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inductive load switching

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jnnewton

Member
Hello,
I do a lot of relay circuits that switch dc solenoids / motors. normally, to avoid having to grossly oversize the relay (b/c it is rated for a resistive load), a simple diode (know as a flyback i think) is placed across the relay / coil of the solenoid or motor to prevent the relay from failing prematureley. Works great. Here's the new problem: i've been asked to switch a motor load that is 120VAC. I need a similar solution, otherwise the relay is going to be bigger than the entire control solution. Thanks for any help.

I read this on wikipedia:

In AC circuits a rectifier diode snubber cannot be used; if a simple RC snubber is not adequate a more complex bidirectional snubber design must be used.

How does one know if an RC snubber is not adequate without testing to failure (10's of 1000's of cycles)
 
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Grossel

Well-Known Member
I didn't get it. Why can't you just use a relay to control the motor on/off?
 

ericgibbs

Well-Known Member
Most Helpful Member
Hello,
I do a lot of relay circuits that switch dc solenoids / motors. normally, to avoid having to grossly oversize the relay (b/c it is rated for a resistive load), a simple diode (know as a flyback i think) is placed across the relay / coil of the solenoid or motor to prevent the relay from failing prematureley. Works great. Here's the new problem: i've been asked to switch a motor load that is 120VAC. I need a similar solution, otherwise the relay is going to be bigger than the entire control solution. Thanks for any help.

The diode across the relay coil has nothing to do with the relay sizing.!

Its to clamp the back emf voltage when the relay is de-enegised, this helps to suppress EMI and also to protect the relay drive circuits.
 

crutschow

Well-Known Member
Most Helpful Member
It seems like you are concerned about snubbing the coil, not the load. Is that true?

Is the relay coil voltage to control the AC motor to be AC or DC?
 
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