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Wednesday, October 19, 2011

Oh No! Short Circuit Danger with Anodized Heat Sinks?

Our friend Brent out in Minneapolis sent me some pre-Halloween HB frights yesterday. I'm still concerned. Am I living dangerously with my anodized but collector-connected heat sinks glued to the ground plane?
What say the HB gurus?

--- On Tue, 10/18/11, KD0GLS wrote:

From: KD0GLS Subject: Re: JBOT Amp
To: "Bill Meara"
Date: Tuesday, October 18, 2011, 9:38 PM

Bill,

Well, it is nearly Halloween, but it wasn't my intention to frighten you!

I think it's hard to argue with the fact that it works as-is. The heatsinks you bought look like common anodized aluminum to me. Out of curiosity I dug a couple of similar ones out of my parts bins and measured them. I found exactly what you did - wide open on the surface, and, not surprisingly, a dead short just underneath. I've always assumed that anodize finish was metallic and conductive, but now I know otherwise. I like to learn at least one new thing each day, so thanks for providing today's opportunity!

The clearances are no doubt small, but I think with the low voltages present in your circuit you'll probably be fine. The only potential problem might be if movement or vibration might someday break through that coating. I'd certainly recommend a fuse in the power supply line if you don't already have one.

Anyway, it looks great sitting there in the middle of your old rig. It's inspiring.

Brent


On Oct 18, 2011, at 19:42, Bill Meara wrote:

Brent: You scared me with your question but I think what I did is OK. Maybe. I looked more carefully at the clip on heat sinks I bought. They are covered with a non-conductive coating, but f I scratch them up a bit I can reach the conducting part. So i guess you just have to treat them gently if you intend (as I did) to super-glue them to the PC board. I notice that Farhan did the same thing (see his JBOT page) so I guess I'm OK. I put some heat sink compound in there too. And then there is the super glue. I have no short circuits from the collectors. I guess I could have used a Dremmel to isolate the copper just below the transistors. Or I could have put some mica between the heat sinks and the boards.

The sinks are deeper than the transistors, so there is no direct contact between the top of the transistor and the PC board.

What do you think?
--- On Tue, 10/18/11, KD0GLS wrote:

From: KD0GLS >
Subject: JBOT Amp
To: "Meara Bill" >
Date: Tuesday, October 18, 2011, 5:27 PM

Bill,

The amp looks mighty FB, and maybe I should try it as well. But I'm curious how you've insulated the collectors of all those transistors from the ground plane. From the picture, it looks like all of the metal heat sinks are sitting right on the ground plane copper.

.73,
Brent, KD0GLS, Minneapolis

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1 comment:

  1. For insurance, you could simply shove a square of double-sided copper-clad board under each xistor. Super glue it if you wish. No more potential problem (no pun intended).

    73.......Steve Smith WB6TNL
    "Snort Rosin"

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