Dual Coil SPST Relay

Thread Starter

foscam123456789

Joined Nov 3, 2023
1
Hello all,

I make a lights to music Christmas display every year and think I need a Dual Coil SPST Relay as a solution.

I want to control one strand of lights with two separate 120 V AC sources.

If one source is on, the lights will be on, the lights will also be on if both 120 V AC sources are on.

If there were a relay with two coils (one for each input) and one NO terminal for the output, there would be no issue.

Could this solution be reached with multiple relays? If so, could a diagram please be provided?

Thanks!
 

crutschow

Joined Mar 14, 2008
34,047
Are the two AC sources of the same frequency, amplitude, and phase?
If not, then you cannot connect them together, unless you want to exercise your line breakers/fuses.

Which source will be powering the lights, one or both?
 

BobTPH

Joined Jun 5, 2013
8,664
@crutschow has a very good point. If you are in the US, and the two sources come from different outlets, you could create a 240V short circuit.

If they do come from the same outlet, or you can guarantee they are on the same phase, then you still have to be very careful about the neutral and live. You would also have a problem if either circuit had a GFI, because the live and neutral could be imbalanced in either the two circuits.

All these problems are circumvented if you use a third, non switched source to power the string. Is that possible?
 
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Ian0

Joined Aug 7, 2020
9,502
You need a DPDT relay.
Screenshot from 2023-11-04 09-11-37.pngNo Residual-Current Circuit Breakers will be tripped!
(I need to draw a new SPICE symbol for a lamp - that one is ugly)
 
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Ian0

Joined Aug 7, 2020
9,502
It's a "lights to music Christmas display" it's probably already blinking annoyingly!

If its real lamps then the filament thermal intertia will reduce the blink. If it's controlling a power supply for LEDs then there will be enough capacitance in the power supply.
 

BobTPH

Joined Jun 5, 2013
8,664
Observation: This is a good example of why posters should describe their problem, not their solution that they cannot make work.

My thinking was influenced by his request for a dual coil relay. If he has simply stated the problem, I would probably had thought along the lines of @Ian0. After all, it is the same solution used to power something from either line or generator, which I already knew about.
 

Ian0

Joined Aug 7, 2020
9,502
Observation: This is a good example of why posters should describe their problem, not their solution that they cannot make work.
It's like every time one is asked to assist with fault finding.
Listen to narrative of all the things that have already been done, then ignore it and start from the beginning with "is it plugged in? Is the power switched on?"
 

MisterBill2

Joined Jan 23, 2018
17,814
Dual coil reed relays are available, and it would be rather simple to make one, even. Driver polarity would matter, even with AC, because the fields will either add or subtract. For AC operation the double-diode scheme will be required to avoid buzzing.issues.
 
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