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Old 11-21-2010, 02:50 PM   #1
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Is this a NO or NC Circuit?


HI;

I need to run a small electric heater to keep a room that I store my plants in at 20 degrees F. 20 degrees F is lower than what the built in thermostats go to on normal heaters. So I have to use an external controller.

I've looked at a couple different controllers, but they are all rated something similar to this:

Relay Output Ratings (SPDT)
  • Normally Open (NO): 16 Amps @120Volts
  • Normally Closed (NC): 8 Amps @120Volts
I would want it to run the heater only when it gets too cold, so would this be considered a Normally Open Circuit? The heater draws about 9 Amps, so if it is considered a Normally Closed circuit, then I would be over the limit.

It makes more sense to me that it would be considered Normally Open, since it would not be running, except then the conditions (too low of a temp) trigger it. I've read the manuals for these devices and it isn't very clearly defined what is NO vs NC. I just want to make sure I get a device that can properly handle the heater I am putting on it.

Thanks very much;

Jamie

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Old 11-21-2010, 04:01 PM   #2
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Is this a NO or NC Circuit?


NO(normally open) and NC(normally closed) refer to the condition of the contactor when in the non enegergized state.

So in your case you would want normally open so when power is applied to the coil(when the temp gets below 20F) the coil will engergize and the contact will close and allow your heater to turn on.

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Old 11-23-2010, 10:34 AM   #3
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Is this a NO or NC Circuit?


Quote:
Originally Posted by darren View Post
NO(normally open) and NC(normally closed) refer to the condition of the contactor when in the non enegergized state.

So in your case you would want normally open so when power is applied to the coil(when the temp gets below 20F) the coil will engergize and the contact will close and allow your heater to turn on.
Thank You.

Jamie
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Old 11-23-2010, 10:49 AM   #4
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Is this a NO or NC Circuit?


I see you moved this back to the top since the controllers you are looking at have you confused, right?
Temperature controllers work on a different principle than a simple relay does. The controller does have a relay, but for cooling it must be programmed to "reverse" acting in the parameters. For heating you would select "direct" acting.

Give me the specs on the controllers you speak of and we can fix you up.
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Old 11-23-2010, 11:23 AM   #5
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Is this a NO or NC Circuit?


Quote:
Originally Posted by J. V. View Post
I see you moved this back to the top since the controllers you are looking at have you confused, right?
Temperature controllers work on a different principle than a simple relay does. The controller does have a relay, but for cooling it must be programmed to "reverse" acting in the parameters. For heating you would select "direct" acting.

Give me the specs on the controllers you speak of and we can fix you up.
I think I have found the proper unit, and should be within it's amp rating.

I am looking at the Johnson Controls A419 Series 120V model.

I found the directions for the change I need to make. Based on this, I think
I will need to set it for Cut in for heat mode:



So I will set the jumper to Heating Mode Cut-In at Setpoint, then set the
temp to 20 degrees, then the unit will engage the relay, sending power to
my heater when it drops to 20 degrees.

Does this sound right?

Thanks,

Jamie
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Old 11-23-2010, 02:36 PM   #6
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Is this a NO or NC Circuit?


don't forget to set the differential to cut out when it reaches max temp.
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Old 11-24-2010, 10:53 AM   #7
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Is this a NO or NC Circuit?


Take a look at this type. They will require a separate contactor, but are the most versatile. You can get an IEC contactor from the same web site for less than $30.00. You would need to mount this stuff in a small cabinet too. Your choice may be better. Check prices.

http://www.automationdirect.com/adc/...ss_Controllers

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