Blown bed heater switch circuit - would like to fix it?

Hi ( Have emailed support re this but appreciate there is probably quite a stack in front and would like to get on since I'm oh so nearly there now!)

Here is the nub

Sorry to have to report a faulty Duet Board. My RS Ormerod machine number is 318.

Got to "now connect ATX power supply unit" in the Commissioning section. On switching on and checking the bed temperature at start up it was warm. I also saw that D6, the Bed Heater Mosfet LED, was lit. On investigation it looks like resistor R18 on the Duet Board is short circuit ( should be 1k) and there appears to be a bubble in TR1 ( R18 is connected from TR1 drain to +5v) so TR1 has obviously blown.

Looking in the spares box I have some BSS138 transistors which I reckon would work ok in place of BSH105 in this application and having a much higher drain source resistance cant be a bad thing to slow the turn off of TR2.

OK to go ahead with this so I can move on?

regards
Andy

Edited 1 time(s). Last edit at 12/30/2013 08:43AM by kwikius.

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Ormerod #318
www.zoomworks.org - Free and Open Source Stuff

Hi Andy

How odd for the driver to blow rather than the power device. I am thinking I don't want the bed supply sharing the common ground with the electronics. There is an output connector to use an external Mosfet. Tr1 is still required so that doesn't help you.

Rory

Whatever I do I have to remove R18, since it is shortcircuit so shorts the gate of TR2 to 5V keeping it on permanently. I would guess its just a solder bridge. Looks a bit messy.. see pic. Also see hole in TR1 at bottom! I thought I smelt something when I switched on for the first time!



Either I get on with it or wait for a replacement

regards
Andy

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Ormerod #318
www.zoomworks.org - Free and Open Source Stuff

Andy

Happy for you to have a go at fixing it. We will still replace it under warranty, so long as you haven't smashed it to pieces with a hammer! (Or done anything else too irreparable...)

Ian
RepRapPro tech support

Quote
droftarts
Andy

Happy for you to have a go at fixing it. We will still replace it under warranty, so long as you haven't smashed it to pieces with a hammer! (Or done anything else too irreparable...)

Ian
RepRapPro tech support

Hooray DiddleeAY ...

Now... where's that hammer?

regards
Andy

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Ormerod #318
www.zoomworks.org - Free and Open Source Stuff

Success
Replaced TR1 with a BSS138 mosfet, which may even have a beneficial effect on switching. Just used desolder braid on R18



Got through the bed heater tests, xyz tests, etc. Looking good. Now for a break, before ( hopefully) the real fun begins!

Thanks for all the help so far!

regards
Andy

Edited 1 time(s). Last edit at 12/30/2013 04:11PM by kwikius.

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Ormerod #318
www.zoomworks.org - Free and Open Source Stuff