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Constant High-Current Source

A companion project to last month's stepper motor controller, this simple high-current source can also charge batteries.

By Ross Tester

Click for larger image
This shot shows the two halves of the project opened out - the Pentium II heatsink with its integral fan on the left and the controller itself on the right. The lower two resistors are chosen according to the output current.
Last month, you will recall we presented a mini stepper motor driver. (Incidentally, our apologies for the gremlin which got into the system and caused most of the earth symbols and one resistor to disappear. No, we don't know why either!)

That stepper motor driver operates from about 8-35V DC but as we pointed out, a stepper motor really likes to have a constant current source so that the motor current (and therefore power/torque) remains constant throughout the stepper's speed range.

This, then, can be regarded as a companion to the Stepper Motor Controller. It is capable of delivering more than 10A with suitable heatsinking - and we cover that shortly.

However, there are a lot of other applications for a constant current source. Nicad battery charging is one which immediately springs to mind. Anything where the constant colour temperature of a globe is important (such as phototographics) is another. And in electronics, there are countless occasions where constant current circuits are used.

So while we're presenting this specifically for the Stepper Motor Controller, it could be used in a raft of projects and circuits.

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