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Connect a home thermostat to a small electric heater?

J

Jono

Jan 1, 1970
0
Sorry for the cross post, I think this may be the correct group to post
this. Would it be possible to connect a home thermostat to a small
portable electric heater? I am trying to control the temp in another
project inside an insulated box, and was trying to get more precise temp
control than the built in thermostat on the heater.
 
A

Al

Jan 1, 1970
0
Jono <[email protected]> said:
Sorry for the cross post, I think this may be the correct group to post
this. Would it be possible to connect a home thermostat to a small
portable electric heater? I am trying to control the temp in another
project inside an insulated box, and was trying to get more precise temp
control than the built in thermostat on the heater.

Sure, with the proper relay, no sweat. I did it with a window mounted
A/C. The A/C on-board thermostat sucked, so I got a cheap house unit,
built a relay box, and voila, great A/C control. I has set it to cool.
With the heater, set it to heat. Mount the thermostat within the
insulated box.

Al
 
J

Jono

Jan 1, 1970
0
What kind of relay would I need? Sorry but I know only very very basic
electronics.
 
A

Al

Jan 1, 1970
0
Jono <[email protected]> said:
What kind of relay would I need? Sorry but I know only very very basic
electronics.

OK. This may be too much of a project for you then.

I had disassembled an old oil burner controller. I used the relay and
the transformer from it to build a circuit that the home thermostat
could operate. I built it into a project box and inclueded safety
features such as a circuit breaker and properly crimped connections. The
24V relay-tranformer-thermostat circuit controlled the high current
requirements of the A/C. Peak current at start up was about 10Amps and
running current was about 4.5Amps. Since you are controlling a heater,
you may need a beefier relay than you can get from an old oil burner
controller.

Al
 
D

default

Jan 1, 1970
0
What kind of relay would I need? Sorry but I know only very very basic
electronics.

The relay has two ratings coil voltage and contact amperage/voltage.
The coil voltage has to be compatible with your control circuit
(thermostat side). The contact rating has to switch the current of
the heater (or higher) at some given voltage rating.

At minimum you need a single pole single throw relay normally open
(that is, it will call for heat when the coil is energized). Many
thermostats are designed to run on 24 Volts AC (coil voltage your
relay would need to be, along with a transformer to power it)

The 24 VAC coil rating is used in the states because it doesn't
require heavy wire with a 600 volt insulation rating the way
electrical code does. There will usually be a small 10 Volt-Ampere,
24 VAC transformer located on the heater, as a rule, to step down the
voltage for the control circuit (thermostat).

A 24VAC mechanical thermostat probably wouldn't survive switching
120/240 VAC for very long (if at all). A 24 VAC electronic thermostat
would die instantly with 120 or 240 on it.

There are heating thermostats that will run on 120/240 directly and
would need no relay (just wire the contacts in series with the heater)
You can find them in hardware stores for baseboard electric heating.
They are rather primitive, and use a bimetallic switch and don't
usually allow one to adjust the hysteresis ("dead band" differential
between turning on and then turning off again - a degree or two as a
rule - otherwise the thermostat tends to turn on and off frequently
and that kills the contacts)

Some low voltage thermostats (but not all) will allow one to adjust
the hysteresis.

If you need tight control use a solid state relay. They switch as the
voltage of the ac crosses zero and don't have contacts that will die
the way an electromechanical relay will - you can set the hysteresis
to zero - turns off and on at the same temperature (or damn close)

An electrician could tell you what you need to do . . . if that's too
confusing.

For good tight control a "proportioning" controller is used. It
switches on and off regularly (like every 5-30 seconds) and just stays
on longer to bring the temperature up. They tend to cost more, and
the contacts won't last forever if asked to switch a heavy current.

Some folks call them "proportional" controllers, but a true
proportional control modulates the power by switching like a lamp
dimmer or varying the voltage from 0-100%.
 
J

JeffM

Jan 1, 1970
0
Sorry for the cross post,
Your understanding of the term is severly lacking.
(What you did was WORSE.)
http://groups.google.com/group/sci....*-*+*-*-*-*-two-groups-*-*-aren't-*-different
..
..
Would it be possible to connect a home thermostat
to a small portable electric heater?
::I know only very very basic electronics.This isn't even electronics. It's Electricity 101.

Until you know how to get similar things working with small batteries,
it's best to leave these things to a pro
--or at least someone with some experience.
At the VERY least, before applying power,
have it checked out by someone knowledgeable.

Starting out with lethal voltages is not a way to learn.
The possibility of a house fire is yet another issue.
 
J

JoeSP

Jan 1, 1970
0
Jono said:
Sorry for the cross post, I think this may be the correct group to post
this. Would it be possible to connect a home thermostat to a small
portable electric heater? I am trying to control the temp in another
project inside an insulated box, and was trying to get more precise temp
control than the built in thermostat on the heater.


Very simple. I did it in about 10 minutes with an electric baseboard heater
thermostat, a plastic switchbox and an extension cord. No relay required.
I snipped off the female end of the extension cord, wired the black and
white directly to the thermostat. At the other end, I wired a make plug,
and a short cord coming out of the plug to a female plug. The thermostat,
which is rated for 20 amps, simply breaks the black wire circuit, switching
on and off whatever is plugged into it.
 
J

JoeSP

Jan 1, 1970
0
JoeSP said:
Very simple. I did it in about 10 minutes with an electric baseboard
heater thermostat, a plastic switchbox and an extension cord. No relay
required. I snipped off the female end of the extension cord, wired the
black and white directly to the thermostat. At the other end, I wired a
male plug, and a short cord coming out of it to a female plug. The
thermostat, which is rated for 20 amps, simply breaks the black wire
circuit, switching on and off whatever is plugged into it.
 
Jono said:
Sorry for the cross post, I think this may be the correct group to post
this. Would it be possible to connect a home thermostat to a small
portable electric heater? I am trying to control the temp in another
project inside an insulated box, and was trying to get more precise temp
control than the built in thermostat on the heater.

Yes, long as the thermstat is rated at or above the heater current. If
not, you'll need a relay as well.

NT
 
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