Monday, November 28, 2005


Join the resistance

An idea I've been kicking around is to use the resistance of the nichrome heater element in the MkII Polymorph Extruder to measure its own temperature.

Adrian's circuit chops the current going through the coil. It might, I thought, be possible to measure the resistance of the coil while there is no heating current going through it. A quick experiment with a lighter showed a nichrome coil varying from 4.9R to 130R when I waved the lighter underneath it.

This isn't really my area of expertise, and I wondered if anyone else might have some thoughts on how it might be done. If we can remove the need for a thermistor, connectors, added assembly steps and wiring, then the device gets simpler - even if we have to add another regular component or two to the board.

Vik :v)

Platinum resistance thermometers use a resistence/temperature conversion regime. They're also very, very accurate. The trick to the application of the principle that you suggest, I suspect, is accurately measuring resistance whilst concurrently running a current at an arbitrary and varying voltage through your nichrome wire. Never thought about having to do that before.

Bye the way, just a detail question about the heated extruder head. It appears that you have your brass extruder with a layer of tape, nichrome and then another layer of tape. From what Adrian said that unit draws some 6 watts. Unless I missed something it looks to me that most of the heat would escape outward instead of being applied to melting the polymer. Is there a layer of insulation preventing that I missed seeing somehow?
Perhaps if you're chopping the current used for heating you could use a standard circuit for measuring resistance that cuts in when you aren't in heating mode.
I've considered giving it a fibreglass blanket to keep the heat in.

What circuit?

Vik :v)
You can find that sort of thing in a standard cookbook for electronic circuits. If you haven't got one of those I suspect I could find you one on the web.

You'd just need to toggle it with the heating input and then wait for a few moments while the nichrome wire cools down after a heating cycle then take a reading.
If you could find one, that would be nice. I've been searching the web with no luck.
Give me a couple of days and I'll find you one. What range are you looking for? I'm guessing about 0-500 ohms?
Hmmm... looks like you might have the circuitry you need already. You are using a thermistor, right? Thermistors are resistance temperature sensors that are different only in that in most cases resistance drops with increasing temperature instead of increasing like it would with your nichrome wire. Take a look at this...

The resistance ranges seem to be similar, though I don't know the characteristics of your particular thermistor. I assume you have some sort of A/D chip in the V(out) on the circuit diagram.
Thanks, the idea definitely looks practical. Adrian seems to have an idea on how to go about this one, suggesting a current sensing resistor in series and efforts to avoid using an op-amp. I'll leave it to his tender ministrations.

Vik :v)
I am not positive, but I think there was a circuit for this using an LM317 in one of the datasheets I read.
On the power wastage question - yes, the extruder does shed heat, and needs about one amp at 12v to run on a 50% duty cycle. We could certainly insulate it more and thereby use a lower wattage. But in one way it's rather nice to have it over-powered; that means that you can (if need be) melt the polymer at a faster rate than we do at the moment, so making the machine run faster in the future would not need a change in design.
I think I'm going to try that idea out with the piston extruder.
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