heat resistor damaged

Hi everyone,
I'm new on reprap community and just finished my reprap i2 after a few problems.

I resolved them, bought some pla and did my first print ( to calibrate, so the object was not pretty :p )

Anyway, after a few tries (without any problem, 200°C reached and maintained for minutes) I had a little problem with my heat resistor ( 6,8 ohm, 3w, I have two of them) that burned a little as shown on the picture. In fact, I saw it really incandescent on the last try, lot of light even by day. So I stopped it immediately and saw that even the kapton had burned a bit !! ( see picture)
Is 19v too much for one resistor? How could the resistor reach 400°C ( kapton melt temperature)??
I tried to look on the internet but I don't find any voltage info, just wattage . And I don't understand the meaning of wattage for a heat resistor, as we exceed it pretty easily, even with 12v : 12^2/6,8= 21W.
A last strange fact : I measure 9-10ohm on my two resistors ( the one of the picture and one unused) , but it indicates 5% tolerance, is it normal?

thx guys, sorry if there are dumb questions, I'm new, and sorry for my english I'm french

Usually they have 12 volts across them not 19 volts which is far too much. Using under-rated resistors as heaters for hotends seems to be happening a lot but they are not meant to be used that way!
I would suggest using one of these instead:
[www.mytechno3d.fr]

Edited 1 time(s). Last edit at 06/29/2014 01:09PM by Ralph.Hilton.

Hi thank you for your answer .
I understand better why I could not find any wattage indication if it is not how they are meant to be used ! And what you suggested could be used with 19V? ( I saw they sell one for 24v )
and it is not normal to have 19v for the hot end ( i use a ramps, which I supply with an old laptop power supply ( 19v, 3,6A))?
I saw other people doing this on the web.

thanks again for your help

Edited 1 time(s). Last edit at 06/29/2014 05:22PM by steph4394.

Youre right with the 21 watt calculation for 12 volts, and I can't explain why its OK to exceed the 3 w rating of the resistor, but at 19 volts, the wattage leaps to over 53 watts.

I have never seen anyone put 19 volts onto a hot end, and most are engineered for 12. If you want to go for 19 volt, then you should recalculate a new resistor value. Using the same 21 wattage figure you calculated before, this comes to about 17 ohms.

I have no idea if this is an appropriate value, and maybe some one here can comment. Proceed at your own risk.

I really appreciate that you gave me a french dealer, and I am gonna go with your solution of ceramic hotend ! ( the incandescent resistor made me want to try another way :p ) But I think you're right for the 17ohm resistor .
but could the 19v damage the ramps ? ( non speaking of the resistor)

thanks again

3W resistors are OK running at 20W as long as they have a good thermal connection to the heater block. The 3W rating is in free air. When you take the heat away with a heatsink it keeps the temperature down.

Don't use Kapton as it isn't a good conductor. I have used aluminium foil and that works fine but I prefer exhaust putty for permanent fix and also for the thermistor so there is no chance it will fall out.

Don't use 40W cartridge heaters as they are not safe under a single fault condition as they can get hot enough to start a fire. 20W is sufficient power and self limits to about 300C if the control fails.

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[www.hydraraptor.blogspot.com]

your fire thing is making me change my mind nophead :p
And you made me realize that I totally prevented the sink of the heat of the resistor with the kapton :s but still, the kapton tape melting really surprised me.
But so you say, as you encircled your resistor with aluminium foil, you managed to isolate the legs of the resistor?

For the 19v issue, think I'm gonna try using a old power supply in an old computer I have, a basic 500w power supply .

thanks guys

Kapton is only rated for 400C. A 3W resistor dissipating 53W will get much hotter than that. They get hot enough to melt aluminium if they don't burn out first.

I trim the foil back to be level with the end of the heater block, which is also the end of the resistor in my designs, so there is no possibility of a short.

---

[www.hydraraptor.blogspot.com]

Ok I am gonna try this with my last 3w resistor (no fail accepted :p), with alu foil and with a 12v power supply then.
again thanks a lot for your help