Switchmode Constant Current Source


Operating a stepper
motor using a fixed (constant) voltage supply results in poor torque at
high speeds. In fact, stepper motors tend to stall at fairly low speeds
under such conditions. Several approaches can be used to overcome this
problem, one of which is to use a constant current supply in place of
the more conventional constant voltage supply. A disadvantage of many
constant current supplies is that simple circuits are inefficient but
that doesn’t apply to switchmode supplies such as the circuit shown
here.

Basically, this circuit is a conventional switchmode regulator
adapted for constant current output and is specially designed for
stepper motor drivers – although it could be used for other applications
as well. The circuit works as follows: IC1 (LM2575T) and its associated
components (D1, L1, C1, etc) operate as a switchmode power supply.
Normally, for constant voltage operation, the output is connected –
either directly or via a resistive divider – back to the feedback input
(pin 4) of IC1.

Switchmode Constant Current Source

Switchmode Constant Current Source

In this circuit, however, Q1 senses the current flowing through R1 and
produces a corresponding voltage across R3. This voltage is then fed to
pin 4 of IC1. As a result, the the circuit regulates the current into a
load rather than the voltage across the load. Only one adjustment is
needed: you have to adjust VR1 for optimum stepper motor performance
over the desired speed range. The simplest way to do this is to measure
the motor current at its rated voltage at zero stepping speed and then
adjust VR1 for this current. The prototype worked well with a stepper
motor rated at 80O per winding and a 12V nominal input voltage. Some
components might have to be modified for motors having different
characteristics.
Author: H. Nacinovich – Copyright: Silicon Chip Electronics


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