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Topic: which micro relay to use? (Read 695 times) previous topic - next topic

heaintdead

hi im trying to switch 120v ac motors for a claw machine on and off and think micro relay would be the most simple way of doing so with digital outputs from the arduino, so im looking for a 5v coil relay capable of handling 120volts. Im new to arduino and would appreciate the help. thanks for your time. 

MalcolmV8

If your A/C motors are small and draw a tiny amount of current you could use these from radio shack

http://www.radioshack.com/product/index.jsp?productId=2062478

They are 5 volt relays and draw only 20 mA so you can drive them directly from the Arduino.  I have a few on some projects I've built.  However they are only 1 amp relays but switching current is just 0.5 amp.  Meaning as the relay switches from on to off there can be no more than 0.5 amp going through it or you'll melt or weld the contacts.  I use them on low powered circuits.

If your motor draws more current you may have to use a transistor to drive a larger relay capable of handling your motor.  I'm sure others with experience driving A/C motors  have even slicker solutions and will chime in and help you out.

retrolefty


hi im trying to switch 120v ac motors for a claw machine on and off and think micro relay would be the most simple way of doing so with digital outputs from the arduino, so im looking for a 5v coil relay capable of handling 120volts. Im new to arduino and would appreciate the help. thanks for your time. 


Certainly possible, but you need to make sure the relay's contact ratings are well above the motors operating voltage and maximum current draw.

Lefty

heaintdead

thanks for the help guys i appreciate it another question came to mind from the first response so how much current can an arduino Digital out put pin handle before somthing fries ?

BillO

The rule of thumb for induction motors is about 6:1.  If the running current is 1 amp, the relay should be able to handle at least 6 amps.   For DC motors, just measure the resistance across the terminals to determine the approximate starting current.  This only gets you in the ball park and each motor is going to be a bit different as inductance is a huge factor as is the inertia of the system, the nature of the load on the motor and the impedance of the supply power.

In practice I usually over-rate the relay contacts by a factor of 10 or more times the normal running current.

Some relays/manufacturers provide HP ratings for their relays.  This make the choice a bit easier, but usually only applies to fairly large motors (1/2HP and up).
Facts just don't care if you ignore them.

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