[R-390] OT: Pass Transistor Question

[2002tii]

Barry wrote:

> > I want to mount the TO-3 pass transistor(s) and TO-3 regulator on heatsinks
> > that are open to the backside of the cabinet.  The problem is 
> that the cases of the various devices are not common and they are at
> > something other than ground potential so I'd like to insulate 
> them from the back panel.
>
>Ahh, I see what's going on now.  If I use a transistor socket, it 
>solves the issue for me.  Without that, I'd need some shouldered,
>insulating washers, but a socket makes a lot more sense.

Yes, the TO-3 case (conventionally the collector of a bipolar device) 
is insulated from the heatsink by an insulating washer (whether you 
are using sockets or not).  These can be mica, silicone with fillers, 
thin plastic (Kapton), hard anodized aluminum, or ceramic.  The 
filled silicone ones are used dry, all of the others are used with a 
dollop of heatsink compound on both sides of the washer (it is 
thermally conductive but electrically insulating -- the good stuff is 
a messy white paste; avoid the clear stuff).  The hard anodized and 
ceramic ones are the best, thermally speaking, but the anodizing can 
get scratched and the ceramic is brittle and prone to crack if you 
aren't careful tightening the hardware.  Mica is a good 
compromise.  Kapton and filled silicone are prone to being cut if you 
overtighten the hardware, and  the filled silicone insulators have 
rather mediocre thermal performance (though the convenience of dry 
installation has made them popular).

If you are not using a socket, you use plastic (generally nylon or 
teflon) shoulder washers on the opposite side of the heatsink.  A 
screw passes through each hole in the TO-3 case, through the 
heatsink, through the shoulder washer, and is capped with a nut.  One 
nut uses a standard star lockwasher, the other uses a solder terminal 
with lock teeth in the screw hole.  You solder to the two pins (each 
sticking through its own hole, generally not insulated except by air) 
and to the solder terminal.

As you have figured out, using a socket can make things 
easier.  Generally speaking, however, you will have fewer problems 
with solder joints than with socket connections to the pins.  Some 
companies used to make what were essentially sockets but with no 
contacts for the pins.  The pins stuck out the other side and you 
soldered wires to them.

Finally, note that the device cases may not be at cabinet potential, 
so if you drop paper clips or other metal objects onto the heatsink, 
you may let the smoke out.  You can buy plastic insulators that snap 
over the installed devices to prevent this.  The better ones have a 
tiny hole in the center so you can poke a probe in to check the 
voltage on the case.

Best regard,

Don