# Using LM56 to control heating element

Discussion in 'Electronic Basics' started by Hammy, Jul 28, 2007.

1. ### HammyGuest

I'm thinking of using the LM56:

http://www.national.com/ds/LM/LM56.pdf

My question is what is typically used as the heating element. I'm
planning on using it to switch in a heater when the temp drops below a
certain threshold and to switch it out when it hits another threshold.

The temp here in the winter time can hit below -20C not including the
wind chill. Is it possible (practical) to keep an enclosure a weather
tight (PVC) (1.25" x 4") at a minimum of say 10C under such
conditions? Or should I be looking at another device or method.

What I'm thinking of doing is like fig 7 of the application note on
the LM56.

2. ### John PopelishGuest

The heater depends entirely on what must be heated and how
well it is insulated. This thermostat could control the
heat to keep your entire house warm, if the output relay was
large enough and you were willing to pay for the power.

So lets start at the beginning. What are the dimensions of
the enclosure?

What is it insulated with? Can you come up with an R value?

What waste (watts) heat is already being produced inside the
enclosure?

What is the warmest and coldest ambient temperatures the
package will be exposed to?

What are the temperature limits of the package?

Until you have gone through that list, you do not even know
if you have a heating or a cooling problem, or both.

3. ### John LarkinGuest

That would work. I'd replace the 2N3904 with a mosfet and put the
heating element, a power resistor maybe, instead of the relay coil.

I think you could also do what you want without the IC... just a fet,
a thermistor, and two resistors should do it.

You might get a power resistor, stick it inside the actual box, apply
some voltage, and calculate the temp rise factor for your box, in
degrees C per watt, so you can figure out how much power you'll need
to raise it from -20 to +10.

What sort of power is available?

John

4. ### HammyGuest

I'll try to better explain what I'm doing. I've built a dual beam IR
barrier similar to this.

http://www.guardall.com/product.asp?PageID=245

I'm putting the Tx in a 1 ½" x 5" weather tight (outdoor electrical
junction box) which is made of PVC or hard plastic. The Detector will
be going in an identical enclosure. Everything works fine (Thanks Ed).
I'm just trying to figure out the most efficient way to regulate the
temp for the brutal winters we get here (Canada). From a heat stand
point everything should be fine unless the enclosure interior exceeds
55C. There are no major sources of heat in the detector. The
transmitter has a 7808 (TO-220) for the LEDS the driver and fet, the
timer TLC556 (DUAL) gets a 78l05 (T0-92).The detectors have a
78l05(T0-92) this supplies two PNA4602M and a dual comparator and
single AND gate. This is all being supplied from a printer PSU 30Vdc @
0.5A or 15W. I'm dropping this to 12Vdc with an lm317.

The Tx draws 1.05W (the bulk of this is dissipated in the 7808 and LED
current limiting Resistor) the RCV draws 0.150W.

I was looking at the LM56 because I could use it to switch on a heater
with one output and use the other output for thermal shutdown.

I was thinking of using an old oven element I have in the garage? Just
provide a two inch length with enough power to keep things warm. I
just thought there may be something available that came with mounting
hardware that someone may know of. Are large power resistor what is
typically used?

The temperatures in the winter can get to -25C and lower. Averages in
the negative early teens (-12 to -15) to dam cold. Live here for a
winter then tell me what you think of the Global warming theory (;.

5. ### John LarkinGuest

It will probably work fine cold, without a heater, until all the snow
blocks the beam.

LEDs can get a lot brighter when they get cold, assuming the current
stays fixed. Zap one with some freeze spray and see!

John