# 12 V relay instead of 9 V?

My RC car uses 2 relays (9 V, SPDT) for it's forward/backward movement, as image below indicates:

However, RC car isn't able to be driven anymore. Specifically, servo works fine but motor can't spin. Instead, there is a sound came from the relay(s). I 'm thinking of replacing both relays to see if it works.

RC car powers up from a Ni-Cd battery (8AA) 700 mAh 9.6 V.

My question is:

Can a 9 V relay been replaced from a 12 V relay? In other words is it possible for a 12 V relay to be energised in this case?

• According to datasheet the pick-up voltage is 75% of 12 V = 9 V. At 9.6 V it may work. – Cornelius Jun 3 '14 at 12:20

According to the data sheet, the pick-up voltage is 75% of nominal, so if you put just over 9V in then it should just switch on.

That said, there doesn't seem to be any reason to change the relays for ones which only-just work at the given voltage where the same range has 9V relays ( which should work for 75% to 130% of 9V ). The 9V voltage is the coil voltage, not the what the contacts are rated to, so choosing a relay with a higher coil voltage will make the car less reliable - at best, it will stop working immediately the battery dips below 9V.

Given the relays click when operated, indicating that they are mechanically sound, you should be worrying more about whether the motor is working rather than the relays. ( it is possible that a contact has failed in the relay, so a relay which clicks might have failed, but usually contacts fail by getting stuck in one position, so if you hear them click it's more likely that it's the motor ). Disconnect the motor and try it directly connected to the battery.

• Motor works fine. I guess is not a good idea replacing with 12 V relays. – dempap Jun 3 '14 at 19:43

Why not use a 5V relay and put a resistor dropper in series with the coil.

A 5V relay is much more common than a 9V relay and probably about as common as a 12V relay so, getting hold of a 5V relay will be the "same" problem as getting hold of a 12 volt relay. If you look at the relay spec linked in the question there is a 5V version and strangely enough there is a 9 volt version but, maybe they are out of stock?

To make the 5V relay work from 9V you'll need a dropper resistor that "loses" 4V and given that the 5V relay coil has a resistance of 70 ohms, something that "loses" 4V will have a resistance of 80% of 70 ohms when placed in series with the 5 volt relay coil i.e. chooses a 56 ohm resistor and rate it at maybe 0.5 watts.

There are plenty of other relays that are less "coil-hungry" than this type so maybe take a look around for one.

• This would work better as a comment rather than an answer. It's kind of thin, and it starts with "why not". – Nick Alexeev Jun 3 '14 at 17:18
• @NickAlexeev I can thicken this up for sure - just thought that it was so obvious it didn't need much else. – Andy aka Jun 3 '14 at 18:39
• Thanks for expanding your post. I don't argue that the original 1-liner was obvious, and worthwhile too. The problem was not with that post per se, but with potential long term effects. After seeing that 1-liner, somebody who is not very familiar with EE.SE customs might think (and argue) that quick and dirty answers are a norm on EE.SE. As a result, we would have more moderation and explaining to do to him. – Nick Alexeev Jun 3 '14 at 18:56