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Optocoupler Circuit Overheating

Posted: Sat Sep 03, 2016 4:32 pm
by djjohnson70
I must be missing something. I tried to design a simple Optocoupler circuit. I have a 24V power supply which powers a 7805 Voltage Regulator. A simple switch powers the Optocoupler. I have a delay timer (555 timer) in between the input and the output Optocouplers.

I put this on a breadboard, but when I leave the input pushbutton pressed, the 7805 Voltage regulator gets very hot, very quickly. The circuit performs the function that I expect, but I didn't think I would draw that much current, as the 7805 is rated for 1.5A.

What can be drawing so much current that the regulator cannot supply the required voltage? What have I messed up in this circuit?

The attached image shows the circuit. NOTE: Everything in orange is the bottom layer of the PCB design, which is all ground.


Re: Optocoupler Circuit Overheating

Posted: Sat Sep 03, 2016 11:25 pm
by skimask
Yep, operating completely normally. much current is your circuit pulling from the 24v source?
7805 hmmmm...
READ the datasheet, the whole datasheet, top to bottom, then READ it again.
So far you didn't really mess up anything in the circuit, just in your overall assumptions on how things should work vs. how they actually work.

Re: Optocoupler Circuit Overheating

Posted: Sun Sep 04, 2016 7:53 am
by davep238
The 555 timer's power supply can be up to 16 volts, so why not use a 12 volt regulator (7812) instead of the 5 volt one? The 7812 will dissipate less heat since there will only be a 12 volt drop across it vs 19. A small heat sink would be a good idea too.

Re: Optocoupler Circuit Overheating

Posted: Tue Sep 06, 2016 8:59 am
by stellajohn
Only major things to heating circuit is due to uncontrollable flow of current from source. Try to reduce the high current flow from source or use the heat sink for IC like 555 , voltage regulator IC.