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<mrdarrett@xxxxxxxxx> wrote in message
news:1155092195.721314.127360@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
>
> pschoen@xxxxxxxxxx wrote:
>> mrdarrett@xxxxxxxxx wrote:
>> > mrdarrett@xxxxxxxxx wrote:
>> > > I have an old laptop (366MHz) which takes in 18.5V @ 3A by external
>> > > (switching?) power supply. (It didn't have a battery with it when I
>> > > bought it - got it 2 years ago for about $200.)
>> > >
>> > > I recently built an external battery pack, powered by 16 Energizer
>> > > 1.2V
>> > > 2500mAh NiMH AA cells, to power my laptop in lieu of the external
>> > > power
>> > > supply, and it does work. So far I have been able to run the laptop
>> > > for about an hour and a half, and still going strong. (Decided to
>> > > stop
>> > > stress-testing at 1:15am, since I had work the next morning.) A far
>> > > cry from paying $150 for a "new" laptop battery, when the laptop
>> > > only
>> > > cost me $200. Although, my 16 NiMH AA battery pack screams "GEEK!"
>> > > at
>> > > anyone who glances at it... ;-)
>> > >
>> > > For my next project, a DC-DC converter.
>> > >
>> > > So... I'd like to convert 12VDC from a jump-start car battery to
>> > > 18.5VDC, at 3A.
>> > >
>> > > After reading a recent thread about avoiding Maxim like the plague,
>> > > I
>> > > went over to TI's website, entered my parameters (Input V: 12VDC;
>> > > output V: 18.5 VDC; current: 3A.)
>> > >
>> > > I found this from the search result:
>> > >
>> > > http://focus.ti.com/lit/ds/symlink/tps5430.pdf
>> > >
>> > > but I was a bit confused why this would have been suggested, since
>> > > it
>> > > seems to be a step-DOWN regulator.
>> > >
>> > > Any suggestions...?
>> > >
>> > > Thanks,
>> > >
>> > > Michael
>> >
>> >
>> > Eh, no response from sci.electronics.basics; belatedly cross-posting
>> > to
>> > SED. Better than multiposting... ;-)
>> >
>> > Michael
>>
>> You could use an isolated 12 V to 6 V DC-DC converter and add the
>> output to your 12 volts from the battery. You could probably run it
>> with a full square wave without regulation to simplify feedback. Just a
>> high frequency transformer driven by something like a UC3526 and a
>> couple MOSFETs, and a simple rectifier and filter on the isolated
>> output in series with the battery voltage to get what you need. You
>> could use the PWM to make it regulated if you want.
>>
>> Paul
>
>
> Say... how would I get the 6V DC isolated? I was thinking of using a
> transformer driven by a PWM circuit I built awhile back and just happen
> to have in a box, but where could I get a 12VDC@xxxx (primary) to
> 6VDC@3A (secondary) transformer? Jameco doesn't seem to carry such a
> beast...
>
> Michael
>
You will probably need to "roll your own", but it should really be nothing
much more than a small toroid about 1" diameter, wound with a couple dozen
turns of wire, if you use about 50 kHz PWM. #22 to #26 AWG wire should be
OK for 3 amps or so. Otherwise, you could probably use parts of an old
computer power supply. Of course, you can just buy a DC-DC converter, but
18 watts will probably cost at least $50.
Paul
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