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Author Topic: 12v to 5v using 7805  (Read 1730 times)
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Chile
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I'm using the 7805 ic to source 5v from 12v inputs. Just two capacitors and it works perfectly.
Now I don't like the amount of heat they build, and since I'm not using them to full capacity, only arround 200mA, I thought that a resistor in series with the 7805 input would help, since it would sink the current that goes in.

So, with my few hobbyst knowledge I "tried to apply" Ohms law and figured out that a 8 Ohm resistor would lower the voltage in 1,6V, and that way reduce the heat produced by the 7805
Is that correct?
And how much power would the resistors have to tolerate? 1,6V*0.2A= 0.32W
Is that correct?

Thanks in advance
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You are correct on all counts. A 0.5W resistor would be appropriate.

Note, though, that you're not really lowering the total heat generated by the system, you're just spreading it over two components (resistor+regulator).

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Why not try a switching regular instead? Will run cooler that way.
pololu.com has some nice ones.

Ah, Argentina - well, obtainability could be an issue.
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Chile
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Ah, Argentina - well, obtainability could be an issue.

 smiley-mr-green
You are right, we have to improvise a lot, or wait two months for whatever you buy from ebay.

Meanwhile, I found this other solution, which I totally don't understand.




* 3_Term_Prereg.jpg (64.03 KB, 1024x768 - viewed 54 times.)
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It just drops some power in the transistor first. That regulator can't handle 48V so the transistor cuts it down first.
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