EEVblog Electronics Community Forum
Electronics => Beginners => Topic started by: Jebnor on July 03, 2013, 01:10:03 am
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I'm in the process of making a 'beginner' Power supply for myself. Nothing too fancy, +- 9-15v 1A variable. I'd like to mount my TO-220 transistors to the metal case to use as a heatsink. They need to be electrically insulated.
My first (albeit potentially messy) thought was to take some paper cardstock and impregnate it with mineral oil. Use this as the insulation material. I think this should do moderately well for thermal conduction. The hole through the metal case will be bigger than required, and I was going to put a little piece of heat shrink tubing around the screw. Any real risk of the paper catching fire? I'm going to smooth and deburr the hole, so short through heat shrink is unlikely.
Any other thoughts on how to mount them to the case with stuff I might find around my house?
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I think you're risking it .. surely you have an old computer P/Supply or similar somewhere? Those things are packed with proper Silpads
for insulating power devices. They usually clamp the device with a bar of metal across the body, with normal screws at each end.
That way you don't need insulation. OR maybe look for nylon screws? DON'T risk it.
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Get a TO220 insulation kit, it has the silpads for the back of the TO220's and insulating inserts to stop the mounting screws shorting.
You should be able to source those fairly easliy.
here's an example
http://www.jaycar.com.au/productView.asp?ID=HP1176 (http://www.jaycar.com.au/productView.asp?ID=HP1176)
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I agree with digsys. Don't try to fake an insulator. Since budget is apparently a driving force, go to your local computer or electronic repair place and see if they've got any dead power supplies that you can have. You'll either get a mounting kit for your TO220 or maybe a TO220 that's totally enclosed in plastic so you don't need an insulator.
Ed
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This is madness. If you can't spare 10 cents for this: http://www.digikey.com/product-detail/en/HF115AC-0.0055-AC-54/BER167-ND/307789 (http://www.digikey.com/product-detail/en/HF115AC-0.0055-AC-54/BER167-ND/307789), PM me your address and I'll send you one.
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Seriously. Pm me your address and I'll mail you one. This is crazy talk.
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Okay everyone Relax! Thanks for the offer of sending me some. I can afford the $5.
On a related note, was there not oil impregnated paper in really old electronics?
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Not in the heat sinks! And not *really* old either, I just bought an HV power supply from the 70s with two massive paper-in-oil HV caps (the smaller is the size and shape of a Spam tin).
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Okay everyone Relax! Thanks for the offer of sending me some. I can afford the $5.
On a related note, was there not oil impregnated paper in really old electronics?
In sealed body capacitors and transformers Oil is going to dry out and turn into furniture varnish when exposed to heat and air for any length of time.
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Might not have been oil...
http://en.wikipedia.org/wiki/Polychlorinated_biphenyls (http://en.wikipedia.org/wiki/Polychlorinated_biphenyls)
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pick yourself up some nylon machine screws, nuts and washers.
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Originally thin sheets of Mica (http://en.wikipedia.org/wiki/Mica) was used as electric insulation for power semiconductors, occasionally it still is actually. If the Mica is frayed at the edge, it might at a glance perhaps look a bit like a piece of cardstock, damp with some liquid or other.
If this is a PSU you intend to use a lot, then I would suggest perhaps rethinking the cooling of the 78xx/79xx devices. The back of my envelope claims each would dissipate around 10W or so of power when delivering 9V @ 1A, and the back of the chassis will almost certainly not be up for the task. It probably wont outright kill the regulators if you just want the power for brief moments at a time, as they will just shut down when hitting their temperature limits. But it might be annoying if you intend to power more substantial things for long periods of time.
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If you absolutely need an insulator and can't wait for a store then check your microwave oven. Open the door and on the side you will see a thin squarish shaped paper, well it isn't paper it is mica. You can cut a piece of that and use it but will need to later replace the part you used from the microwave. It keeps all the food bits from getting into the waveguide/magnetron.
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If you absolutely need an insulator and can't wait for a store then check your microwave oven.
This is why people think engineers are weird.
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If you absolutely need an insulator and can't wait for a store then check your microwave oven.
This is why people think engineers are weird.
LOL
I had to work before in an area where parts could take weeks to arrive and when something breaks you have to get creative. I have done crazy stuff , like a capacitor made from chewing gum wrappers, resistor from scribbling on a paper with #2 pencil, diodes from a needle on a piece of quartz rock or zinc steel, and lots more.