Saturday, 17 January 2009

Yet another quick heater hack

The ideal off the shelf heater would be a cartridge heater but they tend to be at least 1" long, need mains voltage and are very expensive. Here is a cheap 12V alternative: -

It is a vitreous enamel wire wound resistor that can handle surface temperatures up to 450°C. It is a 6.8Ω RWM 6 x 22 rated at 10W, but I am overloading it somewhat to get 240°C.

I bought a pack of five from RS. Farnell and Newark also stock them I believe.

I drilled a hole to accept it in a 19 x 19 x 8mm block of aluminium with an M6 tapped hole for the heater barrel and a small hole for a thermistor.

The tapped hole is at right angles so that the hot zone is as short as possible. It could be made parallel to get more contact area.

The outside diameter of the resistor measured 6.3mm so I drilled a 1/4" hole for it. That was too tight so I drilled it out to 6.5mm. I then wrapped aluminium kitchen foil around the resistor to make it a tight fit and rammed it in.

Here it is under test with a random bit of tube to simulate a heater barrel.

It needs about 11W (8.7V) to get to 240°C. 14.7W (10V) gives 300 °C. I haven't run it for very long so no guarantees it will last, but I can't see why not.

Compared to the aluminium clad resistors I tried before, these are cheaper and you get a more compact heater with a smaller surface area to lose heat from. Also making connections should be no problem with normal solder because the wires are long enough to cool down.


  1. Thats the type I meant by old green ceramic resistors. I have seen them glowing red.
    If they can get to red heat then 300C should not be too much of a problem. Not very scientific just an observation. They were used in many old Valve TV's and amplifiers power stages.

  2. Yes the old ones I remember were a bit more bulbous at the ends and often hollow. These more modern ones are a better shape for doing this with.

  3. yes! what a great hack.

    of course you post this the day after i order $20 worth of ceramic resistors, lol.

  4. Yes I have a pile of ALCLAD resistors left from my previous idea. At least I can use those for a bed heater I have planned.

  5. Hey, I wonder if you could make a big nozzle with holes to insert resistors (heaters) into the same block? That way we'd be heating the nozzle directly. Not sure how that would work; can you mount a block offset in your lathe and machine a nozzle tip offset to one side, with the resistor cavity on the other?

  6. Yes I could do that with my four jaw chuck but it is easier and wastes less material to just make a separate AL barrel that screws in. That could also be the nozzle. I put heatsink compound in the thread so am assuming I don't lose much heat across the join. I may be wrong.

    Also, I am trying to make a design that can be made with just a drill press, some taps and dies and a 10mm end mill bit.

    If I was designing it to be made on a lathe I would use Cerastil and nichrome to make a much more compact combined heater bobbin and barrel / flange. The fanless design's heater is a lot more thermally efficient and responsive than this one. Wrapping the block in insulation will up the efficiency but it will still have quite a high thermal mass. I don't think the matters too much but it increases the warm up time.

  7. higher thermal mass may not be a bad thing. Don't know what the heat of fusion is for the plastics we're using, but the melting process must take in energy. High thermal mass would help to smooth out the temperature variations in the heater block and would also compensate against overshooting the target temperature.
    Better control of the target temp would help minimise thermal decomposition of the plastic, which can lea to brittle jobs.

  8. Hi Marauder,
    I measured and calculated the heat to melt HDPE and it was about 2.8W. This heater has plenty of spare capacity to cope with that.

    My current heater swings +-1C about the set point at about 0.5Hz so I don't think I have a problem with temperature control.

  9. Nop,

    Another great idea. I had no idea that resistors were available that worked to 450 deg C.

    You wrote:
    > ... make a separate AL barrel that screws in.

    Al-to-Al threads can gall (rub material off on another surface) and become very hard to take apart without damage. That's why I used brass for my nozzle inserts.

    If you get Al-Al threads to dis-assemble OK, please post what alloy(s?) of Al you're using.

    -- Larry

  10. I have not had any problem with M6x1 threads in AL but I have only started using them recently. I don't know what the alloy is because I buy it as offcuts on eBay and reuse things I have dismantled.

    I generally use thermal paste or PTFE tape if it needs sealing so I expect they help to lubricate it.

    Greenhouse accessories use AL nuts and bolts with a fairly course pitch and they can be undone, even after being exposed to the elements. At least I assume it is AL, could be zinc I suppose or a special alloy.

  11. You might want to try some copper slip.

    It is used on car brake components and can comp with the high temperatures encountered in brake systems.

