maintaining constant temperature?

I was just going to put the thermistor next to it. There wont be other loads, and it will be in room temp.

But I don't know what kind of thermistor I need or anything. I'd like it to stay at 140F +/- 2F.

What's your target run time? What kind of thermal isolation is there between the metal plate and any other heat sinks? What is the ambient temperature? What are the consequences of failing to maintain 140 F? What other loads are presently being, or are being considered for, the

Yup, you have said it all !!

As a swag, a CMOS 555 hooked up as a variable duty cycle astable driving a saturated transistor switch might work. Try a 10K thermistor from Vcc to DISCH, a 22K between DISCH and THR/TRIG paralleled with a switching diode, and a 0.1 uF cap at TRIG. This won't ever turn completely off, which might be a good thing depending on ambient losses.

Alternately, use the thermistor in a voltage divider compared to a setpoint pot, both input to an LM339 comparator as a bang-bang controller.

Haven't run the numbers on what kind of heating to expect with only

I see...

No, wait-I'm confused. DUMBFOUNDED as it were...

Ok, nevermind all the IC, I can just tweak until a simple thermistor and source work properly. Say for a constant input of 5V and up to 1A, happen to know what sort of thermister would I want to get at radio shack to be able to reach 140F?

The simplest power controller is an infinite gain (on-off) type that turns the power full on if the temperature is low and turns it off it the temperature is too high. A comparator is a device that switches off or on depending on which of two signals is more positive.

Based on the power of the heater, the heat stored in the heater element (that keeps coming out for a while after the power is turned off, and takes a while to come out when the heat is turned on),the thermal mass of the object being kept hot, and the time delay between that mass and the temperature sensor, the temperature will ramp back and forth through the desired temperature. Whether or not it stays within the required bounds under this method of control depends on the the factors I mentioned earlier.

The only way to keep the temperature in much narrower bounds is to pulse the heat faster than the time delay between the heater and the temperature pickup, and vary the duty cycle of those pulses with a more complicated scheme than the simple one I first described. One such more complicated scheme is the PID algorithm (a combination of Proportional gain times the temperature error, another gain times the integral of the error, and a third gain times the derivative of the error.

I think that you may misunderstand what a thermistor is, or does.

A thermistor is a component that varies its resistance depending on temperature. Thermistors are normally used as temperature sensors. They may be used in a circuit that controls a heater, but they don't do the heating themselves.

Radio Shack isn't going to have much that is helpful. But Digikey sells a wide range of thermistors , including some that switch from conducting to insulating over a very narrow temperature range, as temperature rises. This type is called a PTC thermistor for Positive Temperature Coefficient. I use a few of these soldered between a pair of brass ribbons mounted in the front of my motorcycle helmet to act as a self regulating defroster bar. You may be able to use one or two of these connected in parallel and mounted around your heated mass to act as both heater and control system. But it may take more than 5 volts to get it up to its switching temperature, since they have about

The Digikey part number for a 50 ohm PTC thermistor that switches off at 60 C ( 140 F) is 235-1123.

John Popelish wrote in news:yNOdnWRrbdT_YuTeRVn- snipped-for-privacy@adelphia.com:

The devce you are using is a "Switching PTC Thermistor", a name given to it by the manufacturer: Thermometrics.

Most people in the field would not generally refer to this as a "PTO thermistor". The "silister" in the above URL is what most people refer to as a thermistor. The Switching PTO Thermistor curves look like those of "PTC resettable fuses". The are probably the same technology under a different name.

lol. no comment.

Anyway I was intending to just use the thermistor as a heat source, I'm not melting an iceberg. Oh well, I'll figure it out later.

Concert pipe? You need 155C to evaporate the THC. ;-)

Good Luck! Rich