Nichrome hotend question

Hi, I'm building my first reprap/repstrap and I'm wondering how much nichrome to use for the hotend.

I bought some 30ga and I have read that 6 ohms is proper, but when I test that with my 12v supply, it gets too hot and melts the kapton. (koptan, lol)

At that point, it is getting red hot, so I'm wondering if the supply on the electronics board will limit current, to keep the heat down.

I did also try 60cm of nichrome, and that just barely melted the kapton at 12v rather than gassing it.

Thanks,
Legot

Hi,

Just to make sure, when you say "I test that with my 12v supply" you do mean via a board such as RAMPS don't you? Not directly on the 12v supply. Sorry if it's an obvious question only, so far i've never had the nichrome getting so hot it melts Kapton tape!

(Edit) How are you limiting the temperature in your tests? A thermistor or thermocouple acts as feedback to control the power sent to the wire, hence controlling the temperature of the wire.

Cheers,
Gary

Edited 1 time(s). Last edit at 09/13/2011 10:32AM by garyhodgson.

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garyhodgson.com/reprap | reprap.development-tracker.info | thingtracker.net

The wire has to have good thermal conduction to the barrel to take the heat away, otherwise it will get red hot.

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[www.hydraraptor.blogspot.com]

My first hot end was made with nichrome wire (6-7 ohm) around an M6 threaded nozzle. Since there's not much mass to it, it used to get hot enough to melt kapton if connected to 12V continuously. Aside from nophead's question on whether you wound the wire tighly around the nozzle, here are other possible reasons why your tip is getting too hot.

1. You didn't mount the thermistor properly so it's reading lower than the correct temperature
2. You didn't connect the nichrome wire to the correct terminals
3. You didn't connect the thermistor correctly so your electronics is always reading a very low temperature. This would keep the heater on constantly.
4. You have a firmware problem causing the voltage to the heater to be on all the time.
5. You have an electronics problem. Not sure if power MOSFETS end up shorted when it fails.

Ok, so that is my problem, I am running from a 12v supply, not ramps or anything (I am still in the build stage of my Gen 7)

Thank you! Looking back, I see how not regulating the temprature is unwise, live and learn.


And what thermistor would you suggest?
100k with what beta?

Thanks again,
Legot

Just about any type will do as long as it's rated to go to 300C. Don't use ones with axial leads since thicker gauge leads also conduct heat which can affect your temperature reading. If you know what firmware you're using, you can save some trouble editing the thermistor table if you pick the same one that's specified in the firmware.

Edited 1 time(s). Last edit at 09/13/2011 07:28PM by brnrd.

Well I'm going to be using Teacup firmware, but I didn't see a specified thermistor anywhere.

Thanks again.

The file ThermistorTable.gen7.h in the latest version in github states that it uses B57560G104F for the extruder and bed in the comments. But the tables don't exactly match since the person who generated it used the actual measured value for r2 which were slightly different.

Ok, thank you


Vive la réplication,
Legot

BotMill is a joke!!!
I ordered an Axis 2 Plus back on June 23rd 2011. At this time I am still waiting for parts (Sept 19th, 2011). I have a file of 120 pages of emails in PDF format which is attached. I have been lied too and their promises are not honored. I had to assemble most of the parts. The parts that were assembled had rusted bolts on them. I ordered parts from Ultimachine to finish the printer. I was promised 10 hours of voice support, but never could get a phone number to call. A rumor is out that they are selling out to 3D Systems. In August I received the heated bed which was made backwards. The power supply came with a foreign plug which I had to rewire. When I ordered the printer from their website two extruders were included, which later was changed to one. One email promised me the two extruders. I also ordered the printer with yellow parts, but I got some red and some blue. There was no packing list in any shipment.
Parts still not shipped:
Assembled 1.75 mm extruder with motor (finish assembling with motor from Ultimachine)
Assembled 3.00 mm extruder with motor
Correct assembled heated bed (made my own)

I would love to know why 3D systems chose Botmill from everyone else out there.

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Jeff Christiana