# Voltage divider

Discussion in 'Electronic Design' started by Joe.T, Oct 16, 2006.

1. ### Joe.TGuest

Hi
I am designing a photodiode-amplification circuit.
I only have 1 power source (0 - 30V). I was thinking about using 0
and 30V as vee and vcc for my opamp (LM741), and making the photodiode
(fds100) signal float on 15V.

1. Will this work?
2. How do I make a stable 15V 'power source' out of my 30V source,
resistors, and opamps etc.?
My idea was a voltage divider, but how do I make a voltage divider
which is as least dependant on it's load as possible?

Thanks!

2. ### PeteSGuest

For the power source, try using a 7815 (assuming low current as seems to
be the case). These are availabvle in a bewildering array of packages
from a wide variety of manufacturers and are very inexpensive and easily
available.

Cheers

PeteS

3. ### Guest

I don't think I've ever designed a photodiode circuit, or if I did it
was a long time ago. Off hand, I would say that you could use a Zener
diode to regulate the voltage.

http://www.ecircuitcenter.com/Circuits/opreg1/opreg1.htm

4. ### Tom BruhnsGuest

One way to do that is to make the load as stable as possible. I'm
envisioning a circuit in which you have a stable supply voltage, and
just connect a voltage divider with equal resistances from "vee" to
"vcc". That mid-point voltage then goes to the op amp (+) input, which
is low current and reasonably stable. What will you call "ground" in
this circuit? How will you "read out" the output? The photodiode can
feed current to the op amp (-) input, and a feedback resistor from op
amp output to the (-) input will convert that op amp current to a
voltage drop, but the output voltage is referenced to the voltage at
the (+) input.

If I were doing it with a single supply, since the photodiode current
will be in one direction only, I'd get an op amp with low input bias
current, and rail-to-rail input (or at least input common mode that
includes the negative supply). The op amp should then also have
rail-to-rail output or at least output that can go to the negative
supply as well. Then the same circuit can reference everything to the
negative supply voltage, which you can call ground. If you really need
a bias (which would be negative in this case) on the photodiode for
what you are doing, you can use a 9V alkaline battery, which will last
for the shelf-life of the battery almost certainly, unless you're
shining gobs of light on the photodiode all the time.

In any event, the op amp should have input bias current well below the
minimum photodiode current you wish to detect. If you're doing low
light level work, the 741 is probably not a good choice. Op amps are
cheap, and picking the right one can save you a LOT of hassle.

Cheers,
Tom