From: "Sir Charles W. Shults III"
Subject: Re: Help, design behind a variable current power supply, w/o pots.
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Date: Tue, 22 Oct 2002 06:21:48 GMT
NNTP-Posting-Date: Tue, 22 Oct 2002 02:21:48 EDT
Organization: RoadRunner - Central Florida
Okay, let's take a pretty typical design approach- break it down into
First, you have an input section that takes some sort of control signals
such as digital inputs and uses that information to switch something on or off.
In this case, what we switch will depend on how we control our current source.
Next, you will need something that produces a current on demand, and a
simple means of programming it. You may use a simple resistor-programmed
current source, or you may use a voltage to current converter. In either case,
you now have to interface the first part (the input section) with the second
part (the current source).
If you have a voltage to current converter, you will want your input section
to produce different voltages based on your input, then feed that resulting
voltage to the voltage to current converter. This will give you one way of
doing what you want. This also means that your input section in nothing more
than a digital to analog converter (available as a single device).
If you have a current amplifier, you will program it with an input current
and get some sort of output current from it. You may use simple transistors to
do the switching- that is a simple means of doing it. Break your current into
binary ranges, then pick some transistors and resistors that provide those step
A transistor acts as a simple switch if you saturate it- that means drive it
to its input limits. No input signal will be off, an input signal that forces
it all the way on will make it switch as "on" as it can get. The transistor
will have some resistance, and so will a resistor in series with it. Turning
the transistor fully on will allow some sort of current to flow through it, and
the resistor in series with it will be used to program that current to a value
By placing these transistor switches in parallel, you can sum the currents
from switching them on in any combination, allowing you to create any value you
Note that transistors and other semiconductors will change their resistances
with temperature, so if you have a precision application, this will be a poor
solution. For brute force, however, it will work. Also, you can compensate for
temperature change with a diode, but that is a little beyond the scope of this
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