Heat bed thru relay

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Heat bed thru relay

Postby airkuld » Mon Mar 24, 2014 3:43 am

Hey, folks.
I've added a second power supply to my 12 in sq. Hadron dedicated to the bed. I want to turn the psu on and off for heating with a little 30 amp automotive type relay by running the normal bed power lines from RAMPS to the relay. These will toggle the relay on and off which will make or break a connection from the psu to the heated bed. The heater has .6 ohms resistance and the psu is 12 volts. Draw should be about 20 amps. This sounded good on paper to me but I have no practical experience with relays. Does this make sense and does it sound safe? Thanks!
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Re: Heat bed thru relay

Postby fma » Mon Mar 24, 2014 5:58 pm

If your relay can switch 30A, it is OK for 20A.

Just check if it is for DC current, and not for AC current. In AC, average current is "peak current / sqrt(2)", if I remember. So, 30A for AC is only 21A for DC...
Frédéric
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Re: Heat bed thru relay

Postby regressedEE » Mon Mar 24, 2014 6:25 pm

Don't forget a flyback diode or you will fry your MOSFET. Here is a nice explanation.
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Re: Heat bed thru relay

Postby r691175002 » Mon Mar 24, 2014 8:12 pm

I would prefer using a solid state relay for this application, since with PID control a heated bed may turn on and off very rapidly.

A mechanical relay will produce annoying clicking and have a much shorter lifespan.
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Re: Heat bed thru relay

Postby regressedEE » Mon Mar 24, 2014 8:31 pm

+1 on the SSR. Those little Foteks they sell on ebay look okay -- you will need a heatsink if you use the 25A or 40A versions.
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Re: Heat bed thru relay

Postby dave3d » Mon Mar 24, 2014 11:17 pm

I am using Arduino/Ramps 1.4 and Marlin. My bed is just on/off control.

Who uses 3 term PID control for the bed? or am I missing something?

I also want to fit a relay to lower the Ramps board temperature which runs hot.
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Re: Heat bed thru relay

Postby fma » Tue Mar 25, 2014 6:18 am

Even with on/off control, it switches very often, sometimes for half a second or so... I also suggest to use Solid State Relay.
Frédéric
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Re: Heat bed thru relay

Postby airkuld » Wed Mar 26, 2014 3:14 am

Looks like you guys are right - mechanical relay clicks on and off every 4 to 5 seconds. SSR seems to be the way to go. Thanks for your help!
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Re: Heat bed thru relay

Postby dave3d » Wed Mar 26, 2014 12:49 pm

Any recommendations on a specific Solid State Relay to use? I wish to order one.
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Re: Heat bed thru relay

Postby airkuld » Wed Mar 26, 2014 2:22 pm

I looked at the Fotek relays regressedee pointed out and they look fine and affordable on ebay. Not sure how to choose one yet. The bed pulls 19 to 20 amps. Ratings are 25 and 40 on the relays. Is 25 amp rating OK or should I go for 40? Also, some of the relays have a dc rating for the control side and are speced for AC on the load side. Can they be used?
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