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How to use a relay?

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McCheesington

New Member
Hi, I designed a circuit which switches a relay on, and I think that a relay is a switch.

How do I set up something, say, a 3-5V motor, to be switched on by this relay? Or is a relay completely different and if so, how can I power the motor using the relay. Cheers ;)
 

KeepItSimpleStupid

Well-Known Member
Most Helpful Member
A simple definition of an Electromechanical relay:

It, through the use of an electromagnet operates a mechanical switch.
It provides electrical isolation.
The relay coil has parameters such as coil voltage, pull in voltage, dropout voltage, continuous or intermittant duty, actuation time
The relays contacts come in various forms such as SPST, DPDT. They actually might be called "Form C" which is a particular contact arrangement. The contacts can be AC rated, DC rated or even Horsepower rated.
They are divided into various classes such as "definate purpose contactor"
 

davenn

Active Member
Hi, I designed a circuit which switches a relay on, and I think that a relay is a switch.
How do I set up something, say, a 3-5V motor, to be switched on by this relay? Or is a relay completely different and if so, how can I power the motor using the relay. Cheers ;)

ok well you can do something like this....

relay-motor-sw-gif.58079


do you specifically need a relay? you can do it by just switching the transistor alone
and put the motor in the collector circuit of the transistor. making sure the transistor can handle the current drain of the motor :)

cheers
Dave
 

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McCheesington

New Member
KeepItSimpleStupid: I am a complete newbie, so that went right over my head :D Sorry!

Davenn, thanks for the info and the diagram was REALLY useful! :)

Could I use a transistor as well? Or would a relay be better?
 
Last edited:

DerStrom8

Super Moderator
A transistor is basically a solid-state version of a relay. If you can get one with ratings high enough for the current draw and voltage of a motor, you should be fine. Generally, though, transistors are used to switch small amounts of current. For example, the transistor in Davenn's circuit switches just enough current to turn on the relay, which connects the motor to +5v. A simple form of relay has an electromagnet at it's core. When you connect the coil to power, the magnet pulls the metal part of the switch towards it, and it contacts another piece of metal inside the relay, which completes the circuit. Basically, the switch is just activated by the magnet. The transistor might be used to switch power to the coil, and then another power source (connected to the motor) could be switched on by the relay.

Der Strom
 

davenn

Active Member
KeepItSimpleStupid: I am a complete newbie, so that went right over my head :D Sorry!

Davenn, thanks for the info and the diagram was REALLY useful! :)

Could I use a transistor as well? Or would a relay be better?

You're welcome

As Der Strom replied, it depends on your situation and what you are wanting to switch

The circuit like I showed is good for isolating the output of the control circuit from the load of what is being switched ( it protects the control circuitry from excessive voltages/currents during a possible fault condition)
eg .. .say you wanted to switch 240VAC (maybe 120VAC in your country) Then using the above cct is a safety must. But for switching low voltage / low current loads, you coould easily use just a transistor.

Dave
 
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