# convert 12V down to 9V DC

Discussion in 'Electronic Basics' started by [email protected], Oct 3, 2008.

1. ### Guest

My kids' electric scooter has a 6V motor and battery. The 6V battery
doesn't give enough juice, but a 12V battery I installed gives too
much juice (wheels skidding when the pedal is pressed, etc).

The "Gas" pedal is a simple on/off switch.

I'd like to provide 9VDC, at about 5A (a guess - I haven't measured
the current yet - that will take some doing, to determine the startup
surge current with a kid sitting on it).

I was thinking LM317 with pass transistor (I've got a spare 2N3055),
but I'm wondering if I can simplify this schematic a bit (under High
Current Regulated Supply):
http://ourworld.compuserve.com/homepages/Bill_Bowden/page12.htm

Why a 2N3904, why so many resistors? Can they be omitted?

I was also thinking of a buck converter, but I have no P-channel
mosfets.
http://www.daycounter.com/LabBook/BuckConverter/Buck-Converter-Equations.phtml

Thanks,

Michael

2. ### JamieGuest

Just put some large diodes in series, each will give you a ~ 0.6 volt drop./
The simplest method I can think of.

http://webpages.charter.net/jamie_5"

3. ### Tom BiasiGuest

Controlling large currents like that is always a PITA.
For cars and scooters I usually make a "shifter" by tapping batteries at 3,
6 , 9, and 12.

Tom

4. ### Guest

Update - surge current is around 15-20 A actually with the 12V
battery. A linear regulator might not be such a great idea after
all...

Michael

5. ### Guest

Interesting; I'll look into that...

Thanks; I'll look into that. I have a slightly (physically) bigger 6V
battery that I'll try. (None of them spec their surge current
capacity, only A-hr capacity... more research needed...)

Yep, heat... I guess I could be safer by using 20 3V, 1A light bulbs
to allow 20 A to pass, but then that wouldn't be exactly cheap
either...

Yep, especially for 20A...

Thanks,

Michael

6. ### neon

1,325
0
Oct 21, 2006
Motors like batteries don't care about volts . use an lm117 as a current regulator with the 2n 3055 pass then just vary the current easy enough.

7. ### Guest

....zap...

That did it! A fatter 6V battery does the trick.

Thanks,

Michael