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Can I use an SPDT 5 wire Relay in reverse?

artistwantab

New Member
Hey guys,

I am new here.

I have never used a relay this way but I can't understand why it wouldn't work.

So I have 2 12v battery sources. I want to change from one battery to the other when the car is turned off.

Can this be done?

AutoTransfer-Switch.jpg
 

rjenkinsgb

Well-Known Member
Most Helpful Member
Yes, that's fine.

Relay contacts are just like "remote controlled" switches, you can use them in anything you could control with a normal switch, as long as you stay within the voltage and current ratings.
 

Les Jones

Well-Known Member
Most Helpful Member
Is the power for the relay coil being supplied from one of the 2 batteries, the output from the relay common or is it from an independent supply. If it comes from the relay common there could be a problem as there will be a short break in the power as the connection between the NC and NO contact. This could be avoided by using two diodes to provide the relay power from either battery.

Les.
 

artistwantab

New Member
Is the power for the relay coil being supplied from one of the 2 batteries, the output from the relay common or is it from an independent supply. If it comes from the relay common there could be a problem as there will be a short break in the power as the connection between the NC and NO contact. This could be avoided by using two diodes to provide the relay power from either battery.

Les.

You are right...So this....

transferswitch.jpg
 

Les Jones

Well-Known Member
Most Helpful Member
I am assuming that the coil connections are 85 and 86 and the contacts are 87 NO, 87a NC and 30 is the common contact. the diagram in post #1 is more correct.
I am guessing that 85 is connected the negative of both batteries and that the positive switched supply to the relay coil goes to 86. It is this supply that could be a problem if it comes from the common output (Pin 30) because for a very short time when the relay is switching you will loose this supply. I suggest that the supply that switches the relay coil is supplied via a diode from each battery with the anode end to the battery and the cathode of each diode . An alternative would be to use the battery that is selected when the relay is energised to feed the switch which powers the relay coil.
If the relay coil is supplied from a source independent of the batteries then the diagram in post #1 will be fine.

Les.
 

artistwantab

New Member
I see I did goof.

So I have two power sources.
  1. Power is coming from the dash light circuit when the car is turn on.
  2. Power is coming from house batteries
85 I was going to connect to factory ground.
86 Is power to trigger the relay

I want to trigger the relay to change power sources when the car is turned on.

87a NC is power when the car is turned off coming from the house batteries
87 is power from the dash circuit when the car is turned on.

30 is going to the accessories.

So I messed up my direction of the diodes on 87 if I am understanding correct.

BTW...Thank you much.
transferswitch.jpg
 

Les Jones

Well-Known Member
Most Helpful Member
Now you you described the actual function of the circuit there is no need for the diodes. The relay coil will be powered from the car battery when the engine is running. I was considering the two batteries so you could switch over power sources when one battery went flat. Your original circuit will do exactly what you require.

Les.
 

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