Re: How to get a constant current source?
- From: "Tom Biasi" <tombiasi@xxxxxxxxxxxxxxxxxxx>
- Date: Fri, 17 Jun 2005 16:41:14 -0400
<thomson.eric@xxxxxxxxx> wrote in message
news:1119035382.037627.145820@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
>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
>
.
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