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Author Topic: TO-220 Heatsinking - Lessons Learned  (Read 1106 times)
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Just thought I'd post this, as I've made this mistake in the past.. when using TO-220 cased components, don't assume the tab is electrically isolated.  In the case of LM317T, it's connected to the output.  If you are like me, you may forget this, and decide to screw an old heatsink directly to the tab of a component.  In many cases, that's fine.

There's a couple of reasons this is important.  Heat sinks are often exposed to the user, to allow airflow.  Because the case is output, if your heatsink is electrically in contact with it, your heatsink will be output also.  This is bad for a number of reasons, the not the least of which being the heatsink may also be connected to ground.. creating a short circuit.  Output voltage of LM317's are still fairly low, but leaving anything energized exposed can just lead to too many problems.

In addition- this is what I did- it's easy to say to yourself "Hey, I've got a big old heatsink and I need to cool four regulators.  I'll just screw them all in to the sucker, it can take the heat...", but what's really going to happen is that you've connected the outputs together through the heatsink.  This ends badly.. and takes a lot of head scratching to figure out, until you remember that's not just a heat sinking tab....

While on the subject of LM317- when using voltage regulators, be sure you are connecting it properly.  When looking at a schematic, the symbol used for a regulator usually shows the connections as INPUT, ADJUST, OUTPUT if you are going from left to right.  However, if you have the TO-220 (most common) version, the actual pins are ADJUST, OUTPUT, INPUT from left to right.  The schematic symbol is not necessarily going to agree with the packaging, make sure you look up the correct pinout for your device before you hook it up....

If there's a stupid mistake that can be made, I will make it.  If there's not, I'll make a mistake anyway just to prove my point.  One of the nice things about hobby electronics these days is that components are cheap most of the time, and if cooking a dollar component teaches you a lesson, that's pretty cheap education..
« Last Edit: October 07, 2011, 09:41:56 am by focalist » Logged

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Nice writeup, and definitely something to remember. Generally, this kind of connection (case to something) is common in larger power components, from what I've seen. I have some TO-3 cased 2N3055 NPN transistors, which have their collectors connected to the case. Something to think about when making an h-bridge with them...

smiley
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Just thought I'd post this, as I've made this mistake in the past..

All good advice. I like to say, making a mistake is useful, because it will allow me to recognize it when I make it again.
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You know, that might make a very useful thread.. "I made this mistake, watch out for it!".. things that people tend to forget or not think of that end up being "gotchas".

Most of them are newbie errors (like mine) I would expect, but much time and aggravation might be saved.  Some ones I can think of:

Don't forget to connect the grounds...
It's a good idea to discharge capacitors before connecting to them, even if you think they aren't charged..
Heat sinking while soldering is sometimes a good idea..
Specs are generally quoted as maximums, you are better off with wiggle room..
If you NEED one of something, buy three.  If you only WANT it, buy two.  Buying one, without a backup, is a risk..
Components are best purchased in bulk.  Ordering things from China is much cheaper than Radio Shack..
Clean and Re-Tin often..
If in doubt, heatsink it..
Wall Wart DC is most likely NOT regulated.. and is VERY noisy.

etc etc etc. 

There's a TON of experience on these boards, and happily, most of the experts are also happy to admit they've made silly errors.. practical application advice that you won't find on any spec sheet.  I figure if we can get a collection of these together, maybe we could even put it up as a FAQ or something, make it "required reading"... I know I'd read anything like that intently.  Plus, I think it's good for anyone who has gotten good at something to occasionally remember that at one time, they too were completely new and made the most silly mistakes that can be made.  Good for both a giggle and a little humility.
« Last Edit: October 07, 2011, 12:19:33 pm by focalist » Logged

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You know, that might make a very useful thread.. "I made this mistake, watch out for it!".. things that people tend to forget or not think of that end up being "gotchas".

And entertaining besides ;-)  Re LM317, even without a heatsink, if fastened a PCB with a machine screw, be sure the copper is sufficiently relieved around the hole (if a ground plane) or appropriately isolated, don't count on solder mask for insulation.

I had a list of "rules", will have to see if I can find it. The only one I remember is: "Beware the lightening that lurketh in the undischarged capacitor, lest it cause thee to bounce upon thy buttocks in a most ungentlemanly like manner."
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I'd replace the "heatsinking while soldering" with "solder for up to 5 seconds, cool and retry, don't fry!"
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I too have a tip about heatsinking, from a mistake I made years ago. If you're bolting a device to a heatsink, and you are using a mica washer and plastic bush to insulate it from the heatsink, and you drilled the holes for the mounting screw(s) in the heatsink yourself, chamfer the entrance hole slightly with a larger drill bit and then check with a magnifying glass that it is flat. After screwing the device to the heatsink, check with a multimeter that the insulation is OK.

The reason for this advice is that when you drill a hole, the drill may throw up a very shallow crater where it enters the metal, and this crater can pierce the mica washer.
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I think it would be great to start such a collection. Needs to be on the Playground WIKI so we all can add to it.

In about 99 years this could be as cool as Robert Heinlein's "Notebooks of Lazarus Long". Stuff like:

---------------------( COPY )--------------------------
Always store beer in a dark place.

By the data to date, there is only one animal in the Galaxy dangerous to man--man himself. So he must supply his own indispensable competition. He has no enemy to help him.

Men are more sentimental than women. It blurs their thinking.

Certainly the game is rigged. Don’t let that stop you; if you don’t bet, you can’t win.

Any priest or shaman must be presumed guilty until proved innocent.

Always listen to experts. They’ll tell you what can’t be done and why. Then do it!

Get a shot off fast. This upsets him long enough to let you make your second shot perfect.

There is no conclusive evidence of life after death. But there is no evidence of any sort against it. Soon enough you will know. So why fret about it?

If it can’t be expressed in figures, it is not science; it is opinion.

-----------------( END COPY )----------------------

And for more:  http://www.angelfire.com/or/sociologyshop/lazlong.html#inter

...I probably wouldn't have started building stuff and still be building stuff 60 years later if it wasn't for Robert A. Heinlein.


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I liked these: http://www.aikenamps.com/SafetyTips.html

The one about keeping one hand in your pocket is similar to advice to electricians working on a live panel.

I have a friend who's a woodworker. He often mentions keeping blood on the inside.
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facilitate a police state. -- Bruce Schneier

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