Introduction: Variable Motor Driver
This article is about a simple motor driver. The motor driver can be placed into a box with only two terminals. However, you will need to connect a diode across the motor.
The D1 diode is needed to ensure transistor saturation and thus reduce the power dissipation. The second use of this diode is the prevention of transistor collector-emitter reverse biasing that will damage the transistor. The D2 diode is needed to prevent damage due to reverse currents of the motor.
This is definitely not the cheapest solution for a motor driving circuit. The cost of the circuit can be reduced by replacing the Rb and Ro resistors with a short circuit. However, you might need the Rb and Ro resistors to prevent the failure of the transistor.
You might need a power supply of 4.5 V or even 6 V. It depends on the type of motor that you are using.
Components: Power transistor (TO3 or TO220 package), heat sink, bolt, washer, nut, 10-ohm resistor (5 Watt), 10-ohm resistor (1 Watt), general-purpose diode - 2, power source (3 V or two AA/AAA/C/D batteries), 1-kohm potentiometer, solder.
Optional components: Heat transfer paste, masking tape, cardboard box.
Tools: wire stripper, screwdriver, soldering iron.
Optional tools: Multimeter, pliers.
Step 1: Design the Circuit
I used the old PSpice simulations software to draw and simulate the circuit to reduce time.
The minimum voltage between power supply and Rb/D1/Ro node will not fall below 0.9 V (Vd + VceSat = 0.7 V + 0.2 V = 0.9 V).
The power transistor base might have an internal resistance as high as 100 ohms. This resistance is reciprocal to transistor biasing current. Higher biasing currents will reduce the base resistance.
Step 2: Simulations
You can see that the maximum current is very small. This is because the 2N2222 transistor is not a power transistor but a general-purpose transistor. The student edition software does not have power transistor components.
Step 3: Make the Circuit
You need to purchase a power transistor if you are building this circuit. You will also need a heat sink (TO3 or TO220 heat sink). I used a TO220 transistor. TO3 packaged transistors and TO3 heat sinks are more expensive.
I used Ro resistor of 27 ohms because I did not have a second 5 W 10-ohm resistor.
Heat sink calculations are shown here:
Step 4: Encasement
I used an old cardboard box from an eBay electronic components parcel.
Step 5: Testing
Testing showed that the circuit works with 27 ohm Ro resistor.
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