Where to get LM4780 Thermal Pads?

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#21
jacco vermeulen said:



Mr G,

you sure are a thrillseeker.
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Yeah, but not on purpose. I am a boring guy but I was was wearing safety googles at the time.
Jacco! You thermally coupled but electrically isolated Poobah!

Actually I think he is right, the voltage had been gradually approaching the negative DC rail when the thing turned on and I couldn't figure out how that was happenning. But with the heat sink and chassis sitting at -DC nothing blew up (the chassis is plastic with some kind of conducting coating on the inner surface...it turns out). When this all went bad Heat sink was floating from chassis which was floating from ground. Or perhaps not, but that was the intent as I had uncoupled everything. Soo Poobah, the final version will not have all these things isolated (which is why I need one of your pads!) but for testing I had attempted to isolate them to find the problem.

I was so confused I almost hooked up and LED to my case and heat sink to see if they could actually pass current.

I used basic thermal grease on my Krell and other GC, but Arctic Silver on my X-CCS-BOSOZ (it works) and A-X (it blew up and I gave up).

question- what other stuff might be hurt and can I reuse the board or are the traces likely to be vaporized?
 
#22
The components on the board are probably OK.

Just cut all the leads of the chip close to the body. Heat and tweezer the leads out and see what you have left. Contrary to some notions, the board has just about nothing to do with the sound.

You might have to touch up some traces... use bus wire for this... or, copper tape for stained-glass-window-work, works really well.

Did Bergquist not help you out? Either way, if you were able to cleave that mica down... that will work.

Good luck!
 
#23
lgreen said:
isolated Poobah!
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With good reason, the Poobah is pushing kids to smoke,..and me.

The picture with the JAP01 insulator was actually meant to ask the Poobster if that is the India stuff. Are micas on your end that different ?

btw: it's not a question if i have Bergquist sheets, but where i have them. I can live with one less. If you want one you also get the Bergquist sales rep, i keep the catalogue.
 
#29
raromachine said:
Oh - and a good run down of thermal goop incl. Artic Silver and others... yes that's right vegemite and toothpaste!

http://www.dansdata.com/goop.htm
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:rofl: I remember reading that when it first came out! How geeky does that make me? It does, of course, bring home the excellent point that most thermal greases are equally pretty bad.

I was lucky enough to inherit a jar of paste the guys from TA instruments gave us to use with some of our thermal testing equipment. It is the best paste I have used in terms of ease of application, but since I will never have to buy any more thermal paste (it was a 6 oz!!! jar) I guess I'll never know if something better comes along.
 
#30
jackinnj said:
you can use a TO-3 pad, or a pair of ordinary TO-220 pads.

if anyone is interested, there is supplier of mica pads from China which will custom manufacture TO-220 for this format. The pads which would work on the LM4780 or LM4702 -- but I would have to order thousands -- GB anyone????

I should have these manufactured as well, they snug the chip up to the heatsink really well.

An externally hosted image should be here but it was not working when we last tested it.
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With your idea as an inspiration I sought out these from my local hardware store which worked fine!


http://www.stanleyhardware.com/default.asp?TYPE=CATEGORY&CATEGORY=HDW+MENDING+PLATE

For an insulator I used sil-pad 400 which I got as a sample from Berquist after I blew the first chip because of no insulator. Worked great.

Regards

RDV
 
#32
jacco vermeulen said:
I have a jar of Vegemite, that one is going to last my lifetime as well. :clown:
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[offtopic]MAN that stuff is NASTY!!!! Whoever came up with the idea to eat that stuff should have thier head examined!![/offtopic]

I like the idea of the plate to add extra pressure to the chip to ensure good contact, but I would put a little bend in the center of the plate to keep good pressure in the center of the chip instead of just on the edges. I know this won't be easy to do with a steel blank from the hardware store, but if you have the resources, a sheet of spring steel could easily be formed to make a little clamp.

I am sure this is WAY overkill, but still, its an idea!

David
 
#38
Heat sinks

Do you think these will do the job if I isolate them?
 

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#40
poobah said:
The truth be told... you would all benafit by direct mounting these chips and floating the heatsinks.

The temperature difference from chip to sink is significant at these power levels.
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Any problem doing this with two LM4780's on the same heatsink? they are fed by separate transformers. Does this short one channel's rail to the other channel's and produce smoke or shrapnel?

I think a thick layer of vegemite under the heatsink for isolation, and a generous dab of Pocari Sweat to glue it in place. Hmmm... does Pocari Sweat conduct electricity?

..Todd
 
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