Power Supply Woes

Thread Starter

ActivePower

Joined Mar 15, 2012
155
Well, I have been trying for sometime to get a fairly smooth PWM on LEDs and motors with my lab power supply which works fine when switched up providing both +5V and +12V supplies without any hassles.

However, it is when I used two +9V batteries (in series) along with a 7812 and 7805 to power up my PIC16F877A circuit when it all started falling apart.

Individually, the 7812 and 7805 ICs are able to power up my 12V and 5V rails pretty well but when I short the input pins of both to the battery supply, it all goes haywire. The 7805 starts putting out about 7-8V and the 12V supply falls to 3V or so. The battery levels also start draining rapidly. All of it points to a short somewhere but I am not able to locate it. The power circuit is pretty standard and I am quite sure I connected the pins correctly.

The thing that bothers me is if they can work ok separately why can't the regulators work properly in tandem?
 

ErnieM

Joined Apr 24, 2011
8,045
Yeah, post a schematic even though it seems obvious... but a picture of the set up may be better.

You may be "quite sure" your connections are correct, but if that were literally true then it should work.
 

Thread Starter

ActivePower

Joined Mar 15, 2012
155
I am sorry I should have posted the circuit earlier. I am travelling right now and will do it asap. And @atferrari: sorry for the confusion. What I meant was 'why don't they work together'. I'll post a pic of the circuit as soon as I get back.

Thanks for your replies!
 

BillO

Joined Nov 24, 2008
990
Motors! I guess a picture of your experiment and a schematic will help, but likely it is a current draw problem. It would need to be a pretty specific motor to be compatible with a little 9v battery.
 
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