LMAO! Well basically for this part of the project I need to heat candle wax to its melting point. Approximately 150 F. But it need the device to regulate around that temperature. I know absolutely nothing on the electrical end of things. I am a roadway designer so the only electrical design I do is illumination.

 

Yeah , forgot to consider the peltiers low resistance

 

From what I read, peltiers are current hogs. You need at least 1A to get a decent temperature change and they also need 12V EMF (or more).

 

Is it possible/practical to controller peltier output using pwm?


(hope I'm not hi-jacking )

 

If you want to be able to controll the heat aswell then I think the simplest thing to do is to find a power MOSFET or two and attach the backs to the ceramic plate . You will be controlling the amount of heat with a simple potentiometer . You will also need a resistorr on one side of the potentiometer to controll the minimum resistance that the mosfet can have.
That way you got your heating element and controll circuit in 1
Pretty ingenious if I should say so myself

I can draw up a circuit diagram and the parts you need tomorrow , going to go listen to some tunes

 

But it need the device to regulate around that temperature.

If it was me, I'd use an LM35 thermometer IC and a LM339 comparator. (That IC has 4 comparators but I use it for everything. You can never have too many comparators!) This arrangement would act very much like an adjustable thermostat. The output of the comparator will drive a MOSFET acting as a switch to turn your heater on and off, just like your furnace.

This may all sound overwhelming because it's new, but it's easier than it sounds. Using a thermostat will likely solve a LOT of other issues for you.

 

Is it possible/practical to controller peltier output using pwm?

That's commonly used, however it's more efficient to integrate the pulsed current into the peltier so that it's constant. They're more efficient at lower current levels, so full on and off is not as good. And it's probably better for the life of the peltier as well.

The OP doesn't need fancy proportional control any more than you need proportional control of a furnace. On or off will be fine. I built a gizmo that controls to better than 0.1°C using this approach.

 

My bad , I read 'But it need the device to regulate around that temperature' as
'I need to be able to regulate the temperature' for some reason .

Sfountain , do you have a thermometer that can read up to 150 F?

Btw wayneh do you have a schematic of that circuit that you are talking about using the comparator?

 

Just get a thermal switch:

http://www.newark.com/airpax/67f065/thermostat/dp/29M7971?Ntt=67F065

 

A short piece of nichrome wire from a $10 toaster would work. take a 12 inch or so piece of the wire and secure one end of your battery to one end of the wire. attach the other end battery to the other end and after a few seconds, take the temperature of the wire with a thermometer. (fingers are useless above 110-120 F, everything feels hot above that)

keep making the connections closer and closer to each other until you find the distance which gives you the temp you want. You can bend the wire into any shape you want(within reason) after you have found the length that works for you.

 

Just get a thermal switch:

http://www.newark.com/airpax/67f065/thermostat/dp/29M7971?Ntt=67F065

Interesting. Unfortunately it doesn't work well for temperature control since you have a 4º swing between turn on at 60ºC and shut off at 56ºC.

 

The OP just wants to melt wax. I think this will be fine.

 

The OP just wants to melt wax. I think this will be fine.

I forgot about that. OK, but I still think the 67L065 would be better than the 67F065.

 

Btw wayneh do you have a schematic of that circuit that you are talking about using the comparator?

The thermal switch is likely a fine solution, but I took the opportunity to start my very first Eagle drawing (!!). I'm running out of time so it's incomplete. The +in on the comparator would be a variable resistor between the power rails so that you can dial in whatever voltage - temperature - you need. Your adjustment range will roughly be between 0.2V (20°C) and 0.8V (80°C). The pull-up resistor on the comparator output is ~3kΩ and turns on the MOSFET when the comparator goes high, when the LM35 voltage falls below the voltage you've put on the other pin.