Specifications

LM2907, a very low cost speed control can be constructed.

Figure 16

the most simple version is illustrated where the

tachometer drives the non-inverting input of the comparator

up towards the preset reference level. When that level is

reached, the output is turned off and the power is removed

from the motor. As the motor slows down, the voltage from

the charge pump output falls and power is restored. Thus

speed is maintained by operating the motor in a switching

mode. Hysteresis can be provided to control the rate of

switching. An alternative approach which gives proportional

control is shown in

Figure 17.

Here the charge pump inte-

grator is shown in a feedback connection around the opera-

tional amplifier. The output voltage for zero speed is equal

to the reference voltage set up on the potentiometer on

the non-inverting input. As speed increases, the charge

pump puts charge into capacitor C2 and causes the output

OUT

to fall in proportion to speed. The output current of the

op amp transistor is used to provide an analog drive to the

motor. Thus as the motor speed approaches the reference

level, the current is proportionately reduced to the motor so

that the motor gradually comes up to speed and is main-

tained without operating the motor in a switching mode. This

is particularly useful in situations where the electrical noise

generated by the switching mode operation is objectionable.

This circuit has one primary disadvantage in that it has poor

load regulation. A third configuration is shown in

Figure 18.

This employs an LM2907-8 acting as a shunt mode regula-

tor. It also features an LED to indicate when the device is in

regulation.

TL/H/7451–33

FIGURE 16. Motor Speed Control

TL/H/7451–34

FIGURE 17. Motor Speed Control with Proportional Drive