How to get a constant current source?

Question

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?

redbelly · Accepted Answer

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

Tom Biasi · Answer

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

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thomson.eric · Answer

Sorry I posted the same message twice. THe first time somethinghappened with my (Windows) browser and I thought it didnt' go through.Thanks for your help, and first post looks like it generated somesuggestions I will have to work through.

How to get a constant current source?

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