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'[EE]: 12 volts from 24 with backwards charge pump?'
2000\11\29@084916 by Nicholas Irias

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I have a pair of 12 volt batteries wired in series for 24 volts.  In
addition to running 24 volt motors, I want to run around 10 amps worth of 12
volt devices.

I was contemplating something like a backwards charge pump, to alternately
use the two 12 v batteries to maintain charge in a single capacitor.  I
would use a fairly fast switching speed to ensure that rate of discharge for
each battery is approximately equal.

I know that voltage doubling charge pumps are not usefull for high current
loads, but I thought that was due to the fact that only one of the
capacitors can be charged at a time.  In this case, the single 12 volt
capacitor would be getting charge 100% of the time (minus a little time lost
during switching).

Is this a workable idea, or do I need a DC-DC converter to get the high
current 12 volt supply?

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2000\11\29@093215 by Olin Lathrop

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> I have a pair of 12 volt batteries wired in series for 24 volts.  In
> addition to running 24 volt motors, I want to run around 10 amps worth of
12
> volt devices.
>
> I was contemplating something like a backwards charge pump,

Sounds like you want a buck converter.


*****************************************************************
Olin Lathrop, embedded systems consultant in Devens Massachusetts
(978) 772-3129, spam_OUTolinTakeThisOuTspamembedinc.com, http://www.embedinc.com

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2000\11\29@110512 by mike

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On Wed, 29 Nov 2000 05:49:35 -0800, you wrote:

>I have a pair of 12 volt batteries wired in series for 24 volts.  In
>addition to running 24 volt motors, I want to run around 10 amps worth of 12
>volt devices.
>
>I was contemplating something like a backwards charge pump, to alternately
>use the two 12 v batteries to maintain charge in a single capacitor.  I
>would use a fairly fast switching speed to ensure that rate of discharge for
>each battery is approximately equal.
>
>I know that voltage doubling charge pumps are not usefull for high current
>loads, but I thought that was due to the fact that only one of the
>capacitors can be charged at a time.  In this case, the single 12 volt
>capacitor would be getting charge 100% of the time (minus a little time lost
>during switching).
>
>Is this a workable idea, or do I need a DC-DC converter to get the high
>current 12 volt supply?

Forget charge pumps at this current level. You need a switchmode
step-down converter, and a fairly meaty one. If you can split the 12v
loads you may be able to use a few nat semi simple-switcher chips - I
think there ares ome that go to about 5A.

Alternatively could you use the centre-tap between the batteries? -
this would be much more efficient. If you could balance the 12v
devices between the 2 batteries this would be even better.  
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2000\11\29@111143 by Bob Ammerman

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---- Original Message -----
From: Olin Lathrop <.....olin_piclistKILLspamspam@spam@EMBEDINC.COM>
To: <PICLISTspamKILLspamMITVMA.MIT.EDU>
Sent: Wednesday, November 29, 2000 9:22 AM
Subject: Re: [EE]: 12 volts from 24 with backwards charge pump?


> > I have a pair of 12 volt batteries wired in series for 24 volts.  In
> > addition to running 24 volt motors, I want to run around 10 amps worth
of
> 12
> > volt devices.
> >
> > I was contemplating something like a backwards charge pump,
>
> Sounds like you want a buck converter.

What is different about this application is that he has access to the
midpoint of the two batteries.

I would think his backward (or is it inside-out ;-) ) charge pump makes a
lot of sense.

Bob Ammerman
RAm Systems
(contract development of high performance, high function, low-level
software)

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2000\11\29@113435 by Morgan Olsson

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>you may be able to use a few nat semi simple-switcher chips

I have found them rather lossy, as the output switch is dropping some voltage during "on" periods and my app got pretty hot.

I suggest using a suitable controller chip + MOSFET.

/Morgan

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2000\11\29@140324 by hard Prosser

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There is a circuit arrangement that looks very similar to a capacitive
voltage doubler (bridge type)  but uses inductors in place of capacitors to
double the current. This may be more appropriate for the current levels you
are using.
If you're interested I'll try and dig out a copy of the circuit. My main
concern would be it uses a transformer input which somewhat negates its
usefullness unless you're trying to make do with existing components. An
alternative drive method may be possible though.

Otherwise I'd suggest a simple buck regulator - or even have a try at
something more exotic like a resonant type converter for higher effiviency.

I guess you can't just run half the gear off one battery and half off the
other with a common earth at midpoint?

