variable resistance

Tobias

I am working on a temperature project I have got a 1.5k ohm resistor between these two wires which go to a heather unit and the heather goes full blast, I would like to control the resistance between these two wires from 100 ohms to about 1500 ohms what's the best possible way? At 100 ohms between the wires the heather goes really slow and at 1500 ohms it's at full speed. I've got the picaxe reading a temp sensor so with a desired value I could have the picaxe maintain the correct temperature with increaseing and decreaseing the value of the resistance between these two wires.
Thanks for your responds, Toby

remmi870

is this 24v ac? what kind of heater is it? im interested on doing a similar setup for a furnace making a programmable thermometer that runs two furnaces with a program that will allow one furnace to run(like the lower level) durning the day to heat a main level, then heat the upper level at night and lower the temp for the mail level, the whole point is to avoid using two thermostats.

let me know, if your using ac, you could use a triac to control the voltage, dc you could use a mosfet with pwm , or use a digital pot to control a triac or mostfet, using a 555 circuit to adjust pwm. depending on the heater, if you use a digital pot directly it may overload or backfeed into the stamp if your using ac. sometimes you can take ac and use dc to simplify the process, but you could experiement if you can take the heater control apart.

Jonathan

Toby,

Firstly,·this is a Basic Stamp forums. Maybe this post should be moved to the Sandbox. We won't be able to help with interfacing your circuit to a Picaxe.

Secondly, we need more detail. What kind of heater? What voltage is it running at and is it AC or DC? How much current does the heater draw? Does the power to the heater run through the resistor, or is it some kind of control circuit? Answer those and we might be able to help.

Jonathan

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Tobias

The heather is 12 volts dc, It is a wobasto heather don't know if you're familure with that anyway it uses the trucks diesel fuel for energy and the wires that come from the heather are only control wires, I have got a 1/4 watt resistor between these two wires, there is 2 volts on one side of the resistor and .1 volts on the other side. Would the pwm method work for this application?
Thanks toby

Mike Green

PWM will not work. It's intended for generating a specific analog voltage and can't supply any significant amount of current. The simplest and perhaps most reliable way to handle this would be to get a 1K pot, put a 470 Ohm fixed resistor in series with it (to make a maximum of 1.5K and minimum of 470 Ohms) and turn the pot's shaft with a servo motor controlled by the PICAXE. You'll have to consult the PICAXE documentation on how to do that part, but servos are controlled with a variable width pulse, roughly from 1ms to 2ms in width, maybe 0.5ms to 2.5ms depending on the servo model with the pulse width specifying the servo position. If you don't send a pulse about every 20ms, the servo turns itself off leaving the pot in the same position.