STK500 - hot voltage regulators

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Dingo_aus
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Posted by Dingo_aus: Sun. Jan 20, 2008 - 11:09 AM
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I was just wondering how hot the two voltage regulators on the STK500 are allowed to get.

I'm talking about the two near the DC jack, sitting on large gnd planes.

I'm powering the stk500, a mega32, a DS1631 and a backlit 16x2 LCD and the regulators are running too hot to touch.

I am sitting in the middle of an Australian summer without air conditioning so ambient temperature is about 28 degrees C.

Is it ok to let them run hotter than I can touch? Should I add a heat sink?

Thanks,

Nigel (aka Dingo)

Dingo_aus
http://www.users.on.net/~symes/

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AVR Microcontrollers, megaAVR and tinyAVR, ATmega32
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GroovBird
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Posted by GroovBird: Sun. Jan 20, 2008 - 12:41 PM
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How much voltage are you letting in? I'm running my STK500 on 9VDC and it's getting pretty warm too. I suppose running it at 7 would make it waste less (produce heat) but I was too lazy to find a more suitable one.

Running on 6 is perhaps possible, though I don't think it would be stable enough.

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valusoft
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Posted by valusoft: Sun. Jan 20, 2008 - 01:11 PM
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Nigel,

As suggested by "GroovBird", you could always try lowering your DC supply. Perhaps a couple of inline silicon power diodes ...

Ross McKenzie ValuSoft Melbourne Australia

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Bronze MarkJohanEkdahl
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Posted by JohanEkdahl: Sun. Jan 20, 2008 - 02:09 PM
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If you want to lower the power consumption then don't supply the backlight through the STK500. AFAIK the backlight on LCD modules are separated from the rest of the LCD-module. From what you describe I'd suspect the backlight to be the biggest power consumer.

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Ub3r
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Posted by Ub3r: Sun. Jan 20, 2008 - 03:57 PM
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The regulator thats used to power your device, including the LCD, is an LM317. There is another 7805 regulator nearby (further in), but that isn't were your heats coming from.

The LM317's have thermal shutdown, and will never destroy themselves due to overheating. The closest components are quite far and will easily withstand the dissipated heat. You don't need to worry about the capacitor, as its a solid cap, and will also withstand the heat.

Solution: You can just decrease the target voltage from the stk500 board settings. Decreasing the voltage to the backlight will lead to a decrease in current drawn by your backlight, thus, decreasing the total power been dissipated.

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Bronze Markjs
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Posted by js: Sun. Jan 20, 2008 - 10:40 PM
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Quote:
Running on 6 is perhaps possible,
I don't believe so. I run mine at 9Vdc regulated. The power input goes through a bridge rectifier first, so you lose about 1.4V there giving you about 7.6V into the regulator.

Any high current part of the ciruit should run from a separate supply.

John Samperi

Ampertronics Pty. Ltd.

www.ampertronics.com.au

* Electronic Design * Custom Products * Contract Assembly

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Dingo_aus
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Posted by Dingo_aus: Mon. Jan 21, 2008 - 03:09 AM
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Yes, silly me...i was running 12V regulated, I'm now on 9V regulated and it seems happy. I think I set it to 12V in the Winter.

Thanks for the info about the LM317 Ub3r.

Maybe I should just move to the airconditioning ;)

Dingo_aus
http://www.users.on.net/~symes/

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