Nichrome Wire red hot

Ok so I hooked up my Nichrome wire as the articles suggest and when I power the board, the nichrome wire starts glowing red/orange hot almost immediately (in a previous thread someone told me it is not supposed to do this). I have the leads hooked up to the +-C terminals on the extruder, which I believe is correct. Do I need to jump the pins for the thermistor? The thermistor is not hooked up yet. Any ideas? Thanks.

Edit1: Ok so after some more searching and thinking about the issue I thought of a few more things. Could it be that the wire isn't long enough? I am using 6ohm Nich wire at about 3.5in piece. Do I need a longer piece?

Currently I just have the wire suspended in air and the leads taped to my build bed so that I can test with out burning anything. Do I need to have the wire wrapped around the barrel? Will this draw off enough heat to make it not glow?

-Steve

Edited 1 time(s). Last edit at 06/12/2010 09:14PM by steveo290.

Its been a while since I built my extruder so my memory of how long the wire was is fuzzy. 3.5 inches sounds a little short though. I'm thinking mine was closer to 6. In reality though, it is the resistance and not the length that matters. If you are absolutely sure that your ohm meter is giving you a correct reading, then 3.5 inches may be what it takes.

Not having the wire wrapped on anything might be causing lower heat dissipation, which would make it glow faster, but you should definitely check your resistance. Touch the meter's leads together to see if your probe connections are good. My crappy $3 meter normally reads around 0.8 ohms even on a 0 ohm circuit. If the switch isn't positioned right and the battery is weak, I can make it jump to 1.5 ohms of error.

Ok update. I just checked the source where I bought the nichrome (http://store.makerbot.com/cables-and-wires/nichrome-wire-30-ga.html) it doesn't say 6ohm, I thought it did. So I may have been mistaken from the beginning. I have never had to test resistance before but I tried to tonight. I was getting a lot of different results with the piece that turns red hot. Not sure if I was doing it right. I had my MM set to 200ohms and touched each lead, would get anywhere between 2.0 and 30.0 for the initial reading before it faded down.

I decided to try a longer piece and read that. I cut a 7 in piece and wired it up, initial reads were about 12.0 before fading. I turned the extruder on and the wire just smoked heavily for about 5 seconds before I turned it off.

Is this closer to what I want the wire to do? Am I damaging the wire by heating it up? I figured I could just use a longer piece and then cut it shorter if there was too much resistance, is that incorrect? I am somewhat new to electricity measurement. I usually just test things for continuity and dc voltage. Thanks for the help so far dazed.

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The thermistor is not hooked up yet.

Can we assume you didn't wind the wire onto the barrel as well? I'm wondering why you're surprised the wire starts to glow in this case.

The heating wire's power is sufficient for melting enough ABS at the desired feed rate. If there is no ABS to melt or not even a barrel, the wire heats up much higher. To avoid this overheating, the heater's power supply "asks" the thermistor for the current temperature and cuts the power off as soon as the desired temperature is reached.

No thermistor means no temperature information, which equals "extremly cold" for the electronics circuit. Accordingly, the electronics ramps power up to the maximum and never stops. A glowing wire is the logical result.

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Am I damaging the wire by heating it up?

The wire is fine as long as the resistance roughly matches. A glown wire usually has a higher resistance, so you're on the safe side. What's likely damaged, though, is the possibly existing insulation of the wire.

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I figured I could just use a longer piece and then cut it shorter if there was too much resistance, is that incorrect?

This is correct. A longer wire means less Amperes, which in turn means less heating power. Not enough heating power means a slower rate of melting ABS, which in turn means a lower possible feed rate. To get started, you're on the safe side with a long wire.

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The cold resistance is the important measure. You want the nichrome to measure close to 6 ohms when cold. With the fiberglass insulated nichrome I got from MakerBot Industries the length was around 30cm.

If the nichrome is very thin (has a higher resistance per unit length) the 6 ohm length may be too short: the power may be concentrated enough to cause the wire to overheat and glow. You can compensate by using two 12 ohm or three 18 ohm lengths in parallel, The power draw will be the same but will be spread over a larger area.

In my case i tried using 2 pieces @12ohms for more surface area... this is the problem i ended up having once i did that...

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Traum, thanks for the information. I was concerned that the wire was heating up because I didnt have the host software on, which to me would mean that the wire should stay off until the software told the board to heat it. But if that is the case then that is fine. This whole process and theory now makes a ton more sense now that I have experimented and read your guys' advise.

As far as damaging the wire goes. My first wire at 3.5 inches definitely didn't have any insulation left on it after I started it up a few times.

Why does the board even power the nicrome when there is no thermistor?
It should give some kind of "thermistor error" and shut down to a safe condition
at once.
(I guess I should look at the sourcecode this week to check the error-handling
and re-assess if I really want to build my Mini-Mendel using these electronics
and not some other.)

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Ok update and ideas requested.

I got my extruder board troubles sorted out and found that the board was borked so that is why it was heating the wire as soon as power was turned on. I have a replacement with the stable version of the firmware installed. The wire only heats when the host software tells it to. I wired up my extruder with the nichrome and thermistor. Hooked it up and started it up. Then tried to heat up the barrel. The wire started to smoke a little and the insulation started to unwrap on the exposed ends of the nichrome wire(near where it is attached to the leads). After about 20-25 seconds the exposed portion of the nichrome wire became red hot and one end broke off. I had a 7 inch piece which did not turn red hot the last time I tested a 7 inch piece.

Qeustions, if the nichrome wire is shorted (wires touching during the wrapping portion) will that create a bridge and bypass the rest of the wire or will the whole wire be heated regardless of shorts or overlapping? Thanks guys I am almost there, I can see the finish line.

... shortings/overlaps will generate bypasses and reduce the resistance of the wire, so more current will burn the passed wire while the overlapped segment stay cold

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Yes VDX that is exactly what I thought. I tried again with a fresh piece and didn't let the insulated wire touch anything on the exposed ends. I now have a functioning extruder! I am off the configure and try to print something.