Contactor's

Dink said:

I was wondering if a 12v auto starter relay could be used for system testing. (50amps max) I was wanting to test the difference between 24v and 36v on my new bike. The first one runs on 24v, and I have an identical motor that I was considering overvolting to 36v.(controller choice) Contactors are made in different amps/voltages. I understand more amps means larger,thicker contacts. But on the voltage side, is it that they have different windings for each voltage to control it? So being, can I run 24v/36v through the main connecters, and use only 12v to turn it on and off?

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Hi Dink,

First off, the automobile starter has a large contactor (which is just a big relay) typically mounted right on top of the starter motor. I assume you are talking about a smaller relay, sometimes looking like a cube about 1.2 inches square. Is that what you have in mind?

Some of these "automotive" type cube relays can be found to be rated as high as 30, 40 maybe even 50 amps. They are good relays. I have used many of them for all sorts of things.

A relay is a switch activated by a coil. The simplest configuration is a SPST, Single Pole Single Throw. It has 4 terminals. The two big terminals are for the main contacts and have the high current rating for the load switching. The two smaller terminals are for the control coil, which is used to pull-in (turn on) and release (turn off) the main contacts.

The coil for these automotive style relays is rated for 12 V DC. It will require less than one amp, typically. If you use more than 12V on the coil, it will overheat and burn up. If your control circuit is higher than 12V, you can put a resistor in series with the coil and use it. Measure the coil resistance and match it, then you could use 24V to turn on the relay.

The main contacts for the relay will have both a voltage and a current rating. The current thru the main contacts depends on the circuit it switches, or depends on the load in other words. And like you said, higher current contacts are larger. Those contacts will also have a voltage rating. This is because when the contacts open they can have an arc which is considered in the relay design. If you exceed the contact voltage rating, that arc can cause damage to those contacts and cause the relay to fail. I think some of these automotive relays have a 32 volt rating. And for testing a bike circuit, might do just fine at 36 volts. It also is dependent on the type of load which you are breaking. Inductive loads (such as motors) will cause more arcing on the contacts. This can be mitigated by the use of a reverse biased diode across the load.

Hope that helps,

major