Friday 9 January 2009

Heater in a hurry hack

The heater in my last design has two layers of Cerastil that take 24 hours each to cure and the bobbin takes some time to machine. Attaching the wires and winding the coil is quite fiddly. Looking for a short cut I wondered if we could use power resistors. I had this one lying around to play with.

Unfortunately it is only rated for operation up to 200°C. In fact the datasheet says "It is essential that the maximum hot spot temperature of 220°C is not exceeded". Curious to see why, I put enough voltage across it to heat it up to 240°C. That turned out to be about 75W. It seemed quite happy at that temperature for several hours.

It is too big really for an extruder so I bought some smaller 10 Watt ones for £1.42 each.

These are only rated for 165°C but what the heck. I heated a 6.8Ω one to 300°C. At about 180°C it produces a little smoke but that soon goes. At 280°C it starts to smell bad, but at 240°C it seems happy. I left one powered up for a few days. The writing disappears and the connection tags oxidise, but its resistance is stable.

To make a heater I cut a 19mm x 19mm x 8mm block of aluminium from a bar, drilled a 5mm hole through it and tapped it to M6 to fit a heater barrel. The mounting holes of the resistor are big enough for M2.5 but there is not enough room for the head, or a nut. M2 is a bit weedy so instead I used 8BA bolts. These need a 2.8mm hole for tapping. A simpler solution would be to just file a flat on the head of an M2.5 bolt, drill a clearance hole and use a nut on the other side.

Here is the full assembly: -

I put heatsink compound on the M6 thread and under the resistor. I attached tinned copper wires with 300°C solder and insulated them with PTFE sleeving.

I have run the assembly for a couple of days and it held up. I am loath to recommend something which is unsound engineering, but it does seem a simple and robust solution as long it lasts a reasonable amount of time, say 1000 hours. Replacement is easy because the most time consuming thing is making the block which is reusable. I expect there might be some matching crimp connections to avoid the high temperature solder.

Quite a lot of heat is lost from the large surface area so some insulation would be a good idea.


  1. I would think that a better way to capture the heat would be to machine the block such that it would clamp around the power resistor. It would obviously take more machining time but a block with two slots parallel to the heater barrel with one on either side. Running the heaters lengthwise along the barrel would increase the length of the "gooey" section of the plastic inside the barrel.

    Needless to say, if it works like it is now then it is no matter.

    Good idea, as always.

  2. Great Idea I also have a few of those RS power resistors hanging about.
    Thinking about the old style green ceramic power resistors drilling a hole or two in the block gluing the resistors in with fire cement.

    I had just found "Bobbins Spools Motorbike" on ebay that looked just right for the previous idea.. (by accident) when cheeking 6mm thread pitch on Goggle. link:-

  3. Yes I had the same though about ceramic power resistors. Some of them have very high temperature ratings, 450C. I haven't found one just the right size and value yet.

    BTW, the bobbins you linked don't have the standard thread pitch for M6, it is normally 1mm for course and 0.75 for fine, but I am sure you have found that already.

  4. Remembering using 2mm nut & bolt to join a heavy gauge wire with a crimp connector on end when using those power resistors on a project in the 70/80s.
    Maybe a small self tap screws could be used to attach a the spade of the crimp connector. (Thou not as neat).

  5. Yes I did ;-( its printed in the ISO table on the back of my electronic callipers. Was calculating parameters for the Arduino & Sanguino firmware.

    Been a bit busy with family & preparing for redundancy in March to have blogged much I now have two Repstraps minus a fully complete extruder.
    I have a peek & 35mm heater barrel(untested)
    Not ordered ABS yet been looking for ABS in the UK as I resent paying $50 carriage.I'm very tempted to try the L bracket drive from Demented but like the worm gear method from AK47 wont get my Lathe till march though. So brackets and bearings is next job.

  6. Do be careful - I had a 50W resistor in use as a heater for another project, and pushed 90W into it for, oh, 20 minutes.

    It exploded rather violently. One of the connection tags flew out at a high enough velocity to gouge a 1mm-deep scratch in aluminium, and there was a spray of 200+ degree powder (and some flames) coating my desk.

    Good to hear that yours are stable, though.

  7. Wow that is a bit worrying! Did you exceed the voltage rating? I could imagine it exploding if it was across a high energy supply and it went short circuit causing a massive current to vaporise the metal.

    Other than that I can't think why it would explode unless it is a gas tight seal and there is some gas inside.

    Perhaps I should test one to destruction to see how it fails (and keep my distance).

    It might be why the data sheet says "It is essential".

  8. Wow

    Most excellent Nophead Sir.

    It's always the obvious things that pass us by.

    The ceramics sound like a good way to go. 400 degrees is plenty enough.