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relay not de-energising

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RMIM

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Dear All,

I have come across a phenomenon that hope someone can explain – it’s stopping my relay (I think ) from de-energising.

I have 2 loads:
Load 1 and load 2.

Sometimes load 1 needs to be on. Sometimes loads 1 AND 2 need to be on. (Load 2 should never be on without load 1).
Control unit ‘A’ signals when load 1 should be on. Control unit B signals when loads 1 AND 2 should be on. These 2 control units can only give out a LIVE (or if modified will give out neutral).

Everything is ac - The loads use ac. The relay coil is ac.



So I went out and bought a double pole double throw relay with ac coil.
1. I wired a neutral to one side of the coil and control unit B (which gives out a live) to the other side of the coil.

2. To the common side of both contacts I wired live.

3. To the other of the normally open contacts one was wired to load1 and the other load2. The other sides of the load had neutral wired.
When control unit B came on so did loads 1 and 2. When unit B went off so did Loads 1 and 2. As expected.

To get Load 1 to come on by itself I wired control unit A direct to Load 1. – when control unit A was on so was load 1, when it was off so was Load 1 off as expected.

The problem arose when control unit A was on (and so load1) and remained on. Then control unit B was turned on, this would then bring load 2 on as well, as expected – but when control unit B was turned off Load 2 would remain on despite no live current being supple to the relay coil.

Control unit A was stopping the relay from releasing I think.

FIX1:
I thought maybe if I use 2 separate relays this would help. It did not.

FIX2:
This fix did work.
I modified control unit 2 to give out neutral and made the relay coil switch with neutral. Also I made Load 2 neutral switching, Load 1 remained live switching.

This time load 2 did not stick on when load 1 was already on.

What is going on here? And are there other modifications to overcome this problem?

Thanks
 
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Post a wiring diagram of your circuit.

What are the loads?
 
Can you post a schematic? It's much easier to diagnose such problems with a drawing.
 
Neutral should never be switched.
 
Can you post a schematic? It's much easier to diagnose such problems with a drawing.

relay diagram.jpg

thanks for the reply ChrisP58

edit - i have labeled control unit 1 and 2 the wrong way round - but hopefully you will get the idea.
 
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could it be to do with the following?

Capacitive noise

When an AC cable is lying close to the signal cable, there is a capacitive link formed between the two cables. This link, sometimes called crosstalk or pickup, enables a small current to flow from the AC cable to the signal cable. When the signal cable is in the off state, this small current may affect the I/O relay concerned.

For instance, this may cause the electromechanical relay or optocouler to "latch on", LED can glow when the signal is off or in worst case it can switch the relay on even when the signal is off.
 
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Thanks for the reply Inquisitive - do you mean from a safety point of view?

Yes, from a safety stand point it is not safe or recommended. Switching the neutral line can be dangerous Generally, the hot is only switched and the neutral is always a continuous return to the feed in the AC world.
 
Some relays will stay in the energised position with very little coil voltage. You don't say what the control units are, but the fact that they can switch 3 A and can only switch the supply makes me think that they are could have suppression components in them. The leakage thought the suppression components could hold the relay in. It is also possible for the suppression components to form a tuned circuit with the relay coil, which would increase the voltage on the coil. The coil inductance will increase a lot when the relay is in the energised position, which will alter the tuning.

Try measuring the coil voltage when the relay is staying closed when it shouldn't.

I had a problem with a thermostat controlling a relay. There were large switching spikes, so I added suppressors (something like https://www.farnell.com/datasheets/95080.pdf) across the contacts. I then found that the relay would energise correctly when the thermostat closed, but would stay energised when the thermostat opened.

When I measured the voltage across the relay coil with the thermostat open, it was larger than the supply voltage. The coil formed a tuned circuit with the suppressor, and when the relay was energised, the inductance was enough to be resonant near mains frequency. When the relay was not energised, the lower inductance meant that the resonant frequency was far from mains frequency, so there was far less voltage on the coil.

I fixed the problem by moving the suppressor to being in parallel with the coil.
 
Dear All,

. . . . . . . .

2. To the common side of both contacts I wired live.

. . . . . . . .

Thanks

Can you confirm the above? If by any chance you have connected the common side of both contacts to the other terminal of the control switch, you will have this behaviour.

Ramesh
 
I had a problem with a thermostat controlling a relay. There were large switching spikes, so I added suppressors (something like https://www.farnell.com/datasheets/95080.pdf) across the contacts. I then found that the relay would energise correctly when the thermostat closed, but would stay energised when the thermostat opened.

Thank you for the reply Driver.

Funnily enough the control units are 1 hotwater timer and unit 2 is a wireless thermostat. Loads 1 and 2 are a gas boiler and the other a central heating pump. It's an old boiler that requires the boiler to be on before you put the central heating on. And when the boilers on it heats the water.

The best solution is to bin the boiler and fit a combination boiler or even a diverter valve - that takes a lot of time. I just needed a quick fix so I could use modern controls.

My fix of using neutral switched coil seems to have worked, but I wanted to know why, as I saw nothing wrong with my original wiring. I now believe it's some sort of interference. (Capacitive noise? common mode noise?) I always seems to have lots of problems in my circuits (erratic behavior) when there's ac about.

I think your suppressor idea is what I should be looking into.
 
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Can you confirm the above? If by any chance you have connected the common side of both contacts to the other terminal of the control switch, you will have this behaviour.

Ramesh

Thanks for the reply Remesh.

I have posted a circuit diagram if you missed it.
I have connected both commons to live. The control switch is connected to one side of the relay coil.
 
No RMIM, I did go thru' the Ckt Diag. But after going thru' the discussions, I mentioned the possible reason (what I think!)
I can only suggest that this be checked again.

Ramesh
 
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