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Old 16th April 2003, 01:02 AM   #1
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Default Help - Mounting transistor to heatsink

Hi there
I am trying to build an Aleph 3 amp and have all my parts together (finally). when I mount my transistor (Kristijan's PCBs) to the heatsink, should my mounting screw and heatsink be isolated from each other? I know that I have to use some form of insulator(I am using mica insulators with thermal grease) between the transistor and the heatsink. However, once I use a screw to mount the transistor to the heatsink, the screw and the heatsink will have an electrical short. Is this ok?

what about isolation between an aluminium chassis and the heatsinks? Does there need to be electrical isolation between these two?

Finally, do I need insulating bushing between the screw and the transistor? Can the screw end up being in physical contact with the metal part of the transistor through the screw mount hole?

I am a novice and I hope these questions make sense



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Old 16th April 2003, 01:31 AM   #2
PassFan is offline PassFan  United States
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What kind of transistor are you using. Some are plastic and some are metal. If you are using, say, an IRFP240/9240 combination then the bolt will not touch any of the metal of the transistor and that will be fine. If you have any doubt, after you bolt the transistor down, and before you solder it to the card use an ohmeter to check from the screw to each leg of the transistor. You should not get a reading if everything is okay. If you get a reading then you must buy an insulator.
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Old 16th April 2003, 01:41 AM   #3
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Although I'm a complete novice as well. And I have similar questions. I believe the following will answer one of them:

Click the image to open in full size.

It appears that the hole in the TO247AC package is isolated. Or insulated? It sounds like you need not worry about a bushing.

Regards,
Dan
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Old 16th April 2003, 02:04 AM   #4
PassFan is offline PassFan  United States
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Quote:
Originally posted by dantwomey

It appears that the hole in the TO247AC package is isolated. Or insulated? It sounds like you need not worry about a bushing.

Regards,
Dan
Be careful Dan, your assuming his devices are TO247AC. He has not stated what he is using yet. I know a few people that are using 610's and 510's as output devices and you must use a bushing with them.
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Old 16th April 2003, 06:12 AM   #5
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Default Mosfets in use

Hi there
Thanks everyone for replying so promptly. The devices i am using are the IRFP244. I want to say that this is a TO-247 package but I am not very sure. They do however look like this -

http://www.irf-japan.com/package/TO247AC.pdf

Can I do the continuity check on the PCB itself (check for electrical continuity between each of the pin legs and the screw hole ?)

Do I need to keep any leg grounded? Will the continuity checks in anyway ruin the boards?

thanks in advance
VSR
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Old 16th April 2003, 10:25 AM   #6
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Default Heat Transfer Abilities

Which is better thermally; the mica and heat conductive grease or the silcone pads?
If using the mica you spread the grease on both sides for coupling, is this an issue in cooling the hot little chips.

George
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Old 16th April 2003, 12:47 PM   #7
PassFan is offline PassFan  United States
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Default Re: Mosfets in use

Quote:
Originally posted by vsr123
Hi there
Thanks everyone for replying so promptly. The devices i am using are the IRFP244. I want to say that this is a TO-247 package but I am not very sure. They do however look like this -

http://www.irf-japan.com/package/TO247AC.pdf

Can I do the continuity check on the PCB itself (check for electrical continuity between each of the pin legs and the screw hole ?)

Do I need to keep any leg grounded? Will the continuity checks in anyway ruin the boards?

thanks in advance
VSR
If you look at the top of the page you linked it clearly states TO247AC




You need not worry about it. On the top where the screw goes through is black plastic and underneath you can see a ring of black plastic around the hole and then metal. Mount this on your choice of insulative pad (mica w/thermal compound) and let her rip. You do not have to do a continuity check if you don.t want and once you have these mounted on insulators your heatsink will not cause a short when you attach it to the rest of the chassis.

The problem with checking a device after it is on a board is, can the meter read what I want it to read...ONLY. That is, will it read through something else in the circuit and make me think I have a problem when I don't, and how long will it take me to figure out I am chasing my tail 1 megaohm to ground is different than 1 ohm to ground. You will be fine but if you want to double check take a device not soldered to the card, say a spare, and bolt it up and look. Just a little peace of mind, but you will see all will be well. You may blow it up from something else but not this Have fun
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Old 16th April 2003, 03:12 PM   #8
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Does heatpad (sold by digikey BER178-ND) can replace the usual mica for TO-247? It seems like a mix between insulation and grease.
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Old 17th April 2003, 04:13 AM   #9
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Default Re: Heat Transfer Abilities

Quote:
Originally posted by Panelhead
Which is better thermally; the mica and heat conductive grease or the silcone pads?
If using the mica you spread the grease on both sides for coupling, is this an issue in cooling the hot little chips.

George
Yes.

Quote:
Originally posted by François
Does heatpad (sold by digikey BER178-ND) can replace the usual mica for TO-247? It seems like a mix between insulation and grease.
Yes.






Actually, I think that the Mica is slightly better thermally, if I remember correctly. Try Bergquist's site, I think that the data is there. Or alternately, Digikey. They have data on everything that they sell. I think that any difference is only a problem if your heatsink is too small to begin with.

I don't quite understand the question "is this an issue...?" Just put it on both sides. It's the right way. That sauce serves to fill any microscopic gaps between the surfaces of the heatsink, mica pad, and transistor. That is its one and only purpose.

Now that I look again, maybe it's not a question. It's worded like one, but it has no question mark (but I notice that they are being done-away-with here )


For Francois, the answer is Yes. Particularly in your case with your extra-mega-huge heatsinks.

I think that mica insulators in that size are somewhat difficult to get. You can get them, but most don't carry them.

Here are two sources you can read to become a real expert:
http://www.chomerics.com/products/do...hermalPerf.pdf
http://www.irf.com/technical-info/appnotes/an-1012.pdf
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Old 17th April 2003, 04:33 AM   #10
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quote: I don't quite understand the question "is this an issue...?" Just put it on both sides. It's the right way. That sauce serves to fill any microscopic gaps between the surfaces of the heatsink, mica pad, and transistor. That is its one and only purpose.



what he has said is is in his own way is absolutely correct...sil-pad absorbs the compression of the device and fills up any fluctuations in mating surface ...or should I say adhesive properties...mica is a translucent wafer so its "RIGHT THERE"...the compound we use just helps the heat transfer
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