# 7.26V output from a 9V battery

Discussion in 'Electronic Basics' started by [email protected], Feb 11, 2005.

1. ### Guest

Hello,
I am new to electronics and I have a project to interface an active
electrode that need a clean 7.26V (+- 0.02V) supply voltage. I would
like to use a 9V battery. The active electrode comsumes 0.7mA. Can I
simply use a series of resistors or do I need a voltage regulator or a
converter?

Thanks

Daniel Jeffrey

2. ### John PopelishGuest

since the voltage from a 9 volt battery varies from about 9.5 to zero
over the life of the battery, you will definitely need an active
regulator of some sort to hold a 0.7 mA load to a 7.24 to 7.28 volts.
The most common and easy to apply regulators need at least 1.5 volts
to waste, so they are a bit tight for this spec, but some of the low
current low drop out (need less voltage to waste) regulators might be
appropriate.

For example:
http://www.fairchildsemi.com/ds/LP/LP2951.pdf

3. ### MontrealGuest

Thanks I'll look at your example.

Much appreciated.

Daniel

4. ### Bill BowdenGuest

Can I simply use a series of resistors or do I need a voltage
You need a regulator but the input has to be at least a couple volts
higher than the output, so you might want to use a 9 volt battery
and a couple AAA cells so you have 12 volts. There is a low
current 5 volt regulator 78L05 that can be raised to 7.26 volts
with a couple resistors. Connect a couple resistors in series
from the output to ground and tie the common pin of the
regulator to the junction of the 2 resistors. Maybe a 10K
resistor at the output and something around 5K for the other.
Then adjust one of the resistors to get the right output.

But, I'm not sure this will hold the tollerance you want
of +/- 0.02

It may also require a minimum load of more than a couple
mA. In that case you will need to add another resistor
from output to ground so the regulator operates at
5 mA, or whatever minimum is required.

-Bill

5. ### peterkenGuest

that's some *really* weird voltage to use...
might be easiest to just build a simple transistor regulator, say a straight
forward emitter follower or possibly a more complex model controlled by a
rail-to-rail opamp (lm158/258, lm324, or something)
might even use the output of the opamp directly as supply for that kind of
low-power
for reference just use a 5V1 zener @ 10mA, most stable at low cost

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