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Topic: MC7805ACTG getting very hot (Read 855 times) previous topic - next topic

Einherjer-Joe

Hi!

A question regarding working temp of a MC7805ACTG.
In an attempt at a reboot...or rather a resurrection (we're talking night of the living dead here) of my long-time dormant and half-forgotten knowledge of basic electronics I decided to put together a simple voltage regulator, see attached drawing, to power my first Arduino project.

The 7805 is connected to a 31C/W cooling fin (is that the correct English word?), and I don't have a 0.33 uF capacitor at hand, hence the three in parallel at the 9V input side.

So my question is this:
At 150mA at the 5V side the cooling fin is lukewarm, but if I increase the load to 320-350mA it gets so hot that I only can touch it for couple of seconds.
Is this normal, or have I botched something?

BTW, the purpose of the LED is just as a simple power indicator, I've tried with it disconnected also, same result.

Cheers

Zapro

Is your input voltage 9V like the schematic reads ?

You can test the regulator without any load, it should not get hot at all - if it heats up - something is wrong.

I don't know where you got the 330nF from - 100nF on each side is perfect for at 78xx-regulator.

// Per.

dc42

With 9v on the input side of the regulator and 350mA load, the regulator will dissipate (9 - 5) * 0.35 = 1.4W. So if your heatsink is 31C/W then you can expect the temperature of the regulator to rise by 31 * 1.4 = 43.4C. So at room temperature, you can expect the regulator to run at about 65C.

If the input is an unregulated 9v wall wart, you will probably find that it produces somewhat more than 9V when loaded well below its rated load, which will increase the power dissipation in the regulator further.
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Einherjer-Joe

#3
Jun 08, 2012, 07:39 pm Last Edit: Jun 08, 2012, 07:41 pm by Einherjer-Joe Reason: 1

Is your input voltage 9V like the schematic reads ?

You can test the regulator without any load, it should not get hot at all - if it heats up - something is wrong.

I don't know where you got the 330nF from - 100nF on each side is perfect for at 78xx-regulator.


Yes, its 9V from a wall wart.
It don't heat up without load, but thanks for the tip!
In the datasheet its recommended 0.33uF on the input side, in my head that translates to 330nF.


With 9v on the input side of the regulator and 350mA load, the regulator will dissipate (9 - 5) * 0.35 = 1.4W. So if your heatsink is 31C/W then you can expect the temperature of the regulator to rise by 31 * 1.4 = 43.4C. So at room temperature, you can expect the regulator to run at about 65C.

If the input is an unregulated 9v wall wart, you will probably find that it produces somewhat more than 9V when loaded well below its rated load, which will increase the power dissipation in the regulator further.


I guess 65C would feel pretty hot, so its probably normal.
The 7805 is rated up to 1A....maybe I should test it, and then roast some marshmallows     ]:D

Thank you both for input and help!

Cheers

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