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?

 

You should post a picture of your setup, or a schematic.

 

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.

 

Obviously 9V are too puny for ur projects.

 

9V batteries are not intended to deliver any current much greater than 50mA.

 

Is space at a premium because, as already stated, perhaps PP3's aren't suited to your application. Could you not use AA's, you can get 6 way battery holders with a PP3 connector.

 

"In tandem". What is that?

No circuit?

 

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!

 

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.