Redoing grease under transistors shorting out now - diyAudio
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Old 5th February 2013, 04:16 AM   #1
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Default Redoing grease under transistors shorting out now

Hello everyone, I have a mtx amp that had some type of silicone pads under the transistors. They we're somewhat dried up. So i used ceramique 2 by arctic silver, Suppose to be non conductive. After getting a layer under every transistor and hooking the amp back up it blew all 4 40amp fuses on the amp. I took it out of car and opened it up and found nothing burnt so im pretty sure its shorting cause of the thin layer of this thermal grease compared to before. My question is what do i use to properly redo the grease? Silicon pads? Thick layer of Silicon grease?

Oh and the old pads removed had like threads in them, The last photo is after i redid the pads.
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Old 5th February 2013, 04:58 AM   #2
DUG is offline DUG  Canada
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Silicone pads with fiberglass threads...

If you don't want to use them you still need an insulator...just grease doesn't do it.

You could go back to the old standby of mica insulators with the grease...they do insulate...they use them in toasters to support the heating elements.

Another option is to use the aluminum oxide insulators...highly thermal and insulative as long as you don't scratch them.

http://www.aavid.com/sites/default/f...g.pdf#page=104

Have fun.
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Old 5th February 2013, 02:50 PM   #3
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How about putting a layer of kapton tape on the heatsink then a layer of silicon grease or thermal paste? http://pages.ebay.com/link/?nav=item...d=150981840751. That's the stuff i would use. I didn't think they couldn't touch the heatsink I was doing it as hpw I would a cpu.
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Old 6th February 2013, 04:06 AM   #4
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Old 6th February 2013, 05:17 AM   #5
DUG is offline DUG  Canada
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There are Kapton pads available.

I think that the glue on the Kapton tape will insulate (thermally) more than you would get from straight Kapton.

If you have it try it.

It will certainly insulate the transistors electrically.

If you have the tape, you might consider washing the glue off with alcohol or googone or some other solvent that works on the glue.

Thermal paste helps fill the gaps normally occupied by air. (heat insulator)

You know what is needed now...

Have fun playing

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Old 6th February 2013, 05:32 AM   #6
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You need mica or insulating washers to electrically isolate the collectors from the heatsink which is why they have all shorted out. You also need to make sure the screws are insulated electrically from the heatsink.
I would hazard to say they are probably gone now. I would test all of your transistors.
Thermal grease is applied to both sides of the insulator washer to increase the thermal transfer of the heat from the collectors to the heatsink.
On computer CPU chips there is no electrical isolation required as the CPU base is not connected to anything other than the die itself so only thermal heat transfer is needed.
Hope that helps!
Sorry for your loss but lesson learned....
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Old 6th February 2013, 05:39 AM   #7
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stutla22, do you have a schematic of the amp?

The transistors might not be blown. (or maybe only half of them)

Post schematic and I can take a look at it and see what may have problems.
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Old 7th February 2013, 05:18 AM   #8
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Im confused on the whole insulating thing and thermal thing. I want it simple and easy to do. Im pretty sure nothing is burnt it just popped all the fuses from the mosfets touching the heatsink. You said the tape would work but remove the glue? What would the glue effect? Also if i did use the tape would i just stick it onto heatsink and put paste between tap and mosfets?

What about these ?

100 Thermal Insulator Silicon Rubber Pad to 3P Size 25x20x0 3mm RoHS Uni Home | eBay
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Old 7th February 2013, 05:23 AM   #9
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Well non of the transistors had burn marks on them. How would I test them with a voltage meter?
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Old 7th February 2013, 04:02 PM   #10
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Ok here is how to put it simply.

Tabs on mosfets are not like heatspreaders on CPUs. The tabs are NOT electrically isolated at all. The ceramic paste may be non-conductive but all it takes is a tiny sliver of space where all of the paste has been pushed apart and you have a direct short between tab and heatsink. If you put enough paste to prevent this, the paste is actually working as an insulator. The paste works by filling the microscopic airgaps where the metal doesn't touch the heatsink. Not by transferring all of the heat. So in this case you need something that is thermally conductive but electrically insulating. Mica is a very common option.

Here is an example of what you need:
To 220 Mica Insulator Set MICAX10 Nylon WASHERX10 | eBay
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