Re: but I just need to lower 1 volt

optimizer wrote: 
Unfortunately I just need to lower it around 1 volt.
I had some NiCD batteries, and got NiMH's to replace. The NiCD used to
work fine. The NiMH's seem to charge to a total of around 8 volts or
more even though they are two 3.6 volts batteries.
Anyway, if the input needs to be 3 volts higher, I'm missing 2 volts.
Thanks


I think you may be able to get this to work with just 1 cheap
part.

If all you need to do is lower it (your battery pack) about
1 volt,  and you want to do that as simply as possible, put
a 1N4001 diode in series with the pack and the device, with
the banded end of theconnected to the negative side of the
battery.  That will reduce the voltage about .7 volts.
*However*, the pack will soon reduce to the nominal 7.2 volts,
and the diode will further reduce that to about 6.5.  If your
device is "happy" with that, you can use this simple approach.
It does not precisely meet your stated requirements (#1 and #2,
as it will allow about 7.4 volts output, not 7.2 as you
stated, and it might stop the device from working by reducing
the voltage to the device to 6.5, even when the battery is not
low.)  But it is the simplest you can get, and it stands a good
chance of working for you.

Here's what is going on that is causing your 7.2 volt pack to
provide 8+ volts:  a 7.2 volt pack consists of 6 cells whose
nominal voltage is 1.2 volts per cell (6 * 1.2 = 7.2). When you
charge the batteries, they get charged to a hair over 1.4 volts
per cell (as they should be), and the pack voltage is over 8
immediately after the pack is taken off the charger.  It soon
reduces to 7.2 when used.

Finally - if the single diode approach won't work, use the
LM317 circuit already posted. The input voltage will need
to be over 10 volts - which you can get by using 3 3.6 volt
packs, or a 12 volt DC wall wart supply.

Ed
.

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