Rubber insulation & transistor trimming

[kuksul08]

I have two questions.

1. I'm putting a circuit that I built inside some 1" square aluminum tubing. There are dozens of solder connections sticking out from my perforated circuit board. I have some bicycle tube rubber (~1/16") that I cut into a rectangle and placed over the bottom of the board to insulate it from the aluminum housing. Is rubber a suitable electrical insulator, or will it cause issues?

2. Same circuit as before, I have two TIP122 transistors that are a little too tall. Is it okay to trim the metal on the heatsink part of it? I was planning on cutting through the center of the hole.

Thanks!!

 

[McAllan]

That is no problem electrically wise.

However you should have in mind that that plate is also functioning as a cooling plate. If your transistor is not driven very hard or still have sufficient cooling (glued with thermal epoxy by the rest of the plate) then no problem.

 

[kuksul08]

Thanks^

Can anyone confirm, before I go cutting it up?

The transistor is driven at 500mA at about 10Hz approximately. It barely gets warm.

 

[Th232]

If it's thermally fine (and it definitely sounds like it), then go for it.

Regarding the insulation, try pressing down on the rubber with the board, see if anything pokes through. Shouldn't be a problem though.

 

[kuksul08]

Cool, thanks guys :wave:


One more thing - is it okay if the outsides of two resistors touch? They are wirewound Yageo, I believe the outside is some kind of insulating material but I'm not sure.

 

[VidPro]

agree, you can clip and shorten everything on them, as long as the heat sync part (back plane) gets its heat out somewhere as needed.
(as needed: meaning mondo parts run with minimal amps , or backing it up with proper heat syncs if running "amps" through them).
if things like those are heating up (finger burn heat), they will eventually split and crack over time :-( so if its cool your cool, if its getting hot fix it today with good sycning or it will die over time.

resisters can "rub through" the epoxy, so in a stable design you can have them hit eachother fine, but in a design where there it motion of the parts a rub through can cause undesirable conduction.
my solution has been to heat shrink them, which then insulates them PPPP so next solution is to use 4x power need, and then its no problem, the size and heat removal over a larger area and all.

 

[kuksul08]

agree, you can clip and shorten everything on them, as long as the heat sync part (back plane) gets its heat out somewhere as needed.
(as needed: meaning mondo parts run with minimal amps , or backing it up with proper heat syncs if running "amps" through them).
if things like those are heating up (finger burn heat), they will eventually split and crack over time :-( so if its cool your cool, if its getting hot fix it today with good sycning or it will die over time.

resisters can "rub through" the epoxy, so in a stable design you can have them hit eachother fine, but in a design where there it motion of the parts a rub through can cause undesirable conduction.
my solution has been to heat shrink them, which then insulates them PPPP so next solution is to use 4x power need, and then its no problem, the size and heat removal over a larger area and all.

Great thanks!

 

[Mr Happy]

Be careful, though, to avoid mechanical stresses and distortion if you try to cut the tab off a transistor. The same goes for bending the legs of components -- always support the leg next to the case and avoid putting any stresses on the case itself.

 

[louie]

Rubber is a good insulator, but I would use something else. Most rubber will dry up and crumble away sooner than you'd like. I'd use a sheet of plastic, or even cardboard, which is common in consumer electronics.

Note also that a lot of electrical component heat dissipates through the leads as well as the heat sink tab.