[Elecraft] OT: heat sinking resistors

[Tom W8JI]

I've used those resistors here and they are fine up to at 
least 30 MHz sandwiched.

They will not crack if he uses common sense on materials. 
The heatsink should be rigid and very flat, and the clamping 
metal preloaded with some sort of tensioning or spring 
washers. A couple split ring washers in series at opposite 
ends (one under head, one under nut) tightened to just 
collapsing would be good. If someone is really worried about 
it they could put fish paper or a layer of thin Teflon 
between the holding plate and the resistor, so long as the 
tension washer stack can keep pressure steady. There are 
always Belleville washers which are best, but simple spring 
washers (split ring) would work in a non-critical system 
like this.

The only time anyone would get into trouble is if they use 
improper hardware, like stainless screws and a rigid or very 
high pressure lock washer like a star or internal tooth 
type.

There will be very little difference in performance with a 
heatsink floating in air, or with the resistor attached to a 
heatsink that is massive and grounded. Whatever is 
mechanically best will work OK.

Anyone without good mechanical sense needs to stay away from 
clamping things.

73 Tom


----- Original Message ----- 
From: "Geoffrey Mackenzie-Kennedy" <gm4esd at btinternet.com>
To: "Vic Rosenthal" <vic at rakefet.com>
Cc: "Elecraft Discussion List" <elecraft at mailman.qth.net>
Sent: Friday, May 21, 2010 8:21 PM
Subject: Re: [Elecraft] OT: heat sinking resistors


> Vic,
>
> I would suggest that you do not clamp each resistor 
> between a couple of
> pieces of aluminum, because some types of uncapsulated 
> thick film power
> resistors are quite fragile. They may not appear to break 
> if stressed, but
> there is the risk of creating a hairline fracture across 
> the resistor
> "element".
>
> For the two power attenuators that I use in my Rx IMD test 
> setup before the
> combiner (precision attenuators not required in this part) 
> I used 20 watt
> thick film power resistors in TO-220 packages, with their 
> individual
> heatsinks sitting upright, well separated from any 
> grounded metalwork.
>
> 73,
> Geoff
> GM4ESD
>
>
> Vic Rosenthal wrote on Friday, May 21, 2010 at 9:27 PM:
>
>> I just received a few Ohmite thick film power resistors 
>> for an attenuator
>> I'm making,
>> rated 20 watts. They are 15mm x 10mm x 3mm. Obviously 
>> they have to be
>> heat-sunk
>> (heat-sinked?) if they are going to dissipate that much 
>> power.
>>
>> How do you do this? There's no hole in the middle...I can 
>> just clamp them
>> between a couple
>> of pieces of aluminum, but is there a right way to 
>> accomplish this?
>> -- 
>> Vic
>
>
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