Newb question on Diodes

Thread Starter

hang glider

Joined May 27, 2009
9
I want to control a circulation pump. I'd like it to have 3 settings:

Off
On
Auto

the problem is with the "auto" function, because the input could be from two different circuits ('too hot, turn on!' or a 'too cold, turn on').

A two-stage temperature controller would be sending the signal, it has a high range and a low range, and both a heater and chiller would be attached to control a water bath at a fairly precise temperature.

I thought about a 4 way - off, on, auto cool, auto heat ---however I may not know which circuit is engaged, and I don't want to manually switch the pump back and forth.

so, to prevent energizing the wrong circuit, I'm thinking a diode on each line feeding the input leg of the switch would prevent the heater energizer from energizing the cooler and vice versa.

pump's current draw is under 10 watts, 120V
would this work, and what size diodes?

thanks a million!
 

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crutschow

Joined Mar 14, 2008
25,440
If you are trying to control 120VAC a diode won't work. Diodes only conduct DC.

To isolate AC you can use relays with AC coils to control the motor.
 

crutschow

Joined Mar 14, 2008
25,440
That power supply puts out 24W which would be sufficient to power relays.

But I don't think you need to go to DC. Just use AC relays and have them isolate the inputs. The "cool" signal controls one relay and the "heat" signal controls the other relay. The relay contacts (in parallel) then power the pump.
 

Thread Starter

hang glider

Joined May 27, 2009
9
okay, I can embrace that suggestion but I'm having a bit of difficulty finding a 120VAC input relay to control 120VAC load, which is what sent me on the whole 24vdc path... (load on 'heat' is about 1A, load on cool is just a glycol pump (10w maybe?) -
 

crutschow

Joined Mar 14, 2008
25,440
okay, I can embrace that suggestion but I'm having a bit of difficulty finding a 120VAC input relay to control 120VAC load, which is what sent me on the whole 24vdc path... (load on 'heat' is about 1A, load on cool is just a glycol pump (10w maybe?) -
I don't understand that. If you Google 120VAC relay you get a page of hits. Even Radio Shack sells one.
 

Thread Starter

hang glider

Joined May 27, 2009
9
Yeah, I guess I could over-size it - I was trying to "fit" it with the Digi-key filters to get more precise.

thanks for the help!
 

Thread Starter

hang glider

Joined May 27, 2009
9
the 'apply filter' button - really a criteria filter

when you search relay, you have hundreds of 'filters', from current draw to coil voltage to brand name... it's just a tedious process, because selecting the coil at 120v might only show ranges of 50 amp $60 relays. It can be done, but it takes a lot of back and forth, selecting those criteria, then unselecting and trying something else.

I finally narrowed it down to the tyco pclh-206A series.

again, thanks !
 
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