Possible to replace 15v coil relay with 12v coil relay?

[acsam]

Hi everyone.

I am working on a hot tub board that is using a T-91 style relay that is rated for 15v on the coil. I am having a really hard time finding the same
replacement. I do see there are a lot of T-91 relay with coil rated for 12v.

My question is this. Can I use a 12v coil relay instead of a 15v coil relay with the same amps and volts on the switches, or will the 15v fry the 12v coil relay?

Thanks

 

[Nigel Goodwin]

For a start you might try measuring the voltage supplied to it? - it might only be 12V?.

If it is 15V, that's most likely not an issue - relays are pretty sturdy things - but if you're worried about it?, then simply insert a small resistor in series with the relay coil (worked out with ohms law from the current the relay takes from 12V).

 

[Ian Rogers]

If you are worried at all just place two diodes in the feed to the relay thus dropping the coil voltage by 1 ~1.5v ..

 

[Pommie]

If the relay is energised most of the time, such as a power on relay, they may have used a 15v relay on 12v so it runs cooler. Try the 12v and see if it runs hot.

Mike.

 

[acsam]

Thanks for the information guys. I figured as much, due to I have seen 12v turn on using 10.5v-15v

This power relay is used in a hot tub spa for the heater. The switch side of the relay sends 115v through to the heater element which pulls 17amps- 20amps, depending on what wattage the heater is

The low voltage is just for the relay coil. I have seen both 12v, 15v, used on these hot tub PCB board.

 

[crutschow]

I recommend a resistor in series with the coil to reduce the coil voltage to 12V.
That will keep the coil cooler, and extend its operating lifetime.

 

[acsam]

Stupid question, I'm still trying to understand resistors and the best ones to use.

Which resistors should I use?

 

[Pommie]

Are you able to measure the resistance of the relay coil? If so, you need a resistor that is the coil resistance divided by 4.

Mike.

 

[acsam]

Are you able to measure the resistance of the relay coil? If so, you need a resistor that is the coil resistance divided by 4.

Mike.

Ah ok, Ill do that. Thanks a ton

 

[Nigel Goodwin]

Are you able to measure the resistance of the relay coil? If so, you need a resistor that is the coil resistance divided by 4.

Nice simple method Pommie - I must admit I'd just have worked it out from basic principles, rather than a nice elegant short cut

 

[Pommie]

One more thing, the wattage of the resistor needs to be 9/resistance or greater. So if the resistor is 500ohm then it needs to be at least 18 mW or more (1/8 watt or more) if it's 50ohm then it needs to be 0.18 watt so a 1/4 watt resistor is needed.

Mike.

 

[Beau Schwabe]

I would not recommend a resistor or diode solution as suggested. Both solutions will put out more heat than you really want to deal with and it might not pass CE testing.

From the perspective of being an industrial design engineer, a 12V relay in place of a 15V design will cause the 12V relay to run 25% hotter.

We had a similar issue on one of our machines because the manufacturer discontinued the 15V relay we were using that the machine had been designed for.
Instead of using a 12V relay we were able to go with a 24V relay so the relay ran 60% cooler.

If you have a bench top variable power supply find out how much low end voltage it takes for your relay to just "pull in" with both a 12V relay and a 24V relay we found that consistently the minimium "pull in" voltage was about 40% of the Relay voltage rating (your mileage may vary so you will need to test this yourself) .

For us, the 12V relay it would pull in at 4.8V and the 24V relay 9.6V, so 15V was more than enough for the 24V relay substitution.

 

[Pommie]

Surely, the resistor and 12V coil will give out the same heat as the original 15V coil, or did I miss something. However, a 24V relay is probably a better solution.

Mike.

 

[Beau Schwabe]

Surely, the resistor and 12V coil will give out the same heat as the original 15V coil

Yes, but since the relay is physically larger than the resistor the heat will be dissipated over a larger area than the resistor.

EDIT: An 18V relay might also be an option for similar reasons I mentioned in post #12 and easier to find than a 15V relay.

 

[Nigel Goodwin]

I would not recommend a resistor or diode solution as suggested. Both solutions will put out more heat than you really want to deal with and it might not pass CE testing.

I don't see what CE approval has to do with it, and isn't it just self-certified anyway?.

However, a resistor in series with a relay is a VERY common occurance in commercial equipment, I suspect it's often done so as to run the relay as the lower end of it's ratings?.

As with Pommie I see no heat issues, in fact as it's removing heat from the relay it's probably even better.

 

[crutschow]

a 12V relay in place of a 15V design will cause the 12V relay to run 25% hotter.

My math says it will be over 50% hotter (15² / 12²).