You have to laugh to keep from crying.

An interesting evening of RepRap activities. I got my 24v power supply in today, so I thought to just hook that up quickly, do a test and then get some other crap done. I hooked it up and set the print bed temp to 110C. WOW it shot up there in about 70 seconds instead of 20 minutes.
Then about 10 seconds later the circuit board started to hiss and smoke.

The MOSFET that drives the high current stuff is good for 60A at 55 volts, but it needs a heat sink to drive 360 watts. It had cooked itself (shorted) besides damaging the circuit board a bit. I only had P channel MOSFETs sitting around. So I stole one from the unused fan control channel next door, then cut down an old 386 heat sink and attached it with Arctic Silver heat sink compound (and an M3 screw). See photo. OK, it's probably a bit much but I was nervous. I think I may get some more reasonable, proper TO-220 heat sinks for it.

With that in place I resumed testing carefully and all was well. The heat sink only got marginally warm. All was good.

For 10 minutes. Then the glass cracked. Thin window glass does not like this kind of heat. The guys on the IRC channel say that 3.5mm glass from Lowes works great and costs < $4 so I'll get that tomorrow.

Documenting this all here because I want to put it all on the Wiki once I make all possible mistakes :)

  • Todd Johnson - Nice rescue!
  • John Ridley - A quick trip to ebay and I have a handful of MOSFETs and TO-220 heat sinks coming. < $10 for the lot, and what the heck, high current MOSFETs are just handy things to have.
  • Todd Johnson - I saw these at American Science a while ago. Glass sheets rated to 1400F! These sure aren't going to crack on you.
    You could probably cut it to size with a tile saw.
  • John Ridley - Yeah that's oven glass. And it's perfect, I'll have to get some and see if I can cut it. I will try just scoring as normal since it's not tempered it may work. If so it's a hell of a find, thanks. If not I wonder if an abrasive wheel on a dremel would do to score it then snap it.
    If I have to buy a tile saw to cut the glass then it's not such a good deal :)
  • John Ridley - Thanks, got some ordered. Even if the stuff I got today works (best I could find is "double strength" or twice the thickness I already had) I'll switch to this. Heck, I could probably cut them to the right size and sell them.
  • John Ridley - I finally found some info online that indicated the stuff can be cut. It's the same as pyroceram, just a different trademark. The post I read said it's incredibly tough stuff but it will yield to a standard carbide blade cutter. It even said it could be ground with a stone or sandpaper.
    I got four sheets anyway - if it proves workable I may try drilling holes.
  • Todd Johnson - I'll bet at least one of our Eastern Michigan friends has a little diamond tile saw.
    Or if you bought one from Harbor Freight for $65 you could make that back by selling high temp glass print beds. ;-)
  • Todd Johnson - On sale, even:
  • John Ridley - +Todd Johnson You just tryin' to spend my money for me ain't ya?
    OK, I'll try scoring and breaking, if that doesn't work I'll go pick one of those up. Then I'll need to come up with a tile project to justify it...
  • dermot dobson - Even with the new glass, is it worth while ramping the temperature up in a controlled manner? Single-sided heating is probably going to impose differential stresses that you could best do without.
  • John Ridley - It's possible but the new glass should be capable of withstanding anything this can put out. It's commonly used on wood stoves where there are flames on one side and cool air on the other. I have four sheets so we'll see how it does.
  • Todd Johnson - I suspect this material has a low enough expansion coefficient that your "40 seconds to 100C" heater can't hurt it.
    As you say, you have enough pieces to play with.