problem when get 5V from 24V

[encoder]

I have a 24VDC source power, and and i use LM7812 and LM7805 to regulate 5V for microcontroller. LM7805 is put on the output of LM7812, and the input of 7812 is 24VDC, see schematic above.
But LM78xx are very hot when i apply this circuit to project. some one can clear and help me to solve problem.

 

[on1aag]

Hi Encoder,

The solution to your problem is a switching regulator.
If space is also a problem check this out:

https://www.electro-tech-online.com/custompdfs/2007/06/pt78st105.pdf

on1aag.

 

[dknguyen]

There is no way around it as long as you are using a linear regulator. Those just take the extra voltage and turn it into heat. So if you are drawing 0.1A and changing 20V to 5V, you must turn (20V-5V)*0.1A Watts into heat. If you draw twice as much current you dissipate twice as much heat.

YOu have to use a switching regulator in that case.

 

[Gayan Soyza]

The problem is your power supply 24V is too high. And the output is 5V.
So the other 19V you are dissipating & produce more heat. You need a jumbo heat sink.

And also why you add two LM IC’s? Why can’t you direct apply to 7805?

Just use 7805 & give input 9V & see you will see the difference.

 

[Papabravo]

The problem is your power supply 24V is too high. And the output is 5V.
So the other 19V you are dissipating & produce more heat. You need a jumbo heat sink.

And also why you add two LM IC’s? Why can’t you direct apply to 7805?

Just use 7805 & give input 9V & see you will see the difference.

Because it is easier to disipate the heat between two packages then to have one package do the whole job. It might also be the case that 12V is used for some purpose. There are other possible combinations of linear and switching regulators. It would help if we knew the requirements.

 

[Hero999]

How much current do you really need?

If it's only 500mA or so you can build a switching regulator using a couple of transistors but I would use this for currents over 1A absolute maximum.
https://www.romanblack.com/smps/smps.htm

Have you checked the datasheet of the LM317?

There's a switching regulator circuit on the datasheet!

 

[kjennejohn]

Hi.
I've found old PC power supplies to be good for digital circuits, especially simple processor projects. You get several different regulated voltages, with plenty of power in each one. They are readily available on Ebay for cheap.Try to get the ones with built in power switches.

These are switchers, so have output noise to some extent. This noise can interfere with analog circuits so extra filtering is needed sometimes.

Hope that helped. 'Luck on your project.
kenjj

 

[encoder]

Thanks for reply
My project needs 12V for other device, and 5V for microcontroller. The current required < 500mA. And the space also is my problem.
I have found this schematic, a simple switching regulator. Someone can show me the quality of current DC output ?

 

[kjennejohn]

Hi.
Is that 500mA altogether for both supplies or 500mA out of each regulator?

I just checked DigiKey to see what they had for power regulators in TO-3 packages. These almost always get installed on beefy heatsinks to dissipate heat due to large power drops, as in your situation. The 5V version (LM340-5.0CT) needs a HUGE heatsink to handle the 3.5 Watts (7V*0.5A), unless you use forced air, as in a fan. They didn't have a 12V version.

So, linear regulators will need space-robbing heatsinks, and possibly a fan, to handle the dissipated power.

So, you either go with a switcher, which can demand extra parts on the output for filtering not shown in any of the schematics displayed here; or, the simplest way, based on your situation, is to go to a power supply closer to 12V, if not 12V itself.

FWIW.
kenjj

 

[encoder]

500mA for both supply.

 

[kjennejohn]

Hi encoder.
So, to confirm, your 12V regulator supplies 500mA to the 5V regulator AND 500mA to another circuit?

Earlier you said the 12V circuit has to feed another circuit as well as the 5V regulator. Are you saying it handles 500mA (5V reg) PLUS something (another 500mA?) to another load? Remember, EVERYTHING goes through the 12V regulator.

If you are pushing an Amp through the 12V regulator you are dissipating 12(Vdrop)*1(Amp)=12 Watts!! Add that to the 5V regulator's dissipation, 7(Vdrop)*0.5(Amp)= 3.5W, and you have 15.5 Watts of heat to get rid of! We're talking serious burns here!

Dude, replace the 24V supply or go to a switcher.
kenjj

 

[Hero999]

Thanks for reply
My project needs 12V for other device, and 5V for microcontroller. The current required < 500mA. And the space also is my problem.
I have found this schematic, a simple switching regulator. Someone can show me the quality of current DC output ?

Check the datsheet.

Did you look at the link I posted for the two transistor switching regulator?

It can be built pretty small; all you need to do is change the zener for a 5.6V unit.
**broken link removed**
https://www.romanblack.com/smps/smps.htm


Why are you that bothered about ripple?
What are you using it to power?

I assume it's a digital circuit and most digital circuits are pretty immune to ripple; that's why computers have switching regulator power supplies.