Intermittent electromechanical relay operation

Thread Starter

Courtesy Flush

Joined Jun 1, 2018
17
Hi,

First post.

I've put together a control system for a small refrigerator to use as a curing chamber for meats. The refrigerator is a simple thing, not frost free, no fans, just a small freezer compartment in the top right corner that cools the whole thing. Per the label, it's a 0.9A load. I have placed a small humidifier (20W), a small dehumidifier (23W) and a small fan (1.25W) inside the refrigerator.

This simplified schematic represents what I have wired inside a box on top of the refrigerator. Add another identical circuit segment to see how the third controller would be wired. I left that out as I felt it was redundant within the context of this inquiry. Minor discrepancies exist in the schematic supplied. They are: an indicator LED is paralleled with outlet 3 that I forgot to add to the schematic and LED 1 and 2 are swapped. My bad. Sorry if any of that causes any real confusion.


The scheme is to use a WH8040 controller (ref. 4 below) to operate the humidifier (ref. 9,) another WH8040 to control the dehumidifier (ref. 8.) One WH8040 can't do both. An STC-1000 temperature controller (ref. 2) to operate the refrigerator (ref. 6.) All of the controllers have relay outputs (ref. 3 and 5.) Notice all controllers are paralleled to relays (ref. 1) that are used to power outlet 3 which is connected to the fan (ref. 7.) The point of those relays is to only power that outlet and fan if any or all of the controlled outlets are energized. If all the controllers outputs are off then the fan/outlet 3 is off too. The LED's are merely 120VAC indicator lights. Everything is wired with 18 gauge stranded wire which is rated for 10A.

Pretty straight forward it would seem. It works fine often. However it fails often too.

What happens?

How it works: If any one of the controllers meets it's turn on set point, it's internal relay energizes and closes it's switch applying 120VAC, the indicator LED comes on, it's controlled outlet gets energized, that load powers up, the downstream relay energizes and the outlet 3/fan comes on. All good. If another controller gets involved, all good too.

How it fails: Sometimes though, when one of the controllers after powering everything for a while meets its turn off set point, that controllers relay switch DOES NOT OPEN and the controlled load stays powered even though the controller indicates that it's output relay has been de-energized. I can hear the controllers output relay clicking when the turn off set point is reached so it would seem the relays solenoid is trying to open the switch.

I've changed all the outlet 3 controlling relays. No change. What is happening and how do I fix it?

1. - Schneider electric RPM11F7 120VAC - 15A / 250VAC https://www.schneider-electric.com/...-relay---zelio-rpm---1-c-o---120-v-ac---15-a/

2. - STC-1000 temperature controller : https://images-na.ssl-images-amazon.com/images/I/B1kOAUuVDWS.pdf

3. - Output relay contact capacity : Cool 10A/250VAC; Heat 10A/250VAC https://www.google.com/url?sa=t&rct...HF3FF_en.pdf&usg=AOvVaw1TYnv_2PXR_TA3j8gweTjP

4. - WH8040 Humidity controllers are these: http://usefulldata.com/wp-content/uploads/2016/05/humidity-controller-mh13100_manual-download.pdf.

5. - Contact capacity : 5A/250VAC https://octopart.com/lz-12vm-k-fujitsu-796258

6. - Refrigerator : 2.5 cubic foot, non-frost free, 120VAC 0.9 amps per label
7. - Muffin fan : 120VAC 5/4 Watts per label
8. - Dehumidifier : 9VDC 23 Watts per manual. Plug in power supply.
9. - Humidifier : 120VAC 20W ultrasonic

I hope I've met the requirements for a respectful and serious inquiry. I need an answer to this. Thanks for any help you might provide
 

dl324

Joined Mar 30, 2015
12,523
Welcome to AAC!

How do you get the failing relay to open? Do you have to replace it?

My guess is that the relay contacts are being welded at turn on due to a high surge current. However, I don't know how they could become "unwelded" if they start working again.
 

AlbertHall

Joined Jun 4, 2014
10,921
Welcome to AAC!

How do you get the failing relay to open? Do you have to replace it?

My guess is that the relay contacts are being welded at turn on due to a high surge current. However, I don't know how they could become "unwelded" if they start working again.
Yes, my first thought too. It could be either the contacts in the controller or the external relay causing the problem.
As it is wired at the moment the full current for everything is switched by the controllers.

To relieve the controller of feeding outlet 3, you could connect the right hand end of the relay contacts direct to mains live instead of via the controller contacts.

https://en.wikipedia.org/wiki/Contact_protection
 

Thread Starter

Courtesy Flush

Joined Jun 1, 2018
17
Ok, thanks for the interest and tanks for the replies. And thanks for the welcome!

Let me take these one at a time.

"So the controller(s) turn off but outlet 3 stays on?" Short answer, yes. Sometimes. The controllers de-energize their output relay when the turn off set point is met. I can hear the relay solenoid when it happens. But the switch within the relay stays closed intermittently. That means mains power is still applied all down the line, including the input of the outlet 3 relay in that controllers part of the circuit.

"When this happens does outlet 1 or 2 stay on as well?" Yes.

"Do you have to replace it?" No. See next answer.

"How do you get the failing relay to open?" The whole thing has one power cord that powers everything. Pulling the plug shuts off everything and plugging it back in starts everything back up. So the controllers read temp and humidity again and their outputs act accordingly.

