Programmable source powers dc micromotors

VK Dubey, JP Rao, and P Saxena, Centre for Advanced Technology, Indore, India -- EDN, 3/1/2001

The circuit in Figure 1 is a simple, economic, compact, and tricky way of using the LM723 as a programmable voltage source to drive dc micromotors. Because of the µPs' accurate positioning and control, these motors are useful in applications such as optical mounts and flexible shaft control, which take advantage of the higher speed and fast movement of servo controls compared with stepper motors. These designs require a stable, programmable dc-voltage source.

The LM723 is a fixed linear regulator, but this application configures the regulator as a programmable voltage source. You can set the output to a value of 200 mV to 6V. The output, an emitter-follower type, provides low output impedance. The circuit limits the maximum output current to the load, or the motor, at 75 mA. The output of an 8-bit DAC and a current/voltage converter provide a variable reference voltage. At the noninverting input of the LM723, you need to adjust the value of R₁ so that the maximum reference voltage does not exceed 8.5V. Because the reference voltage comes from an external source, the circuit doesn't use the internal voltage reference of the LM723. The circuit also incorporates short-circuit current limiting and remote shutdown. Varying the output voltage changes the speed of the motor that connects across the output.

You adjust the minimum output voltage of 200 mV by offsetting the DAC output with zero data, and successive DAC input codes increase the voltage-source output to 6V. You can use a single-chip µC for controlling the speed through the DAC, the direction, and the brake. The no-load maximum speed is 15,100 rpm. By attaching a reduction gear-head with a ratio of 529-to-1, the maximum frequency from the magnetic encoder in response to maximum speed is 2.8 kHz. The circuit feeds back this signal to the µC to measure the speed. The linearity of the voltage source is good over a voltage, temperature, and speed range (Figure 2). With only slight modifications in component values and ratings, you can use this same LM723 configuration in other similar applications for higher output voltages.