakage current (typically µA range)
    • Voltage/current characteristic is not linear (not purely resistive), distorting switched waveforms to some extent. An electromechanical relay has the low ohmic (linear) resistance of the associated mechanical switch when activated, and the exceedingly high resistance of the air gap and insulating materials when open.
    • Some types have polarity-sensitive output circuits. Electromechanical relays are not affected by polarity.
  • Possibility of spurious switching due to voltage transients (due to much faster switching than mechanical relay)
  • Isolated bias supply required for gate charge circuit
  • Higher transient reverse recovery time (Trr) due to the presence of Body diode
  • Tendency to fail "shorted" on their outputs, while electromechanical relay contacts tend to fail "open".

To verify the relay you have is good is simple. Take an incandescent light bulb and hook it in your circuit in series with the contact side of the relay with your power source. When you do this it is a resistive load unlike an home meter. You will se that the light bulb will turn on and off with current on the 120 volt side. I would bet all of your relays are good. I am sorry but the electrical theory is totally relevant on this question. So the correct answer to this question is that with the info you provided and test you have done there is simply NO way to say the relays you have are good or bad. However what are the odds of a relay that was good when taken out of service and not used went bad on the shelf? NO WAY> Not to mention multiple relays!

This said I am not expecting any accept on this. I am simply trying to get you to understand how and why they work like they do. Yes I would have normally done an assist on this. However when they are telling you need to throw something perfectly good away and spend unneeded money I had to step in. If you have any further questions on how or why they work like this please feel free to ask... I will try to explain it as best I can...

jrloar, HVAC Technician
Category: HVAC
Satisfied Customers: 306
Experience: Electrician for 12 years, every HVAC certification available
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Customer: replied 3 years ago.

I appreciate the lesson in circuitry and am always willing to learn. You were correct when you mentioned the relays most likely were not bad. They were not and the funniest thing about this whole situation was that I never needed them in the first place. Since I didn't want to use any 24VAC source from the furnace I purchased a good transformer from my supplier and used the EAC spade from the furnace to power the 110VAC side of the transformer. From there it was a simple matter of wiring the 24VAC thru the humidistat to the solenoid and back to the other side of the transformer. When the furnace fired I got the 24VAC I needed for the humidifier and on it came. However, not end of story. When furnace cycled off I expected the humidifier to stop running but it did not. Turns out I had a bad fan relay which in the Aprilaire 700 has an electronic circuit board turning out 24VAC even though my transformer was not energized. The 110VAC was going directly to the circuit board where the board, being screwed up sent 24VAC back out of the unit and fired up the fan and solenoid. Took me a while to figure this one out.

Thanks for the update, Dave. I never was quite sure how you had these wired to begin with because you didn't go into detail. Typically the first way we wire a humidifier is off the "Hum" terminal on newer furnaces. But some older furnaces do not have that terminal. Then, as you said, the "EAC" terminal can be used to power a transformer as you did.

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