Resistor heating up quickly then blowing out

Hello,

My Huxley is having in issue that is new. The resistor was just fine until today I tried to print and it wouldn't get hot. So I figured that one had hit the end of its life span. So I put in a new one, turned it on, and it got to 200 degrees C in about 10-15 seconds and was smoking and then it the resistor blew out a couple of seconds later and it got cold. I repeated the process with the same result. Is my Sanguinololu bad?

Thanks,
Harry

It can only get hot too fast if the voltage is too high or the resistance is too low. Are you sure the resistance is correct?
P(ower) = V(oltage) * I (current) and I (current) = V(oltage)/R(esistance).

If the heating does not stop when the temperature is reached it could be because of damage to the isolation of the thermocoupler, so that there is a shortcircuit.

After my first month of printing, I kept having resistors blow on my RepRapPro Huxley. Vik Oliver suggested that I try making my own resistor out of nichrome wire, imbedded in furnace cement. This is what he uses for his hot ends, but he hadn't tried using the rectangular block on the Huxley.

What I did was paint furnace cement on the blockwhere I was going to wrap the wire. I had to be careful to keep away from the thermistor. Once I had a dry layer completely covering the outside rim of the block, I wrapped some nichrome wire a couple of times around the block, making sure the wires didn't touch. Holding this in place, I painted another layer of furnace cement and dried with a hairdrier. Then kept repeating this until I had wrapped about 500mm of wire around the block and then gave it a few more layers of furnace cement.

This has worked well for me for about 5 months so far.

Hello,
My Huxley/reprappro has a 6.8R resistor.
I broke the original part , ordered two replacement resistors from reprap.fr ....very short lived .
heses replacement were metoal oxide resistors.
Now I use a resistor from Mouser: 71-CW02B6R800JE12 6R8 3,75W 250 degres max
The hole in the aluminium bloc is slightly larger tha the resistor. I packed the resistor with some turns of kitchen aluminium foil, wetted with thermal grease.
....this is strong enough

Robert

PS there is a better part 284-ACS5SW-6.8 5W 350 degrés !

Hello,-
My printer has been working without any problems for a while, but suddenly the heater didn't work. I bought some new resistors, since the old resistor was dead. When using the new resistors (Resistance 6.8ohms 3W) and setting the temperature to 200 degrees C. the resistor gets hot really fast and then it dies. The voltage to the resistor is always 19.56 V, when I set the temperature in pronterface, its like the temperature is never controlled.

Do anyone know the reason to the problem- could it be the MOSFET?

Thanks,

Nimal

Edited 3 time(s). Last edit at 04/05/2013 03:28PM by Nimal.

Hi,

I have the same issue as Nimal, and I suspect that it is the MOSFET that has been damaged.

I have started another thread (see link), but maybe it could be merged into here instead.

[forums.reprap.org]

- Lars

If it is the MOSFET, it is likely that mine has been damaged too.
My nichrome wire heater is still working fine though. It will happily do a 6 hour print.

Quote

could it be the MOSFET?

You can measure the voltage the MOSFET is driven with. There are 3 pins: one is connected directly to the voltage source, one goes to the heater and the third one goes to the ATmega pin. Measure this ATmega pin connection. It should be 5V while heating up and close to 0V after reaching target temperature. Testing with a target temperature of 50 °C might be more convenient for these measurements.

If the voltage on the ATmega goes down, but the one on the heater doesn't, the MOSFET might be indeed damaged. If these 5V do not go down, the MOSFET is innocent.

---

Generation 7 Electronics	Teacup Firmware	RepRap DIY

Inquisitor Wrote:
-------------------------------------------------------
> My nichrome wire heater is still working fine
> though. It will happily do a 6 hour print.

My temporary nichrome heater was wire and capton tape wrapped around the block, and was only intended to work once as I'd blown my last resistor. I really like your way of constructing a nichrome heater, I will raid the hardware store at the weekend and give it a go.

Back on topic, I will get the multimeter out tonight and check the MOSFET on mine. My other thought was getting my scope on the ATMega pin and see if I could see the PWM signal.

EDIT:

I just checked the Melzi schematic, and it uses RFP30N06LE MOSFETs. These are rated up to 30A, so it could be plausible that a shorted out heater resistor (It I remember correctly, my original blown resistor tested as shorted with a multimeter).

Now I have the datasheet and schematic, I can check it out this evening.

RFP30N06LE Datasheet: [www.sparkfun.com]

Edited 1 time(s). Last edit at 04/23/2013 10:34AM by LarsIslander.

OK, here's what I've found on my Huxley:

• The hot end (resistor disconnected so it does not burn out) has 19.3v across it (measured on the screw terminal) as soon as the printer power supply is turned on.



• With the power off, there is continuity between the source and the drain on the hotend MOSFET.



• With the power off, I confirmed that the top of the SD card housing has continuity to ground, so I am using this as my easy-to-reach ground.



• I then confirmed which pin I was checking on the ATMega644 on the Melzi board diagram [reprap.org].



• I then connected the board via USB to my laptop, opened up pronterface, and tested to see if the pin was going high/low as expected. During this stage mains power was not connected, and the USB power was set on PWR_SEL jumper.



• Once pronterface is connected to the printer, the hotend reads 16C
  
  • Hotend is set to OFF. 3mV between ground and the hotend pin.
  
  
  
  • Hotend is set to 100C. 3.58v read between ground and hotend pin.
  
  
  
  • Either way there is still continuity between the MOSFET source and drain.
  
  

That's all I have time for this morning. I am concluding that the MOSFET has been fused 'open' when the original resistor shorted.

I'd appreciated a second opinion, as I'm a software engineer, and only blag electronics in an emergency

- Lars

Hi all, just a quick update on my progress with fixing the hot-end.

While I'm ordering some replacement MOSFETs, I am currently trying to modify my Marlin firmware to use the FAN output (pin 4) instead of the HOT END output (pin 13). I'm not having much luck.

The pin is only defined once, and I've swapped the two values, compiled and then uploaded to the board. Monitoring the voltage on these pins shows that when turning the hot-end on and off, it is still using Pin13.

TBH I'm a bit stumped.

EDIT: I've sorted it out. Details are available on my blog here: [endoftheworldasweknowit.co.uk]

I am now re-building my hot end and will test.

- Lars

Edited 2 time(s). Last edit at 04/28/2013 08:04AM by LarsIslander.

Just a quick update to let you know that the above worked ok!