Mounting TO-3s

[Devil_H@ck]

I know there's a thread about this, but I didn't find the answer to my question there.

I have a heatsink with 2 L-shaped bars on it. I've drilled the holes for mounting the TO-3s and last week I mounted the TO-3s, so far so good.

However, today, while testing the power supply of my JLH amp some more, on part of the supply was giving 31V and it couldn't be regulated anymore. After some searching I found out that it was the TO-3 that was shorted to the heatsink.

This is how I mounted it:

I also used a "plastic" insulator between the heatsink and the TO-3. I used Arctic Silver 3 as thermal paste. The Arctic Silver 3 does conduct current, that's probably the reason why the transistor was shorted to the heatsink, right?

You can see some pictures of the mounted TO-3s on my website: http://members.lycos.nl/anthonyvh/index.php?page=jlh

So, what should I do? I was thinking along the lines of:
1. Make the holes for both the M3 screws and the transistor legs bigger
2. Get a non-conductive thermal paste

Would this be OK? Is there any other way besides pushing heatshrink tubing up the transistor legs to ensure proper isolation from the heatsink?

[Sch3mat1c]

Looks like you also forgot the insulator between the device's case and the heatsink, as well as insurance that the screws don't contact the heatsink. (If the drawing is to scale, it'll only take a few degrees offset for the screws to make contact.)

Just use the normal old white crap, it's just a transistor.

Tim

[chris ma]

Hi,
A very nice site you have there.
You can use the continuity check on your MM to make sure the heatsink is really isolated from the MJ15003 prior to connect it with power. Makesure you have not overdone it with the paste. I have never try Arctic Silver so can't really comment.

If the holes for the screws are big enough like mine ones I put the screw thru a PVC/nylon sleeve inside the heatsink too.

Good Luck.
Chris

[peranders]

You need this:

http://www.elfa.se/elfa-bin/setpage....se/2008964.htm

[Devil_H@ck]

Originally posted by Sch3mat1c
Looks like you also forgot the insulator between the device's case and the heatsink,...

Read my post again, I used an insulator, I just didn't draw it .

Originally posted by Sch3mat1c
Just use the normal old white crap, it's just a transistor.

I might have that somewhere, though I'll have to search for it .

Originally posted by chris ma
Hi,
A very nice site you have there.

Thanks .

Originally posted by chris ma
You can use the continuity check on your MM to make sure the heatsink is really isolated from the MJ15003 prior to connect it with power.

Well, everything was fine at first. It worked OK for 2 days, then today, I must have handled the heatsink a bit to rough and one of the pins must have bend or something. Off course, once I get myself some alu panels to mount everything I won't have to worry about that anymore.

Originally posted by chris ma
If the holes for the screws are big enough like mine ones I put the screw thru a PVC/nylon sleeve inside the heatsink too.

I'll make the holes bigger and slide some heatshrink around the screws too. I was also thinking about using a plastic washer both above and below the transistor, so the screw wouldn't make any contact with the transistor at all.

One part of the problem is that I didn't use a "model" (can't recall the correct word for the moment) for the TO-3 holes. I used a paper with the holes printed on it and then marked it, but something must have gone wrong, because some holes are off by 0.5-1mm.

Originally posted by peranders
You need this:[url]http://www.elfa.se/elfa-bin/setpage.pl?http://www.elfa.se/elfa/produkter/se/2008964.htm

I have that . In fact, I'm getting 15 extra insulators and 30 extra washers tomorrow .

[peranders]

Originally posted by Devil_H@ck
I have that . In fact, I'm getting 15 extra insulators and 30 extra washers tomorrow .

If the transistor not is isolated, have you checked that you don't have any sharp egdges in the screwholes?

[Keld]

Hi

Arctic Silver 3 does conduct current, that's probably the reason why the transistor was shorted to the heatsink, right?

My advise NEVER use electrical conductive paste when you use insulaters. There will always be a risk that there will be a short from case to sink.

[Devil_H@ck]

Originally posted by peranders
If the transistor not is isolated, have you checked that you don't have any sharp egdges in the screwholes?

Yes, there aren't any irregularities inside the holes (well, except for the fact that some are a bit oval, but that's thanks to my excellent drilling skills ).

Originally posted by Keld
My advise NEVER use electrical conductive paste when you use insulaters. There will always be a risk that there will be a short from case to sink.

Ok, I just checked again and it seems I was wrong:

Not electrically conductive.
Arctic Silver 3 was formulated to conduct heat, not electricity. It is only electrically conductive in a thin layer under extreme compression.

(Quoted from: http://www.arcticsilver.com/as3.htm)
One thing less to worry about .

Btw, I mounted the transistors in a new way today. Check my website. Here's how I did it:
- Plastic washer both on top and below the transistor
- Heatshrink around the part of the screw that's "inside" the hole
- Heatshrink around the transistor pins
- AS3 on the heatsink
- AS3 on the top side of the thermal insulator

I spend 2 hours doing that to 4 transistors, it takes quite a while. Especially the 4th one caused me a lot of trouble. The holes seems to be misaligned a bit too much and the pins keep shorting to the heatsink.
I'll make the holes bigger tomorrow.

Now, it's time to go to bed. I've got a math exam tomorrow .