How do you mount your transistors ?

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If you are mounting TO-3 transistors the best way is to use L or U bracket so you don't have to use nuts on the heatsinks.
 

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lordvader said:
You insulate the transistors with grease, a washer, whatever, but then you screw them down. A screw is made of metal, and it makes contact with the transistor AND the heatsink. Wouldn't that defeat the purpose of insulating in the first place?

There's to ways (i'm aware of atleast) to overcome this. The first one is to use a special plastic "washer" that isolates the heatsink from the screw through it. You place it on the heatsink screw hole and it covers the top and the inside, so the screw floats at the voltage of the transistor but the sink is isolated from it. I use them all the time and never had a problem.
The other one is nylon nuts and screws. Yes, they do exist and are relatively cheap (not as much as the washer thing through), but i had my problems with them... if you, for whatever reason screw and unscrew them a couple of times the threading on the screw just ruins.
 
Re: Screw & Nuts

chris ma said:
HPotter,

Where do you get your hardwares normally? I have so much frustration trying to get some screws and nuts in Toronto. I tried home depot, canadian tire, sayal, active surplus etc.. either they have the screw but out of stock with the nuts or washers. Or they do not have the right length.
Chris

You see that your neighbor HPotter gets his at Home Depot. You could also try a marine store, since you're in a waterfront town. They should have lots of choices from #6-#12, pan head and FH, at least. Also, go to McMaster Carr (online). They have everything, though perhaps not one screw at a time. They ship same day, and you'll have them in 2 days. Not bad.
 
Nuts

I have tried both Home Depot and Canadian tire, they have only big ones not the ones to mount the TO-3, TO-220 on heatsinks,
even for PCB standoffs are too big too..

Also I am looking for some small thru PCB mount terminals (can be tighten by the nut on top of PCB) so that I can remove all the external connections in order to take the PCB out of the chasis without desoldering for parts replacements. For example where the PSU PCB connects to Amp PCB as well as all heavy ground wire connections to star point etc..

Thanks for the suggestions,
Chris
 
I recommend ....

Due to the problem of using the right amount of grease (usually too much is used + the best types are too thick to work with), I recommend going greaseless. I ended up using Bergquist Silpad 2000 which guarantees very high performance.

For best electrical, capacitive and thermal performance, go with BeO types (you still have to contend with the grease problem which can reduce thermal performance significantly). BeO has another problem -- while harmless in it's fired state, ground up dust is toxic and leads to berylliosis if inhaled. This is similar to Astbestosis which is the effect of Astbestos. Strange how materials so dissimilar in thermal property have virtually identical effect on the body. BeO also has interesting radiological properties.

Some people like Aluminium Oxide washers (misnomer -- they are Aluminium which has oxide layer on both sides). These should also perform well IF you mount them properly, but do not have the same fantastic dielectric properties as BeO.

Save yourself the trouble and mess and go with Silpad.

As for mounting instructions and torque, the Aavid/Thermalloy site has a good white paper on this per package type -- incidentally they are also the only manufacturer of BeO type washers I have found to date.

Petter
 
#6-32 screws

I bought all stainless steel #6-32 screws, nuts, washers, split ring etc.. just to get more frustrated when I got home, the #6 screw can not even go thru the Caddock resistor's mounting hole, and it is too big for the heat sink hole too! I think I'll have to find some shop that sells 3mm screws and nuts, I tried Sayal last weekend they have the 2.6mm 2.8mm and 3mm screws but no washers and nuts, have to wait for the next shippment....
No joy.......My mistake, I should have carry the heatsink and the resistors with me all the time..the I save a few dollars.

The other problem I have also is the capacitor mounting clamps. It was so funny when the guy at the shop try and bend those clamps and convince me they will fit my caps that I brought and show him. Now I'll have to buy those clamps from UK too and pay shipping, custom brokerage and import duty too for some simple hardwares ! It is a joke that this industry in Toronto is in such a state!
 
heat sink too

Last weekend I was in Electrosonic on Gordon Baker tried to buy some smallish heat sink too, well they ran out of stock on the ones that I ordered, so the lady was such a good salemans and sold me some other ones that came with a clip, so I thought great, no screws, paid $7.0 a piece for 10 of them to mount on the PCB with the caddock resistors. The caddock resistors are too thin for the clip to work and if I want to use those heaksink I have to increase the oversize of the PCB too because the fins will get in the way of other components too! Well back to the drawing board again.
 
Beryllium Oxide insulators

HPotter,

Where did you find Beryllium Oxide insulators for TO-247s?
As far as I know these are the best heat transfer insulators made. I thought these were not made any more because they are toxic if injested. As if some one woud eat one! Mica is the second best. I am building an Aleph five and started searching for the thinnest micas made. If the Beryllium insulator is not too expensive I would use these. Thanks.

A tip for those using TO-247 cases. The TO-247 hole is not centered. And to help put equal pressure to secure it down I use those small TO-220 heat sinks to hold down the TO-247. I Also drill and tap 4-40 machine screws to secure them to the heat sink. And use a split lock type washers to keep the screw secure.
 
Thermal grease for heat transfer

Grease is used because the interface of all the parts have air gaps between them. These are tiny gaps but they are important to get out because air is one of the best insulators for both heat and electrical exchange. Air between your parts will lower heat transfer. The grease fills the tiny air gaps and is a better heat transfer agent then air it replaces. Too much grease is not better and will reduce heat transfer. Coat the parts with the thinnest layer that will cover the part. You are only filling tiny imperfections between the parts to push out any air. Not all grease is well made. Use a quality grease like Thermalloy's Thermalcote(tm) thermal joint compound 1 ounce tube is plenty part # 249.
 
lordvader

Use insulating nylon washer made for use with metal transistors with TO3 and TO220 devices. A TO247 has an insulated body around the screw hole but needs the Mica thermal washer and grease between the transistor and the heatsink to provide good electrical isolation and good thermal conductivity. When tightening your screws, USE THE FORCE, but not to much force or you can damage the insulating washers and they will no longer be insulators.

H.H. (Heatsinks Hints)
 
T0-247 Insulators

My turn for a question:

Can someone suggest a US source or two for the mica insulators for the TO-247 case devices?

I have seen the (pricey) kapton ones, but not the mica ones. It seems that Keystone Electronics is the major supplier of the mica insulators, but I couldn't find any info about them on their website, and Mouser, Digikey and a few others I checked don't list them in the TO-247 size.
 
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