Simple DC Motor Driver
This simple DC motor driver circuit uses a 741 operational amplifier operating as a voltage follower where its non inverting input is connected to the speed and rotation direction of a potentiometer VR1. When VR1 is at mid position, the op-amp output is near zero and both Q1 and Q2 is OFF.
When VR1 is turned towards the positive supply side, the output will go positive voltage and Q1 will supply the current to the motor and Q2 will be OFF. When VR1 is turned to the negative supply side, the op-amp output switches to the negative voltage and Q1 will turn OFF and Q2 ON which reverses the rotation of the motor's direction.
As the potentiometer VR1 is moved toward either end, the speed increases in whichever direction it is turning.
The TIP3055 Q1 NPN power transistor has a collector current specs of 15A and VCE0 of 60V DC.
The MJE34 Q2 PNP power transistor has a collector current specs of 10A and VCE0 of 40V DC.
Parts List
THE H-BRIDGE
Perhaps the most common way to control DC motors is to use the H-bridge network, as
shown in Fig. 18.6 (see the parts list in Table 18.4). The figure shows a simplified H-bridge;
some designs get quite complicated. However, this one will do for most hobby robot applications.
The H-bridge is wired in such a way that only two transistors are on at a time. When
transistor 1 and 4 are on, the motor turns in one direction. When transistor 2 and 3 are on,
the motor spins the other way. When all transistors are off, the motor remains still.
Note that the resistor is used to bias the base of each transistor. These are necessary to prevent
the transistor from pulling excessive current from the gate controlling it (computer port,
logic gate, whatever). Without the resistor, the gate would overheat and be destroyed. The
actual value of the bias resistor depends on the voltage and current draw of the motor, as well
as the characteristics of the particular transistors used. For ballpark computations, the resistor
is usually in the 1K- to 3K-ohm range. You can calculate the exact value of the resistor
using Ohm’s law, taking into account the gain and current output of the transistor, or you can
experiment until you find a resistor value that works. Start high and work down, noting when
the controlling electronics seem to get too hot.
Pulse Width Modulation DC Motor Control
- R2 adjusts the speed of the oscillator and therefore the speed of M1.
- M1 can be any DC motor that operates from 6V and does not draw more than the maximum current of Q1. The voltage can be increased by connecting the higher voltage to the switch instead of the 6V that powers the oscillator. Be sure not to exceed the power rating of Q1 if you do this.
- Q1 will need a heatsink.
- Q1 in the parts list can handle a maximum of 5A. Use the IRF620 for 6A, if you need any higher.
- This circuit is not a true pulse width modulation control. Because only the frequency of pulses varies, it is really pulse frequency modulation. This works, though not as well as true PWM.
