What temperature is too hot for the 7805?

Hi,
I finished my second Thruthi (smk4 filter) and I think, I works. But I noticed that you cannot touch the 7805 after running serveral minutes…

I have no thermometer, but I think it is over 50°C.

Is this normal or did I messed up somthing?

(I lend the first Shruthi to a friend, so I cannot compare the temperature, and sorry for the bad english)

The temperature comes from the excess energy the Regulator hast to dissipate from the input voltage to the desired +5V. The higher the input Voltage, the more energy the 7805 transfers into heat. It has an internal protection so it won’t die from overheating, but if it gets to hot to touch you probably should switch to a PSU that has a lower output voltage (best is switching power 7.5V 300 or more mA).

hmm, I use an universal power connector running at 9V. Ok I will try a lower setting.

Most cheap Wallwarts output a way higher voltage than rated to be shure the 9V rate will be available at maximum load. Just measure :wink:

hey … good idea …I will measure the voltage under load …:smiley:

no kidding, I does not consider that, stupid me …

Sooner or later you will want a PSU that outputs 7.5V no matter what the load is :wink:

ok I try it in the evening, I will report any success :slight_smile:

Use a high quality PSU at a voltage that is just enough for the regulator. It’s usually 2v or so above the output of the regulator.

If you’re really paranoid about the heat then you can replace the linear regulator with a switching one. But be prepared to spend £30 or so on one.

Simple test, if you burn yourself before your reflexes can draw your hand back, it is more than 60degree celcius… Shitty test, I know… :slight_smile:
7.5V seems to be a good voltage for shruthi’s.

Keep in mind that a super hot vReg is also a symptom of a short on the 5V power rail. You can confirm that by measuring the resistance between +5v and GND, it should be in the kilohm range, if its low, then you have an issue somewhere.

ok, think we got it :slight_smile: The voltage of the power plug is slightly exact. I reduce it to7.5V I measure 7.4V with load and 190mA over all … think this looks good now. The 7805 does not warm up at all now …thank you all

ok, it runs the whole evening … I cannot fell the heat with the finger … only the lips can do … what a different to 9V in … think it was between 50 and 60°c

it runs well :slight_smile: https://soundcloud.com/torsten-vaquet/damage