(Solved) resistors burning out - use vitreous enamel coated resistors!

Hello,

I've got my PRUSA working quite well however my extruder heater resistors keep burning out after a few uses (5 so far!)
I'm using Sprinter on Sanguinololu with Pronterface and RepG.
Is it possible I haven't set up Sprinter correctly, is there a way to reduce the power to the heater extruder?

The resistors work well with other people.
There is good contact with the heater block with exhaust cement.
I am using the correct thermistor chart.

Please help!

Mike

Edited 1 time(s). Last edit at 11/17/2011 10:26AM by michaelc.

Hi Mike,

... you have to reduce the current or enhance the thermal connection to the heater body.

Exhaust cement is a poor thermal conductor - you should have really good contact to the aluminium for 'cooling' the resistors slight below working temp or they will overheat.

Can you measure the real temperature of your heating block?

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Viktor
--------
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Hi Viktor,

I'll be able to measure the temperature once I get some more resistors.. I lost another one this morning. I think the thermistor values are accurate however and the temperature is fairly stable after heating up. When heating it rises to 270 before stabilising. It also only takes around 20 seconds to heat up which seems too fast.

Can I reduce the current within Sprinter? I noticed the PID settings but i'm unsure what to modify.
I will try fire cement later, I had tried thermally conductive adhesive however this had the same result.

thanks,

Mike

//// PID settings:
// Uncomment the following line to enable PID support. This is untested and could be disastrous. Be careful.
//#define PIDTEMP
#ifdef PIDTEMP
#define PID_MAX 255 // limits current to nozzle
#define PID_INTEGRAL_DRIVE_MAX 220
#define PID_PGAIN 180 //100 is 1.0
#define PID_IGAIN 2 //100 is 1.0
#define PID_DGAIN 100 //100 is 1.0
#endif

What resistor are you using in particular? (value and manufacturer)

Take a picture of your extruder, maybe there is something obvious causing the failure.

I can say with complete certainty it is not a firmware problem, as long as you extrude abs/pla that isn't a horrible smoking mess.










-------------------------------------------------------
> Hello,
>
> I've got my PRUSA working quite well however my
> extruder heater resistors keep burning out after a
> few uses (5 so far!)
> I'm using Sprinter on Sanguinololu with
> Pronterface and RepG.
> Is it possible I haven't set up Sprinter
> correctly, is there a way to reduce the power to
> the heater extruder?
>
> The resistors work well with other people.
> There is good contact with the heater block with
> exhaust cement.
> I am using the correct thermistor chart.
>
> Please help!
>
> Mike

---

www.Fablicator.com

It should take about 120 seconds to heat up. If it only takes 20s the thermistor will not have time to react and it will be hard to control.

---

[www.hydraraptor.blogspot.com]

michaelc Wrote:
-------------------------------------------------------
> Hi Viktor,
>
> I'll be able to measure the temperature once I get
> some more resistors.. I lost another one this
> morning. I think the thermistor values are
> accurate however and the temperature is fairly
> stable after heating up. When heating it rises to
> 270 before stabilising. It also only takes around
> 20 seconds to heat up which seems too fast.
>
Like nophead said, it's too fast. How many Volt do you use? More then 12V, then you get much more heat and need to put power down with pwm.
> Can I reduce the current within Sprinter? I
> noticed the PID settings but i'm unsure what to
> modify.
> I will try fire cement later, I had tried
> thermally conductive adhesive however this had the
> same result.
>
> thanks,
>
> Mike
>
> //// PID settings:
> // Uncomment the following line to enable PID
> support. This is untested and could be disastrous.
> Be careful.
> //#define PIDTEMP
> #ifdef PIDTEMP
> #define PID_MAX 255 // limits current to nozzle

PID_MAX = 255 means it can go to full power. Start with 50 in your case and watch timings and end temperature. This should be a hard limit to the pwm output, as far as I understand it.

> #define PID_INTEGRAL_DRIVE_MAX 220
> #define PID_PGAIN 180 //100 is 1.0
> #define PID_IGAIN 2 //100 is 1.0
> #define PID_DGAIN 100 //100 is 1.0

If you need a lower PID_MAX you may need a lower INTEGRAL_DRIVE_MAX and PGAIN and IGAIN to adjust to your lower limit. Leave that for later, only INTEGRAL_DRIVE_MAX > PID_MAX makes no sense, adjust that too.


> #endif

Edited 1 time(s). Last edit at 10/22/2011 01:57PM by repetier.

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I use bang-bang PID with a similar warm up time, and other than a 25C initial overshoot, manage perfectly adequate temp control.

Again, firmware is probably not the cause of the resistors burning out. My bet is he is using a different manufacturers resistor which can't take the high temp.



nophead Wrote:
-------------------------------------------------------
> It should take about 120 seconds to heat up. If
> it only takes 20s the thermistor will not have
> time to react and it will be hard to control.

---

www.Fablicator.com

Hi, thanks for the replies. I've tried a few things over the last few days.

- I started to think it was my fault using the exhaust putty, so I've tried fire cement and aluminium foil to fill the gap between heater block and resistor, still the resistors have blow out.
- This is the resistor i'm using, i've heard a other people have used it successfully.
- I am using a 12V pc power supply. I'm going to try changing the PID settings when I get some new resistors.

Mike

edit: this resistor

Edited 1 time(s). Last edit at 10/27/2011 01:17PM by michaelc.

You should be using vitreous enamel resistors, not silicone. They are only rated for 250C, whereas enamel resistors are rated for 350C.

---

[www.hydraraptor.blogspot.com]

Specifically, this one:

[search.digikey.com]

Or the 4.7ohm if you need more heat.

---

www.Fablicator.com

using 4.7 instead of 6.8 is fantastic, I think nothing of warm-up times with a 4.7

---

-----------------------------------------------
Wooden Mendel
Teacup Firmware

I chose 6.8R because it maxes out below 300C on my extruders, so is inherently safe.

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[www.hydraraptor.blogspot.com]

Hi everyone,

thanks for your help. In the end it was the resistors causing the problem - since moving to the vitreous enamel type the issue has been resolved and temperatures have become more stable.

They are still recommended on the Geared extruder nozzle wiki page. [reprap.org]

thanks, Mike

#edit: admin please move this somewhere

Edited 1 time(s). Last edit at 11/17/2011 10:35AM by michaelc.