7805 running hot but clock is running fine

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1 year 7 months ago #9076 by Pjotrke
Hi,

Recently I built Ian's All in One Rev. 2 clocks I bought about a year ago. I'm what you call an absolute beginner, except for having made some test boards before starting this project.

With the kit finished everything is running fine, though the L7805CV voltage regulator gets too hot to touch. The Mosfet feels ok.

I am currently using a 12V power supply. I have used other supplies with a factory reset after starting up for the first time. Evertime resulting in a hot voltage regulator.

The power led works just fine, all components of the low voltage circuit are in the right way and all other components (RGB/colon leds, tubes and Wifi module work as advertised.

I hope someone can give me some pointers on what could be the cause of the hot regulator. Could a 9V adapter be the solution, even if 12V should be just fine?

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1 year 7 months ago #9077 by Ian
Nice to hear that everything is working, but the 12V supply is not really recommeded. A 9V supply is preferred. The amount of power that the 7805 has to dissipate depends on the voltage drop from the input (12V in your case), minus the target voltage (5V). This results in a lot of power being wasted.

The 7805 can handle it, but it should have a heatsink if you want to continue using the 12V supply.

A more modern solution is to use a buck converter, but the soldering needed for these is slightly more difficult, and so we only put buck converters on "more advanced" kits. The 5 pin TO-220 is a bit tricky to solder. It has 5 legs in the same space as the 7805's 3 legs.

If you use a heatsink: Do not mount both the 7805 and the MOSFET on the same heatsink without insulating them. The metal tabs are connected internally, and you will have trouble if you try to mount them both on the same metal heatsink.

My recommendation would be to use a 9V adaptor.
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1 year 7 months ago - 1 year 7 months ago #9078 by Pjotrke
Thanks Ian for reacting so quickly.

I already went ahead and ordered a 9V supply. Your post gives me peace of mind by confirming I probably did not screw up something.;)
Last edit: 1 year 7 months ago by Pjotrke.
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1 year 7 months ago #9079 by Ty_Eeberfest
This is a good buck regulator in a standard TO-220 3-lead package. I've used them a few times and they seem to work well. They sell for about $5.00, so not exactly cheap.

www.mouser.com/datasheet/2/281/oki-78sr-56393.pdf

Look into it later when the dust is clearing off the crater.
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1 year 7 months ago - 1 year 7 months ago #9086 by Ian
And the same thing from flea-bay for $2.06:

Buck-Module-replace-TO-220-L7805-LM7805-LDO

I might invest in a few of those, just for a play
Last edit: 1 year 7 months ago by Ian. Reason: Link

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1 year 7 months ago #9096 by Pjotrke
Got the 9V supply today and was able to alleviate the problem. The 7805 still gets warm but it doesn't get too hot to the touch. I did have comments of other Nixie clock creators that in their case the regulator hardly gets warm at all though.

And I'll get a few buck regulators anyway. A more efficient clock is never a bad thing.

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