  12. Hmm... The specs on that RS page says they are only rated to 200 degrees C, am I missing something?

  13. The manufacturer's datasheet that I linked to says they can handle surface temperatures up to 450C.

  14. I would like to make an extruder this way.

    RS now offer a 14W Vitreous Wirewound Resistor (RS order no 666-1588)

    Would this be more suitable? or is the idea to give it more power than it can handle and thus overheats?

  15. I would like to make an extruder heater this way

    RS now sell an 14W Vitreous Wirewound Resistor (RS order no 666-1588)

    Is this more suitable? or is the idea to push more power through the resistor than it can handle and thus overheats

  16. Yes you have to overdrive the resistor Watts wise to get it hot enough but it is stil within its surface temperature rating. I have actually used a smaller resistor on my latest extruder and it seems to hold up well so far.

  17. Is it fair to say then, that your main design constraint when choosing an appropriate Vitreous Wirewound Resistor was the dimensions of the resistor? Was there any reason for choosing 6.8 Ohm rather than 1 ohm or 10 ohms?

    For my extruder, I just want to use a 32mm brass M6 barrel, as is used in the Reprap ThermoplastExtruder 2.0, I would like to mill the tip directly into the barrel, I then intend to add an M6 through tapped aluminium block on the end to house the vitreous resistor, and thermistor. Have you any recommendations as to which resistor I should use, or with which would be interesting to experiment with? what size resistor did you use in your latest extruder?

  18. The resistance is chosen to give an appropriate power when connected to 12V. Power is (V^2)/R. 6.8R gives about 20W which has been about right for all my extruders. The duty cycle is about 80% to get 240C.

    The one in the article would be a safe bet. I used a smaller one to reduce the surface area and mass of the heater block, but I haven't run it for more than a about 15 hours yet.

    The smaller one is an RWM 4 x 10 which needs a 5.5mm hole and is about 12mm long. The enamel is very thin so take care to to scrap it off if the hole is too tight.

  19. have you seen this?

    talks about wire-wound power resistor technologies vs thick filme, and etc.

    one point they made was about pulse and high-frequency conditions. They mention that it could act as a low-q inductor and may interfere with sensitive components.

    did you find any of this (noise) to be a problem? or because it's embedded in a hunk o metal maybe it doesn't end up mattering?

  20. Wire wound resistors have some inductance because they are a coil, but it doesn't have a significant effect when used as a heater because the switching frequency is relatively low and the waveform is unimportant.

    At worse it may cause a bit of ringing on the switching waveform, but nothing compared to the noise you get from switching stepper motors for example, which are many orders of magnitude more inductive.

    I can be a problem in audio circuits though and wire would resistors are not recommended for sense resistors in chopper drives.

  21. makes sense, thanks for the response.

  22. I made one and posted it on thingiverse here:

    If you want to make a thing for this, I'll take mine down and post my pics as a copy, or I can just change the license to one of your choosing.

  23. I was looking through resistors on digikey to make another heater, and I found some tubular resistors that look interesting. Here's an example:

    If we could find one of these with the right dimensions and resistance, one of these could turn out to be the ideal off-the-shelf heater, just stick your barrel through it. I'm a little out of my depth with this, though, any thoughts?

  24. They are too long really, the shortest one is 44mm. With a hot zone that long I think you would get more ooze.

    Also the hole is too small for a 6mm barrel and has a wide tolerance, so it would be hard to make good thermal contact. I think you would have to glue it on with a high temperature thermally conductive adhesive.

  25. are these any good ?

  26. "Maximum working temperature must not exceed 125 °C." so probably not.

  27. newbie electronics question....

    I am trying to apply this outside of reprap to heat something -- i've never built a reprap. what is the circuit like to power this resistor?

    the reason i ask is that i hooked one of these up to a variable dc power supply, with just the resistor between the leads, and it draws no current. therefore, it doesn't get hot. why is this? do you have to hook your heater up in series with one of the stepper motors or something in order for the circuit to do anything? perhaps there is some other problem, and my question doesn't make sense. should just a plain old resistor hooked up to a power supply draw current? let me know if i need to clarify.

  28. Yes a resistor connected across a power source will draw a current I = V/R and dissipate power P = V^2/R. If it doesn't get hot it either not connected, or the value of the resistor is too high for the voltage.

  29. i accidentally bought 6.8 k ohm instead of 6.8


  30. How did those resistors hold up? Or did you just hack a new system to do the next cool thing?

  31. I moved on to a smaller resistor and I have used those almost continuously for more than a year with no failures.