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Topic: 12v input to 7805 voltage regulator heating in a second?? (Read 357 times) previous topic - next topic

pixilicous

Sep 22, 2014, 08:59 pm Last Edit: Sep 22, 2014, 09:08 pm by pixilicous Reason: 1
Hi,

I have a design that I uploaded not long ago here:
http://forum.arduino.cc/index.php?topic=260610.msg1840409

i dont think my question will be related so for basically there is an atmega328 with 3 tlc5940 daisy chained and 16rgb leds, the 7805 voltage regulator has got 2 capacitors on input/outputs of 10uf each..

I got the pcbs and i connected everything (dip sockets/crystal/ceramic capacitors/power jack etc)
Instead of waiting to connect all Leds and then turn everything on, i connected only 1 led.
That led turned on once and that's it, the 7805 was too hot to touch, i got burnt.
the power adapter is a 12v 1.5a, i have around 3 which i tried, after that i tried a 5v battery but the 7805 is already dead.

is this related to the fact that the power didn't have where to go so the 7805 was fried as it was disspiated as heat at the regulator?

this doenst make sense to me, as not all the leds are turned all the time as well.. what can cause the 7805 to not be able to handle 12v?? i saw this in another design i made on a strip board


cjdelphi

Wayy too much current... switch to a switching regulator or if you ate within spec of the 7805 slap a big heatsink (computer cpu sized)

JimboZA


Wayy too much current...


.... from one LED?

Maybe it didn't have a series resistor?
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pixilicous

I do not understand the official recommendation then for 7805 on the offical page at http://arduino.cc/en/Main/Standalone


pixilicous


Maybe it didn't have a series resistor?


The led? the TLC5940 had a resistor, there isn't one especially for that LED

JimboZA



Maybe it didn't have a series resistor?


The led? the TLC5940 had a resistor, there isn't one especially for that LED


Well if you don't drop a few volts across the series resistor, you'll get a current up in the amps range not the 20 or so mA that an LED wants, so it's possible you went to or over the 1.5A that I think a 7805 can handle.

The random graph I found (not saying it's the one for your LED) shows the current climbs steeply to 2-3 Amps.
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pixilicous

ok so apparently the horrific heat didnt kill the 7805.. must have been a safe shutdown but still it was so hot!

anyways, i probably had a short circuit somewhere causing this, because now everything is connected and it doesnt heat much..

it works :))

thanks for the help !

pwillard

High heat is pretty normal for a 7805 with 12V on the input pin if you opted to not use a heatsink.  Just saying.

MarkT

For a TO220 device something like 0.5W is the maximum you'd really want to
put into it without some sort of heatsinking.   If you have 12V supply to a 7805
that's suppling 70mA you will be at 0.5W dissipation already.

If you put 1.5A through it the dissipation will be 1.5 x 7 = 10.5W and a medium
sized heatsink and a fan would be required (or a large heatsink).

A DC-DC converter would waste much less power and run at a sensible temperature
without such issues....
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Henry_Best


For a TO220 device something like 0.5W is the maximum you'd really want to
put into it without some sort of heatsinking.   If you have 12V supply to a 7805
that's suppling 70mA you will be at 0.5W dissipation already.

If you put 1.5A through it the dissipation will be 1.5 x 7 = 10.5W and a medium
sized heatsink and a fan would be required (or a large heatsink).

A DC-DC converter would waste much less power and run at a sensible temperature
without such issues....
Or you could use a L78S05 (rated at 2A max), but you'd still need a heat sink.

zoomkat

Incorrectly wiring a 7805 will make it get very hot very quickly if it is supplied from a strong power source. Check your wiring, Two leads are probably reversed.
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C-F-K

The 78XX series actually work by dissipating the extra power into heat to create the output voltage. Therefore the efficiency of those things are the worst ever, just second to a 100W light bulb.

Can you get your hands on a "real" DC-DC converter? Efficiency and output are much better on those and you don't have to worry they get hot, so you don't need a giant heat sink to keep things cool.
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