I have 30 amps of 220 (single phase) available to my shop.
When my electric heater (20 amps) is running, which isn't often because
the shop is very well insulated and my compressor (10 amps) kicks in,
..mmm let's just say.. the lights dim a little.
Neither piece of equipment runs a lot so I see no reason to bust open
the concrete driveway and burying cable to upgrade to a bigger service.
So.... it shouldn't be that hard to develop a relay box that simply cuts
off either piece of equipment when the other is running.... I mean...
shouldn't be that hard for somebody who knows this kinda stuff.
I would be helped a lot if there was name for such a circuit, because
then could start digging for a schematic.
You simply want to throw a switch and have the compressor power up, and have
power cut off to the heater?
Two ways to do it.
Get a double pole double throw switch, and hook it up to the two units.
Thrown one way the compressor will have power, the other the heater. In the
middle neither. It is like a 3way switch, only for 240v.
The more elegant way is to use a 240v relay with normally on contacts. When
the compressor switch is off, the relay will allow power to the heater.
When the switch is on, the relay will open shutting off the heater.
The DPDT switch is certainly easier, but it may require you to move some
wiring around. With the relay everything can stay where it is, with a small
cable connecting them.
Toller in another post has pointed you in the right direction; however,
there is even a simpler way.
Install a DPDT non fused, 250V, 30Amp, disconnect switch that you manually
Leave the switch in the air compressor position.
When you get cold, switch to the heater.
When things warm up, switch back to the air compressor.
Low cost, simple and reliable.
I agree that this is by far the simplest(best) solution if I didn't
forget to turn the heat back on before locking up for the night.
My solid surface sheets need a minimum temp of 60F in order for the
adhesive to work... to wait for that in the morning is a PITA....or so
Let me ramble for a bit here. As I understand, you want power to the
heater available at all times when the thermostat calls for heat and
when it does you want to interrupt the power to the compressor it it
will not run until the heater goes off. So, what you need is a relay
that is operated by the thermostat. Put the coil to the relay across
the (probably) 24 vac circuit that will operate the relay contacts.
Run the power to the compressor through the relay contacts so that they
are open when the thermostat is calling for heat (furnace is on).
Simple DPDT relays are cheap and available from Newark.com as well as
Grainger or others. I'd get 15 amp contacts for the compressor power
and a relay matching the voltage of your thermostat circuit. Simple
Isn't that backwards? Wouldn't you want power available to the heater
at all times except when the compressor needed to run (seldom and only
for a few seconds) Otherwise, you might have to wait until the room
came up to temperature before you could drive a nail with the compressor.
The concept would be the same, but pick up the signal from the
compressor to switch a relay rather than the thermostat. The higher
priority, lower duty cycle device should interrupt the lower priority
higher duty cycle device.
Rob Mitchell Feb 3, 7:19 pm show options
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Date: Thu, 03 Feb 2005 22:19:31 -0500
Local: Thurs, Feb 3 2005 7:19 pm
Subject: Re: Priority switch? (Lektricuty question)
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Mike in Arkansas wrote:
<Isn't that backwards? >
Maybe, but it was my understanding that because the heater only came on
so rarely that he could live without the compressor for the very short
time it was on. BTW, I was assuming that there was an external
thermostat involved in the previous suggested method. If not, and the
heater has an internal thermostat without 24 vac, then the relay coil
would need to be 240 volts and would be wired directly to the heater
element connectors inside the furnace. Be glad to fax a drawing to OP
if it will help.
I e-mailed my fax number to you.
There must be other folks in here who could make use of this solution,
When the heater runs, the compressor can't kick in till the heater stops.
When the compressor is running, the heater can't start till the
compressor is done.
I appreciate your help.
Which is probably backwards from what you want. The heater will run
regularly and potentially for long periods. The compressor will run
rarely and for short periods. Would you rather wait 2 minutes for the
heat to come back on while the compressor charges, or an hour for the
compressor to come on while the heater runs?
If you want the opposite to happen, that is, disable the heater when
the compressor starts or is running just parallel the 240 relay coil to
the compressor motor wiring. So when the compressor motor is running
the same 240 volts will open the normally closed relay contacts that
control the voltage to the heater. Either way, it's a simple thing to
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