Is this good enough? (heatsink)

Discussion in 'General Electronics Chat' started by davidGG, Mar 23, 2013.

Dec 22, 2012
51
0
Hi, I want to use my AC wall transformer (120v -> 21V), 'plugged into' a rectifier (21V -> 19V) into a LM317T power regulator (19V -> 5V) to power a microcontroller.

Will a heat sinks like so suffice for prolonged use?
http://www.ebay.com/itm/400275309095

Average current will be 100mA to maybe .900, won't exceed 1A ever.
Power dissipation ~ (19-5)V*500mA = 7W (this is where i'm becoming doubtful of a small heat sink being able to dissipate 7W)
Rectifier is rated at 400V, 1.5A
Regulator rated at 40V, 1.5A
Transformer rated at 240VAC, 10A (not 100% sure about the amp rating but I remember it was in the teen area).

Also, this may seem stupid, but I am just getting into digital logic, but...
a) can integrated circuits (NAND,AND,NOR,.. gates) be powered with AC?
b) what is the voltage drop on these integrated circuits? can't find any information on the datasheets.

and, any good guides on how to connect to the internet using a microcontroller? Atmega32a is what I am using.

Thanks, I appreciate the help.

2. spinnaker AAC Fanatic!

Oct 29, 2009
5,078
1,179
It should be fine but my suggestion is that you try to find a 9V DC walwart. They are cheap. You would be dropping much less voltage across your LM317. Plus you don't need the rectifier.

Dec 22, 2012
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Never heard of the term walwart, will look into.

4. spinnaker AAC Fanatic!

Oct 29, 2009
5,078
1,179
No digital gates cannot be powered with AC. How would that work? It does not even make sense.

Voltage drop for digital ICs would be found in the datasheet but it is not worth worrying about since it would rarely factor into the design for any hobbyist.

My suggestion is before you do anything you review the lessons at the top of this forum.

Oct 29, 2009
5,078
1,179
6. poopscoop Member

Dec 12, 2012
139
16
Why not just use a cell phone charger?

Most chargers are SMPS rated for 120v 60hz to 5v DC at anywhere from 500ma to 1A. I have two sitting right here, one of which is an 850ma and one of which (HTC) that is 1A.

I have a feeling that a cell phone charger will be much smoother than the set up you intend to use and you'll save money on a heat sink.

7. JohnInTX Moderator

Jun 26, 2012
2,394
1,051
The 317-T has a thermal resistance junction to ambient of 50 degC/watt. At 7W, the junction will be about 375 degC (assuming 25C ambient, 50*7 + 25). The max junction temperature is 150degC so the device will thermally limit (by dropping the output). Without a heat sink, its a no go.

To see how much sink you need, you have to go through the steps: the junction to case thermal resistance is 5degC/W. At 7W you get 35+25 = 65 degC (assuming an infinite heatsink attached to the case). If you have a sink rated at 10degC/Watt at 7W you'd have 35+25+70 = 135degC (assuming perfect conditions). You derate according to what the thermal conductivity of any insulator is etc. A healthy de-rating is recommended i.e. don't assume you can run at 135degC before considering that ambient rises in the case, sunshine etc etc etc. Plus touching the heat sink will burn skin. A sink rated at 5degC/W should be OK for what you describe, maybe at something less than 900ma but do the math with your particular sink.

The sink you pictured is not spec'd but its way more that 10degC/W.

Here is a description of how to chose a heat sink. Its a little thick in places but will get you started. Page 4 of the LM317 datasheet lists the thermal resistances of the various packages. Browse the AAVID site under board level products-TO220 for a big list that you can sort by thermal resistance. Mouser has them.

Your choices are more heat sink, LM317K with heat sink, lower input voltage, lower output current, big fan or another supply completely.

Have fun.

Last edited: Mar 23, 2013
8. Evil Lurker Member

Aug 25, 2011
117
24
Or you could just be a cheap bastard like me and scavenge a an old Socket 7 or 462 CPU cooler or perhaps a northbridge heatsink off a junk obsolete computer, drill a hole in it where you want your mounting screw to go, slap on some Arctic Silver 5 on the back of your regulator, and mount the sucker. And if you really want some serious cooling you can always leave the fan on it.

9. JohnInTX Moderator

Jun 26, 2012
2,394
1,051
You could just bolt a lot of metal onto the tab.

But how much?

Actually, for hobby work, Evil's approach is probably OK, too.

Last edited: Mar 24, 2013
10. takao21203 Distinguished Member

Apr 28, 2012
3,578
463
totally no. too small. try 5v electronic transformer.