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Reducing current via photoconductor

B

Bill Wayne

Jan 1, 1970
0
Hello,

For a small project, I need to keep current inversely related to a
photoconductor's conductivity. In other words, current can flow in the
circuit when the photoconductor is *not* conducting. Note that the
voltage and current is coming from a different part of the circuit,
like a capacitor or battery. My last plan was to connect a zener diode
to it so that when the current is high enough, the diode would reverse
it's permitted direction of flow and therefore stop the current. When
light is not shining on the photoconductor, it resists enough to drop
the current below the level of the breakdown current.

Then I found out that zener diodes work via voltage, not current. So I
still need to find a way. I understand it's possible by using a couple
of transistors, but I'm not sure how. So, how would I go about either
A) using transistors to solve my problem, or B) using another way?

Thanks,
Bill
 
Hello,

For a small project, I need to keep current inversely related to a
photoconductor's conductivity. In other words, current can flow in the
circuit when the photoconductor is *not* conducting. Note that the
voltage and current is coming from a different part of the circuit,
like a capacitor or battery. My last plan was to connect a zener diode
to it so that when the current is high enough, the diode would reverse
it's permitted direction of flow and therefore stop the current. When
light is not shining on the photoconductor, it resists enough to drop
the current below the level of the breakdown current.

Then I found out that zener diodes work via voltage, not current. So I
still need to find a way. I understand it's possible by using a couple
of transistors, but I'm not sure how. So, how would I go about either
A) using transistors to solve my problem, or B) using another way?

Thanks,
Bill

What exactly is the project?

I am working on something that sounds similar to what you are working
on.
 
E

Eeyore

Jan 1, 1970
0
Bill said:
Hello,

For a small project, I need to keep current inversely related to a
photoconductor's conductivity.

You mean regulate the current according to the *resistance* of your photocell.

That's easy enough. How large is the current you want to regulate What's the
load, what's your power supply ?

Graham
 
B

Bill Wayne

Jan 1, 1970
0
What exactly is the project?

I am working on something that sounds similar to what you are working
on.

I'm working on a coilgun. I'm trying to use a light tripwire by
setting up the circuit so that when a photodiode isn't getting any
light, the circuit completes. It's a little more complex than that, so
the capacitors used for the gun aren't going to be running through the
photodiode (or anything else that can't withstand huge voltages/
currents).

Bill
 
B

Bill Wayne

Jan 1, 1970
0
You mean regulate the current according to the *resistance* of your photocell.
Yes.

That's easy enough. How large is the current you want to regulate What's the
load, what's your power supply ?

I'm not looking at a very large current. The power is coming from a
capacitor, a 100 microfarad 100 volt one at most (although more likely
from a 10 volt one).

Bill
 
E

Eeyore

Jan 1, 1970
0
Bill said:
I'm working on a coilgun. I'm trying to use a light tripwire by
setting up the circuit so that when a photodiode isn't getting any
light, the circuit completes. It's a little more complex than that, so
the capacitors used for the gun aren't going to be running through the
photodiode (or anything else that can't withstand huge voltages/
currents).

So you actually want the current to be *switched* not regulated ?

Connect your photo-detector to a comparator and use the comparator to drive a
power device.

Graham
 
B

Bill Wayne

Jan 1, 1970
0
So you actually want the current to be *switched* not regulated ?

Connect your photo-detector to a comparator and use the comparator to drive a
power device.

I did some reading on comparators but I'm not to sure I understand how
they work, or how to apply them. Is the following correct?

The comparator has five connections: two switches, two inputs, and one
output. When Switch A has a higher voltages that Switch B, the output
becomes Input A. The same applies for Switch B and Input B. Therefore,
to achieve what I want, I would connect the photoconductor to Switch B
and the power supply to Input A.

Bill
 
F

Fred Bloggs

Jan 1, 1970
0
Bill said:
Hello,

For a small project, I need to keep current inversely related to a
photoconductor's conductivity. In other words, current can flow in the
circuit when the photoconductor is *not* conducting. Note that the
voltage and current is coming from a different part of the circuit,
like a capacitor or battery. My last plan was to connect a zener diode
to it so that when the current is high enough, the diode would reverse
it's permitted direction of flow and therefore stop the current. When
light is not shining on the photoconductor, it resists enough to drop
the current below the level of the breakdown current.

Then I found out that zener diodes work via voltage, not current. So I
still need to find a way. I understand it's possible by using a couple
of transistors, but I'm not sure how. So, how would I go about either
A) using transistors to solve my problem, or B) using another way?

You must be dreaming if you think *anyone* can figure out what the hell
kind of circuit you're talking about. Just skip your idea of
abstraction, Einstein, and tell us more specifically about how and what
you have hooked up and what it's supposed to do.
 
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