"My guess is that the relay contacts are being welded at turn on due to a high surge current." See above. They are being held closed somehow against the will of the solenoid within the relay.

"It could be either the contacts in the controller or the external relay causing the problem." Maybe it's the relays on the boards in the incredibly cheap controllers. I swapped the relay 1 and 2's with no effect.

"you could connect the right hand end of the relay contacts direct to mains live" The load here is a muffin fan drawing 1.25 watts.

"are those relays (relay 1 & 2) the relay shown in REF 3?" No. "Notice all controllers are paralleled to relays (ref. 1) that are used to power outlet 3 which is connected to the fan (ref. 7.)" Relay 1 and 2 are referred to in reference 1.

"are they really energized using 120VAC?" Everything in the circuit schematic is 120vac except the controllers relays which have 12VDC inputs but even the controllers are powered by 120VAC. The dehumidifier has a wall wart, 120VAC to 12VDC, that plugged into one of the outlets.
 

AlbertHall

Joined Jun 4, 2014
10,921
I have just spotted a problem with the circuit. If both controllers are on together then one of them switches off everything will stay energised as the controller which is still on will supply power via both the relay contacts to the other circuit.

You could prevent this by rewiring the relay contacts as I suggested in post #4.
 

Thread Starter

Courtesy Flush

Joined Jun 1, 2018
17
"If both controllers have switched off but outlet 1 or 2 is still energised then the problem is in the controller for whichever outlet is still energised."

I would tend to agree, but what is happening and how do I fix it? Replacing all the controllers over and over doesn't seem like a very technical solution.

"If both controllers are on together then one of them switches off everything will stay energised as the controller which is still on will supply power via both the relay contacts to the other circuit."

I currently disagree. Lets assume controller 1 and 2 are both calling for action and all the relays' output switches are closed and outlet 3 is powered. Now controller 2 is says it's time to switch off. It's output relay opens, no line side power applied to the input of relay 2 anymore so that relays' switch goes open too. Chase the juice around Albert and then please explain how mains power can reach any device on the controller 2 side of the circuit if the output relay of controller 2 and the output of relay 2 are both open. Explain the hot side path please.

I'm not trying to be confrontational. Remember Albert, this circuit works much of the time. Loads being switched on and off, fan blowing when it's supposed to and not blowing when everything is quiet, like it's supposed to. But if you can convince me, well then I will be convinced.
 

AlbertHall

Joined Jun 4, 2014
10,921
Now controller 2 is says it's time to switch off. It's output relay opens, no line side power applied to the input of relay 2 anymore so that relays' switch goes open too.
No.
Power is still applied to relay 2 via controller 1, relay 1 contacts, and relay 2 contacts. Relay 2 is latched on by power coming from realy 1.
 

ebeowulf17

Joined Aug 12, 2014
3,275
"If both controllers have switched off but outlet 1 or 2 is still energised then the problem is in the controller for whichever outlet is still energised."

I would tend to agree, but what is happening and how do I fix it? Replacing all the controllers over and over doesn't seem like a very technical solution.

"If both controllers are on together then one of them switches off everything will stay energised as the controller which is still on will supply power via both the relay contacts to the other circuit."

I currently disagree. Lets assume controller 1 and 2 are both calling for action and all the relays' output switches are closed and outlet 3 is powered. Now controller 2 is says it's time to switch off. It's output relay opens, no line side power applied to the input of relay 2 anymore so that relays' switch goes open too. Chase the juice around Albert and then please explain how mains power can reach any device on the controller 2 side of the circuit if the output relay of controller 2 and the output of relay 2 are both open. Explain the hot side path please.

I'm not trying to be confrontational. Remember Albert, this circuit works much of the time. Loads being switched on and off, fan blowing when it's supposed to and not blowing when everything is quiet, like it's supposed to. But if you can convince me, well then I will be convinced.
I'm pretty sure Albert is right. As long as only one controller output is live at once, everything will work fine (thus, it works fine most of the time.) Once two are on, you've created a latching circuit. Neither can release unless BOTH controller outputs turn off. If both controllers turn off before you notice the behavior, they all release at the same time, even though one should've turned off sooner, again creating the illusion that everything is fine.

Sorry, no time to draw it, but I'm pretty sure I see it the same way as Albert.
 

Thread Starter

Courtesy Flush

Joined Jun 1, 2018
17
OK, we're getting somewhere then. I don't mind being wrong. That's fine. I have to admit I'm not getting it though. I would like to.

If someone would be kind enough to only show me how to rewire it, I will, gladly and be happy. I do work better with pictures.

I'll sort out how it works for myself somehow later if you don't want to stick around in this thread and learn me.

I've got nothing to offer you but my thanks and you'll have plenty of that.
 

Thread Starter

Courtesy Flush

Joined Jun 1, 2018
17
Revisiting the 'all controllers off and outlets still on' inquiry. I've seen so many different combinations of things good and bad with this over thing over the last several days that I can't answer that question.
 

AlbertHall

Joined Jun 4, 2014
10,921
Connect the right hand end of the relay contacts in your diagram direct to mains live instead of via the controller contacts. This will break the latching effect and all will be well.
D4ce0Mh.jpg
 
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Thread Starter

Courtesy Flush

Joined Jun 1, 2018
17
Thank You Albert! I can do that and I will right after dinner. I will then power it up and with cold beer in hand and the radio on, I will watch and see what happens.

My Friday nights... Exciting!? No?
 
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