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Adjusting heater current

Posted by jbayless 
Adjusting heater current
February 23, 2010 09:54PM
Hello all, quick question.

Is it possible to adjust the current to the nichrome heater in software? Or can it only be done by changing the nichrome resistance.

I've noticed that our heater briefly pulses red hot, cools quickly, and then pulses again a few seconds later. I think it would be preferable to have longer, less intense pulses to avoid burning out the wire insulation.

Re: Adjusting heater current
February 24, 2010 03:29AM
if you used two lengths of nichrome, each twice as long as usual and connected in parallel, you would spread the same thermal energy over 4x the wire length, reducing instantaneous peaks while maintaining heating ability.

You may not need to do this though, perhaps use a longer piece of nichrome so the thermal peaks don't reach red hot zone?

creating better thermal contact between nichrome and barrel would help, but could be problematic to implement.

Better thermal contact between nichrome and thermistor would also help, but you may end up with a less accurate reading of barrel temperature.

Wooden Mendel
Teacup Firmware
Re: Adjusting heater current
February 24, 2010 04:34AM
Yes if your nichrome gets red hot it is not going to last. None of my nichrome heaters got that hot, but I have always embedded it in some form of cement. I never left any exposed.

You might want to consider using a vitreous enamel resistor wrapped in AL foil, pushed into an AL block with a tapped hole for the barrel: [hydraraptor.blogspot.com].

I have found this method totally reliable, even with smaller resistors: [hydraraptor.blogspot.com]. I have made a complete set of Mendel parts using the above extruder, with no breakdowns, and the heated bed used the same resistor technique: [hydraraptor.blogspot.com].

It is a lot less fiddly to make as well, but you do need basic metal work tools. I.e. hacksaw, file, drill press and M6 tap. The resistors, AL bar and AL foil are arguably easier to obtain than insulated Nichrome and Kapton. Another advantage is that you can get away with bang-bang control as the temperature changes slow enough for the thermistor to react. The only disadvantages are that it takes about 100s to warm up instead of ~12s and is a harder shape to insulate. You can get away without insulation though unless you are using a fan.

Edited 1 time(s). Last edit at 02/24/2010 04:41AM by nophead.

Re: Adjusting heater current
February 24, 2010 10:12AM
In the firmware, in extruder.cpp, there is a variable called high_heat, which gets set to 255, I believe. If you decrease this number you will decrease the maximum PWM duty cycle, which will slow down the heater and give you the desired results. When I had a 4 Ohm heater I had to turn this down to 200. I also had to reduce the low_heat variable because it would always heat to 180 C when I turned the heater on to a temperature lower than that.

I believe that PID control would give you the same effect, except at startup.

Darwin clone, Gen 2 electronics, Arduino Duemilanove w/ AtMega328, 5D Firmware, Pinchwheel extruder
Re: Adjusting heater current
March 08, 2010 02:52PM
how much current does the nichrome wire draw? I'm battling to find the 14A nif5003 mosfets, but can get ones with less current capability (7.2A) would these be sufficient?
Re: Adjusting heater current
March 08, 2010 10:00PM
2A for a 6 Ohm heater running at 12 V.

Darwin clone, Gen 2 electronics, Arduino Duemilanove w/ AtMega328, 5D Firmware, Pinchwheel extruder
Re: Adjusting heater current
March 09, 2010 02:48AM
thanks for the reply mccoyn smiling smiley
Re: Adjusting heater current
March 17, 2010 09:28AM
I have started to experiment with my nichrome wire and the driver I will use. I see that apparently 2A should do. My question is, what is the temperature that the SW regulates to? Or what temperature do I need to melt the plastic? Has anybody measured this with a thermo couple? Thanks for your input!
Re: Adjusting heater current
March 17, 2010 11:11AM
About 220C to extrude ABS and make it stick to itself. About 200C for PLA. It varies a bit with the type of plastic and where the thermistor is mounted.

Yes I always calibrate my thermistor using a thermocouple. Some people use a thermocouple instead of a thermistor.

Re: Adjusting heater current
March 25, 2010 11:35AM
220C or 220F? 220C sounds a little bit high, don't you think?

But if it is 220C, then it will have to do. I am using a 6 ohm nichrome wire and I can get it to 220C, however, the thermal jacket I am using is rated at 200C. I wonder how dangerous would it be to extend the usage to 220C?

What other thermal jackets can the group recommend? I am using a zeusinc PTFE sleeve that I bought out of McMaster Carr. I have seen the fire cement technique but that item is nowhere to be found here in Dallas TX. Unless, I am wrong...
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