I have a 24 volt wheelchair that runs on 2 12 volt batteries in
series. I need output not to exceed 13 volts for my cpap machine.
There are many DC-DC converters that are labeled 24 to 12 but it seems
all of these put out 13.5 or 13.8 volts. The cpap manufacturer clearly
stated that any voltage above 13 may damage the equipment. Is there a
way to reliably shave a volt or so off of the converter output?
Perhaps by using a diode in series? The Cpap is rated at 40watts max
or 3 amps at 13 volts. Any suggestions are welcomed, the more specific
the better.
Thanks

dpondoc@gmail.com wrote:
> I have a 24 volt wheelchair that runs on 2 12 volt batteries in
> series. I need output not to exceed 13 volts for my cpap machine.
> There are many DC-DC converters that are labeled 24 to 12 but it seems
> all of these put out 13.5 or 13.8 volts. The cpap manufacturer clearly
> stated that any voltage above 13 may damage the equipment. Is there a
> way to reliably shave a volt or so off of the converter output?
> Perhaps by using a diode in series? The Cpap is rated at 40watts max
> or 3 amps at 13 volts. Any suggestions are welcomed, the more specific
> the better.
> Thanks

The AM 1301-7R is rated 12 volts at up to 4 amps out with 8
to 35 volts in.

http://www.power-one.com/resources/prod ... et/mdc.pdf

They run about $500, though.

Digikey shows lots of other choices, but most, including
this one are not in stock. But their search choices can
give you some part numbers to look for. If you want good
reliability, pick a unit that is rated for a bit more than
you need. 50 to 100 watts is possibly a good max rating for
a 40 watt load.

--
Regards,

John Popelish

On Feb 29, 7:36=A0am, dpon...@gmail.com wrote:
> I have a 24 volt wheelchair that runs on 2 12 volt batteries in
> series. I need output not to exceed 13 volts for my cpap machine.
> There are many DC-DC converters that are labeled 24 to 12 but it seems
> all of these put out 13.5 or 13.8 volts. The cpap manufacturer clearly
> stated that any voltage above 13 may damage the equipment. Is there a
> way to reliably shave a volt or so off of the converter output?
> Perhaps by using a diode in series? =A0The Cpap is rated at 40watts max
> or 3 amps at 13 volts. Any suggestions are welcomed, the more specific
> the better.
> Thanks


I have a couple of questions. Why do you have to use the wheelchair
batteries to power your cpap machine? It would make more sense to
have a 12 volt golf cart or forklift battery sitting by your bed,
dedicated to cpap use. Charge it during the day.
How do you power your cpap machine now?
You would need a good reason to use a battery bank that has an
inappropriate voltage and then down-convert it again, with the added
complexity and losses (and expense) that entails. If you power the
cpap machine directly from a suitable battery you run almost no risk
of a sudden power failure. Add a dc-dc converter to the setup and you
increase the risk.
A dc-dc converter would make sense for use away from home. But if you
stay in hotels, then using a 12 volt power supply that simply plugs
into the wall would cost a lot less and wouldn't draw down your
wheelchair batteries at night.
Just throwing out alternative ideas. But perhaps you have your own
reasons for wanting the converter, like simply taking naps in your
wheelchair, in which case plenty of engineers on this board can advise
you. I see John P has already come up with a suggestion.

<dpondoc@gmail.com> wrote in message
news:88d84610-c5bd-4690-8866-17c202422f31@u10g2000prn.googlegroups.com=20
> I have a 24 volt wheelchair that runs on 2 12 volt batteries in
> series. I need output not to exceed 13 volts for my cpap machine.
> There are many DC-DC converters that are labeled 24 to 12 but it seems
> all of these put out 13.5 or 13.8 volts. The cpap manufacturer clearly
> stated that any voltage above 13 may damage the equipment. Is there a
> way to reliably shave a volt or so off of the converter output?
> Perhaps by using a diode in series? The Cpap is rated at 40watts max
> or 3 amps at 13 volts. Any suggestions are welcomed, the more specific
> the better.
> Thanks

Is there some reason why you can't simply take
a tap from the center of the two battery=20
connection?

Greg Neill wrote:
>
> <dpondoc@gmail.com> wrote in message
> news:88d84610-c5bd-4690-8866-17c202422f31@u10g2000prn.googlegroups.com
> > I have a 24 volt wheelchair that runs on 2 12 volt batteries in
> > series. I need output not to exceed 13 volts for my cpap machine.
> > There are many DC-DC converters that are labeled 24 to 12 but it seems
> > all of these put out 13.5 or 13.8 volts. The cpap manufacturer clearly
> > stated that any voltage above 13 may damage the equipment. Is there a
> > way to reliably shave a volt or so off of the converter output?
> > Perhaps by using a diode in series? The Cpap is rated at 40watts max
> > or 3 amps at 13 volts. Any suggestions are welcomed, the more specific
> > the better.
> > Thanks
>
> Is there some reason why you can't simply take
> a tap from the center of the two battery
> connection?


The batteries are generally charged in series. It wouldn't take long
to destroy the pair.


--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida

On Fri, 29 Feb 2008 07:36:21 -0800 (PST) in sci.electronics.basics,
dpondoc@gmail.com wrote,
>There are many DC-DC converters that are labeled 24 to 12 but it seems
>all of these put out 13.5 or 13.8 volts. The cpap manufacturer clearly
>stated that any voltage above 13 may damage the equipment. Is there a
>way to reliably shave a volt or so off of the converter output?

Perhaps an internal adjustment?

On Mar 1, 2:47=A0am, David Harmon <sou...@netcom.com> wrote:
> On Fri, 29 Feb 2008 07:36:21 -0800 (PST) in sci.electronics.basics,
> dpon...@gmail.com wrote,
>
> >There are many DC-DC converters that are labeled 24 to 12 but it seems
> >all of these put out 13.5 or 13.8 volts. The cpap manufacturer clearly
> >stated that any voltage above 13 may damage the equipment. Is there a
> >way to reliably shave a volt or so off of the converter output?
>
> Perhaps an internal adjustment?

Stick some diodes each with a voltage drop of around 0.7 volts in
series with the output of the 13.7 volt battery?

# Diode Voltage
Diodes Volt drop. Net from 13.5 to 13.8 v

0 0 13.5 to 13.8
1 0.7 12.8 to 13.1
2 1.4 12.1 to 12.4
3 2.1 11.4 to 11.7