Re: current limiter / voltage regulator question


"panfilero" <panfilero@xxxxxxxxx> wrote in message
news:1170079659.906597.260680@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx


"all" you need to do is scale the 0-5V up to 0 - 30V the 15 ohm
resistance
will convert the voltage to 0-2 A


My load changes non-linearly with the voltage across it, that's when
i'm trying to steer towards a steady current. My load is a Shape
Memory Alloy wire.


Then use a meter to measure the current. Surely the load doesn't change with
a fixed voltage applied to it? Essentially you will be building a constant
current source anyways(ultimately its a voltage regulator anyways and you
can find schematics to build constant current sources in many of the voltage
regulators datasheets).

That is, even if your load is non linear as long as your load does vary with
time then it should be good enough.

i.e., by Ohms law V = F(I). As long as F isn't to complicated you can solve
for I in terms of V. If we assume F is injective on some bounded interval
then we can find the inverse of F.

So how to do this easily in practice? If you know the bounds on your load
then you can plug in a voltage and measure the current and then adjust V
until you get the current you want. Essentially you yourself is the constant
current source.

Say your load registers 1A at 10V but you want to supply 1.5A's. What to do?
Try increasing V to 11V and see how much your current changes and which
direction(although its most likely in the same direction as the change in
current).

Doing this you can put any current you want through the load and as long as
the load doesn't depend on t to much then it will be pretty accurate.

Essentially you can map out the V-I curve doing this and get the dynamic
resistance of your load. I don't know about memory alloys but as long as
they don't get to hot then it will work fine. It really depends on your
application though. You'll just get a different V-I graph for different
temperatures.

If you plan on doing a lot of work with it you will be better suited to get
a constant current source though. It would be much easier to turn then dial
to the current you need instead of measuring the current and voltage.
(although it wouldn't be so bad with two multimeters, one measuring current
and the other voltage.)

Ofcourse there are some issues with my method above as there is no current
regulation. If the load changes with time it could cause some serious
problems. A constant current source is just a voltage source that will
change the voltage to that it can deliver a constant current. Essentially my
quick and dirty method above is where you become the control feedback
circuit. Not the best way but works in a pinch.

Jon





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