12V into 5V regulator?


New Member
I'm building a small robot that will contain an 18A controlling an LCD.

I'm powering the Picaxe and associated circuitry from a 7805 regulator, but I'm running the motors from 12V (8 x AA's).

Is it alright to just tap off the 12V into the 7805, with lots of smoothing caps o'course, or is this too high a voltage to be feeding the little fella. Should I take a tap off, say, 9V instead? (Which could be a bugger, as I'm using two 4 x AA packs and don't want to melt the plastic if I solder onto a spring or whatever.)

The circuitry powered by the 7805 will be drawing around 50mA, max.

Any thoughts appreciated.


Most of the 7805 breed of regulators can cope with inputs as high as 40v so you should be fine.
The thing to watch out for with linear regulators is power dissipation. Your modest current of 50mA should be no problem.
(12v-5v)*.05A=0.35W. It will get quite hot without a heatsink so I would suggest fitting one.
I would also suggest fitting a low value resistor between 12v and regulator input. It will help isolate any motor noise. About 47ohms would do nicely and help spread the heat a little.
Don't forget to add 0.1uF caps as well as 10uF close to the regulator pins and another 0.1uF close to the PICAXE pins.

Edited by - beaniebots on 16/03/2007 12:04:47


New Member
Thank you very much, BeanieBots. I'll do as you suggest.

(I'll also breadboard it all first to keep Stan happy.)

Thanks again.


And get hold of (and read) Data Sheets to keep everyone else happy.... :)

Seriously, getting Data Sheets (and reading them) can save hours.

Michael 2727

Senior Member
Adding the 47 Ohm resistor in your case
probably won't be needed, a small heatsink
should work fine.
A resistor in some applications is needed
but they can cause the regulator to drop out
under heavy loads, running motors from a
small battery supply may do it.
My 2 cents worth, from experience.


I would still suggest the 47R in this particular application more for noise immunity. The heavy (and noisy) load is on the line side, not the regulated side.
The 47R will drop about 2.4v leaving around 9.7v headroom. Even if the 5v load went to double the expected 50mA, there would still be 7.3v headroom, plenty for a lightly loaded 5v regulator.


New Member
Thanks everyone, and thanks for the datasheet reminder, Dippy.

I'm usually pretty good with datasheet checking, but just felt that 7V was a lot to drop and thought I might get some realworld feedback.

Y'never know, someone might have begun a reply with "Are you crazy..." :)

Thanks again.