convert 12V down to 9V DC

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
 
mrdarrett@gmail.com wrote:
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
Click to expand...
I like the buck converter idea, but if I was doing it, I
think I would try to put a control pot in place of the
on/off switch, and rig up a current mode buck controller, so
that the torque was proportional to the pot voltage. That
would allow a wide speed range and smooth acceleration with
controlled motor current, instead of controlled voltage.

You could use a PNP transistor as the switch if you insist
on switching the positive rail.

Even if you go with a voltage controlled buck and the on/off
control, it should still have a current limit to protect
everything in the event of a stall and not strip the gears
on the hard starts.

--
Regards,

John Popelish
 
mrdarrett@gmail.com wrote:
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


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


http://webpages.charter.net/jamie_5"
 
<mrdarrett@gmail.com> wrote in message
news:4bc3ce63-c425-49fd-9e37-1cbb3f72b399@t54g2000hsg.googlegroups.com...
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


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

Tom
 
Jamie wrote:
mrdarrett@gmail.com wrote:
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


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

Of course it is. A PWM speed controller won't waste all the energy,
though.



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The first sign of insanity is denying that you're crazy.
 
On Fri, 3 Oct 2008 14:29:40 -0700 (PDT), mrdarrett@gmail.com wrote:

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
Click to expand...
Big power resistor!

John
 
On Oct 3, 2:29 pm, mrdarr...@gmail.com wrote:
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-Equati...

Thanks,

Michael
Click to expand...


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
 
On Fri, 3 Oct 2008 23:52:22 -0700 (PDT), mrdarrett@gmail.com wrote:

On Oct 3, 2:29 pm, mrdarr...@gmail.com wrote:
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-Equati...

Thanks,

Michael



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
Click to expand...

Big power resistor!

John
 
mrdarrett@gmail.com wrote:
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?
Click to expand...
I'm not sure what the exact function of the '3904 is, I've seen similar
circuits without it. There are better ways (see LM723(?) linear
regulator controller).

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
Click to expand...
That would certainly work.

But before you go there, consider a few things:

(a) the battery may be sagging under load. You may get better
performance just by putting in a bigger, newer, or higher current
capacity (_not_ amp-hour capacity, but higher surge capacity) 6V battery.

(a2) you can check this by measuring the motor's terminal voltage under
load; if it's much less than 6V start chasing around for the lossy parts
of the circuit.

(b) A high-power series resistor from a 12V battery will give you a
better top speed, and some measure of short-circuit protection. It'll
also get HOT. I'd design for a 15-20 amp stall current from 12V, which
would give you the same starting characteristics, plenty of zip, and
lots of heat.

(b2) and you'll need a physically bigger battery if you want to go as
far, because of all that heat you're generating in the resistor.

(c) A buck converter is the 'right' way to go. There are
controller/driver chips that will do this with NMOS parts switching the
positive rail, and you can always switch the negative rail to the motor.
Either way you can control the current in the circuit.

(c2) But if you're at the stage of looking up schematics on the web
you'll have to learn a lot before you could apply a buck converter.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" gives you just what it says.
See details at http://www.wescottdesign.com/actfes/actfes.html
 
mrdarrett@gmail.com wrote:
On Oct 4, 9:38 am, Tim Wescott <t...@seemywebsite.com> wrote:
mrdarr...@gmail.com wrote:
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?
Click to expand...
-- snip --
(b) A high-power series resistor from a 12V battery will give you a
better top speed, and some measure of short-circuit protection. It'll
also get HOT. I'd design for a 15-20 amp stall current from 12V, which
would give you the same starting characteristics, plenty of zip, and
lots of heat.


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...
Click to expand...
-- snip --

I should have thought of that...

Look into 6V car (or motorcycle) headlamps. They should be cheap, and
you can hook as many as you need in parallel to drop down to 6V at the
motor. Even better, the resistance will go down as the current goes
down and they cool, so they'll give you an even higher top speed (I see
spun bearings in your future).

The only downside to putting them in parallel is that if one goes the
other(s) will see higher currents, so they may go pop-pop-pop.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" gives you just what it says.
See details at http://www.wescottdesign.com/actfes/actfes.html
 
On Oct 4, 9:38 am, Tim Wescott <t...@seemywebsite.com> wrote:
mrdarr...@gmail.com wrote:
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'm not sure what the exact function of the '3904 is, I've seen similar
circuits without it. There are better ways (see LM723(?) linear
regulator controller).
Click to expand...

Interesting; I'll look into that...

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

That would certainly work.

But before you go there, consider a few things:

(a) the battery may be sagging under load. You may get better
performance just by putting in a bigger, newer, or higher current
capacity (_not_ amp-hour capacity, but higher surge capacity) 6V battery.
Click to expand...

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...)

(a2) you can check this by measuring the motor's terminal voltage under
load; if it's much less than 6V start chasing around for the lossy parts
of the circuit.

(b) A high-power series resistor from a 12V battery will give you a
better top speed, and some measure of short-circuit protection. It'll
also get HOT. I'd design for a 15-20 amp stall current from 12V, which
would give you the same starting characteristics, plenty of zip, and
lots of heat.
Click to expand...

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...


(b2) and you'll need a physically bigger battery if you want to go as
far, because of all that heat you're generating in the resistor.

(c) A buck converter is the 'right' way to go. There are
controller/driver chips that will do this with NMOS parts switching the
positive rail, and you can always switch the negative rail to the motor.
Either way you can control the current in the circuit.

(c2) But if you're at the stage of looking up schematics on the web
you'll have to learn a lot before you could apply a buck converter.
Click to expand...

Yep, especially for 20A...

Thanks,

Michael
 
On Oct 4, 9:38 am, Tim Wescott <t...@seemywebsite.com> wrote:

....zap...

(a) the battery may be sagging under load. You may get better
performance just by putting in a bigger, newer, or higher current
capacity (_not_ amp-hour capacity, but higher surge capacity) 6V battery.
Click to expand...

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

Thanks,

Michael
 
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