Hello, I want to make a unit that will keep a glow plug at around 400 degrees F. A glow plug, on 12v, produces about 1200 degrees F. The unit needs to be powered by 12vdc (car battery). I will be using two glow plugs and they use about 150 watts each. Any sugestions will be greatly appriciated.

 

Use a pulse width modulator (PWM) and a big MOSFET driver. I'm including a schematic. With the potentiometer, you can adjust the duty cycle (power dissipation of the glow plugs) from zero to 100%. There are also dedicated ICs that can be used for PWM. If you use the MOSFET I've shown, its maximum average power dissipation will be about 3.7 watts, so you'll need a heat sink.
The LM393 is shown as U3 and U5, but it is a dual comparator, so you only need one.
You might be able to use a noncontacting IR thermometer to measure the temperature of the glow plug. Otherwise, I'm not sure how you would do it. It seems like any sort of contacting sensor, such as a thermocouple, would be a heat sink. I have a cheap IR thermometer that I bought at Radio Shack. Its upper limit is 400F, and I doubt it focuses well enough to measure a glow plug accurately. If you already know how to measure the temperature, can you share the method with us?

I edited the schematic. The old one had the glow plugs connected to the positive battery terminal, driven by an N-channel MOSFET. The glow plugs are now connected to the negative battery terminal, driven by a P-channel MOSFET.

Attached Thumbnails

 

 

you could always measure the resistance of the glowplug during the off time. The temperature/resistance curve will be much like a platinum thermometer :lol:

 

I think Clay is working with glow plugs for Diesel engines (Clay?). From what I can gather, they use Tungsten for the heating element. Alcohol fuel (i.e., RC) engines use a Platinum alloy because a catalytic reaction between the alcohol and the Platinum causes the element to get hot and glow, providing ignition for the next compression stroke.
If you were serious about the measurement idea: :?
You can't measure the resistance of a element of unknown dimensions and deduce its temperature from a temp/voltage curve. Imagine a platinum thermometer for which you have a calibration curve. All is well. Now, unbeknownst to you, I sneak in and add another one in series (or parallel). Measure the resistance and look up the temperature. It will be wrong.
If you were being facetious -
Never mind!

 

Ron you are correct in that I am using Diesel Glow Plugs. The glow plugs I am using will be vaporizing Gasoline. Gasoline has an ignition temp of 832 degrees F. The actual vaporization point of Gasoline is 40 degrees F. I think that I can run the plugs at 400 or so degrees and keep the fuel from igniting but yet keep it vaporized. Very simple process I just need a controller to run the glow plugs so they don't overheat and burn up.

 

Ron. I meant that the curve will have the same sort of slope and shape. It doesnt matter if its tungsten, since the idea is to calibrate it anyway. You only need a few points around 400F, then you set up your controller accordingly. You will know when I,m facetious, its difficult to mistake.

 

Hello


Why not use Thermocouple wire for temperature Control/Feedback.

Diameters are available down to .004" (Omega) and they should be able to handle the temperature.


Charles