# How to get a constant current source?

T

#### [email protected]

Jan 1, 1970
0
I have been frustrated for months working through an introductory
circuits book, as I want to build many of the circuits, but many of
them include constant current sources. Why is it so easy to find
voltage sources at your local store (i.e., batteries!), but searching
for current sources on the web leads to a complicated bunch of circuit
diagrams?

Here is my naive question: using thevenin-norton equivalent circuits,
couldn't I transform a battery (i.e., voltage source) into the desired
equivalent current source using Vth=InorReq. That is, can I put a
voltage source in series with a resistor (as opposed to its equivalent,
a current source in parallel with the same resistor)? What is the
problem with doing that?

Does anyone know where I could buy a cheap but reliable current source?
Why is this so hard?

T

#### Tom Biasi

Jan 1, 1970
0
I have been frustrated for months working through an introductory
circuits book, as I want to build many of the circuits, but many of
them include constant current sources. Why is it so easy to find
voltage sources at your local store (i.e., batteries!), but searching
for current sources on the web leads to a complicated bunch of circuit
diagrams?

Here is my naive question: using thevenin-norton equivalent circuits,
couldn't I transform a battery (i.e., voltage source) into the desired
equivalent current source using Vth=InorReq. That is, can I put a
voltage source in series with a resistor (as opposed to its equivalent,
a current source in parallel with the same resistor)? What is the
problem with doing that?

Does anyone know where I could buy a cheap but reliable current source?
Why is this so hard?

A constant current source would keep the current constant as the load
changed. If the load were a small percentage of the voltage source's
internal resistance the small changes in load would be small compared to the
source resistance and the current would change very little.
Examine a 12 volt source with a series resistance of 1 Meg. Ohm. How would
the current change as the load varied from 100 ohms to 1K ohms?
Tom

R

#### redbelly

Jan 1, 1970
0
... Why is it so easy to find
voltage sources at your local store (i.e., batteries!), but searching
for current sources on the web leads to a complicated bunch of circuit
diagrams?

Because batteries involve chemistry, and it is the nature of the
chemicals or matierals used to maintain a constant potential
difference. This is simply how nature works.

To make a constant current source requires (usually) some sort of
active feedback loop, to monitor the current output and adjust the
power source accordingly.

Mark

T

#### [email protected]

Jan 1, 1970
0
Sorry I posted the same message twice. THe first time something
happened with my (Windows) browser and I thought it didnt' go through.
Thanks for your help, and first post looks like it generated some
suggestions I will have to work through.

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