how can solve 'LM7805 heat problem with Relay'

Deleted member 115935

Joined Dec 31, 1969
Take the famous step back,

the 7805 is a LDO voltage regulator.

To a first level, It acts as a variable resistance to keep the output voltage constant.
your putting in 24 volts, and dropping that to 5 volts,
so across the "variable resistance" of the 7805, you have 19 votls,

Good old Watts = Volts * amps.

Say your relay needed 100 mA , then that 100 mA has to go through the 7805.

19 volts times 100 ,A is 1.9 Watts,

So to make the 7805 cooler, you need to either lower the current going through it, or lower the voltage going into it.

Personally, as has already been mentioned,for anything more than a few mA , I'd be looking at a simple DCDC to get me 5v from 24 v,

These are simple to use option

many others around, and from other suppliers.

The other point I think I pick up, is your using a micro controller, Im guessing an Arduino, and want to control the relay.

If I read that right,
then this might be of use


Joined Jun 5, 2013
I still cannot understand why you want to step the 24V down to to 5V When you simply could use a 24V relay and have no problem with heat.


Thread Starter


Joined May 3, 2020
I still cannot understand why you want to step the 24V down to to 5V When you simply could use a 24V relay and have no problem with heat.

You are right i didnt't explane my project fully i'm sorry my fault

I want 24v vcc and atmega328p need 5v vcc so i used lm7805
Atmega328p doing read adc(nct thermistor) and clcd

If i push the button relay turn on.

So relay control 24v wire
and led on

I can't found small 24v push button
And i thought 5v can controlling 24v using relay so it worked but there was a heat issue with LM7805.

Thank you guys all!

I have a colclusion

1.Just change relay, pushbuttln
2. Use LM2576
I'll try first one and i'll keep notice you my works!

I really apriciate your replys thank you all you guys


Joined Nov 29, 2009
I think there's a lesson to be learned here - by us responders! It wasn't until post #21 that somebody took the trouble to take the OP through the problem step by step. As I was reading the responses I was thinking "Why doesn't someone take it from first principles, as it's clear the OP is a novice at this". Yes, it's difficult when somebody posts a request for information but doesn't supply a clear explanation of exactly what's required - but that's what happens when you're a novice! You can't see the wood for the trees. Read through the whole thread again and you'll see what I mean. Still, it looks like he got there in the end!

Deleted member 115935

Joined Dec 31, 1969
I think i should change LM7805 ->LM2576R-5.0
but i wonder that just changing the regulator can solved the temperator problem?
The LM2576 is a switching regulator contorler, so will have lower dissipation than a LDO


Its a controler, not DCDC module , so you now have to design and make the DCDC yourself.

Why not fit one of the DCDC regulators you have been recomended in other posts that do not require any design


Joined Sep 17, 2013
There's an entire product category known as "three-terminal linear regulator replacements" (they're fully integrated switchers) that is typically found under the category "board-mount DC-DC converters". Some are potted, some are just surface-mount devices. Now I plan things a lot but the great thing for me is you can lay out a PCB for one of those three pin .1" screw terminal blocks, maybe you're not planning to draw very much so you stick in one of those linear devices in a TO-220 package, then if you decide to add another part that hikes the current draw and now you can't take the regulator heating you can pull out the linear regulator and drop one of these in. Check out something like the CUI VX7805-500 (.5 ampere).