Richard P






                   Nicholas Irias
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I have a pair of 12 volt batteries wired in series for 24 volts.  In
addition to running 24 volt motors, I want to run around 10 amps worth of
12
volt devices.

<snip>

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2000\11\29@145644 by Bob Ammerman

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What is wrong with the original poster's idea?

Imagine two 12V batteries in series, with a 'center tap' connection.

Now imagine a double pole double throw switch.

The common terminals of the switch are connected to a sizable capacitor and
thence to the 12V load.

When the switch is in one position it connects the capacitor to one battery

When the switch is in the other position is connect the capacitor to the
other battery.

Now, imagine replacing the switch with the appropriate semiconductors and
drive it with a [PIC]:

You can switch as slowly as you want, since the cap only has to hold up the
output during the 'flying' time of the 'switch'.

Rather a simple circuit.

You do have to keep the 12V system's ground isolated from that of the 24V
system.

In fact, relative to the 24V system the 12V system's ground is a 12V square
wave!


Bob Ammerman
RAm Systems
(contract development of high performance, high function, low-level
software)

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2000\11\29@154737 by Olin Lathrop

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> What is wrong with the original poster's idea?
>
> Imagine two 12V batteries in series, with a 'center tap' connection.
>
> Now imagine a double pole double throw switch.
>
> The common terminals of the switch are connected to a sizable capacitor
and
> thence to the 12V load.
>
> When the switch is in one position it connects the capacitor to one
battery
>
> When the switch is in the other position is connect the capacitor to the
> other battery.
>
> Now, imagine replacing the switch with the appropriate semiconductors and
> drive it with a [PIC]:
>
> You can switch as slowly as you want, since the cap only has to hold up
the
> output during the 'flying' time of the 'switch'.
>
> Rather a simple circuit.
>
> You do have to keep the 12V system's ground isolated from that of the 24V
> system.
>
> In fact, relative to the 24V system the 12V system's ground is a 12V
square
> wave!

This is fine if you can tolerate having the 12V and 24V systems at different
grounds.  I just assumed it wasn't fine since that is a bit unusual, and the
original post was a bit too long to read all of.  If the two systems can't
be isolated, then I still think a 24V to 12V buck converter is a reasonable
choice.


*****************************************************************
Olin Lathrop, embedded systems consultant in Devens Massachusetts
(978) 772-3129, EraseMEolinspam_OUTspamTakeThisOuTembedinc.com, http://www.embedinc.com

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2000\11\30@154826 by steve

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> I have a pair of 12 volt batteries wired in series for 24 volts.  In
> addition to running 24 volt motors, I want to run around 10 amps worth of 12
> volt devices.

If your 12V devices are simple loads like light bulbs, solenoids,
etc, you may be able to just provide them with a fixed, open loop 50%
duty cycle, chopped 24V.

Steve.
======================================================
Steve Baldwin                Electronic Product Design
TLA Microsystems Ltd         Microcontroller Specialists
PO Box 15-680, New Lynn      http://www.tla.co.nz
Auckland, New Zealand        ph  +64 9 820-2221
email: stevebspamspam_OUTtla.co.nz      fax +64 9 820-1929
======================================================

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2000\11\30@164047 by Gordon Varney (personal)

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First, never pull a load from one battery in a series configuration. This
will damage the one battery that is supplying the current to the load 12V
load. At 10Amps, any thing short of a switching converter will require a
heatsink larger than the batteries. A simple 50% duty cycle will not work at
these currents and the output inductor and capacitors will be huge. You
require a converter that has been designed for this purpose. That works at
high frequencies to minimize the Inductor and caps.

Contact:
Translectric Inc. 1-800-333-2589
ask the salesman for a SM2412-10 about $89.93 list
This unit is 85-90 % efficient with a load regulation of 200 mv
and less than 25 mv ripple. This unit is a computer quality power
supply with special filtering on the input. The output current
is guaranteed at better than 10 Amps. The Input is from
18 Vdc - 55 Vdc. This is a Industrial unit, that meet military specs.



Gordon Varney


> > I have a pair of 12 volt batteries wired in series for 24 volts.  In
> > addition to running 24 volt motors, I want to run around 10 amps worth
of 12
> > volt devices.

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'[EE]: 12 volts from 24 with backwards charge pump?'
2000\12\01@042041 by Peter L. Peres
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imho 24V->12V@10A (=~ 120W) is very trivial for a DC/DC converter and you
might get away with a single chip minimal part count solution too.

Peter

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