Motor Driver Design

It is necessary to replace the ULN2803/2003 drivers with your own custom designed drivers just in case you'll be using powerful motors other than those found in floppy disk drives. We'll be using standard transistors this time.

Figure 1 (NPN transistor)

Transistors are commonly used as an electronic switch. They can be used to drive stepper motors, relays. solenoid valves, and even LEDs. This tutorial will only discuss the NPN type using common emitter configuration biasing to simplify equations.

The Circuit

Figure 2 (Circuit)

The figure shows a typical BJT (bipolar junction transistor) with a bias resistor and a diode. An input voltage is required at the base of the transistor to activate the load connected at the collector.

The circuit is based on the one made by Tomi Engdahl ( then@delta.hut.fi ) and this is a link to his website. A wealth of information in his pages!

How It Works

The transistor is in the "ON" state when voltage is applied at the base. This will then allow current to flow to the load (output terminals) and the voltage between collector and emitter will be 0. The transistor is said to be cutoff.

The "OFF" state of the transistor follows the conditions: there is no voltage applied at the base, the voltage between the collector and emitter is equal to the supply voltage. and no current goes to the load. The transistor is said to be saturated.

The transistor amplifies the input current by a factor called hfe (in text books it is also referred to as Beta or current gain). Current gain or simply ß is the ratio of the output current over the input current. The output current will always be ß times the input current.

The output signal is not only bigger than the input signal but it is also inverted from its original form. This is the inverting characteristic of the transistor. I provided an illustration below.

Figure 3 (Signals)

The resistor serves as the bias for the transistor. Decreasing the value of the resistor increases the current output while increasing the resistance decreased the output current.

The diode stops any back voltage from motors and any other inductive loads. Back voltage can damage your parallel port.

Customizing The Circuit

Note: In order to redesign the circuit, you will need some guide in finding suitable replacement/s for the transistor that will satisfy your application. An ECG (Electronic Components Guide) will come in handy or any listing that contains the transistor's operating values and its current gain. Better yet, download datasheets from the manufacturer's website for correct ratings.

Situation: I have a 12V 2A load and I want my PC to control it using the parallel port and an external power supply.

Figure 4 (Block Diagram)

The output voltage of the parallel port is 5 volts. We need a 12V power supply that can deliver at least 2 Amperes for the driver circuit (transistor) and the load.

To make it more realistic, let's say that I want to drive these motors:

Figure 5 (Big Motors)

Note: I have an extra PC power supply from an old casing. It has a 12V tap and I frequently use it for testing circuits because it also has 5V leads.

Step 1: Find a transistor which can handle the required ratings. Look for one which can handle 2 Amperes or more and can operate at 12 volts.

For me, I will use the Motorola MJE3055 NPN transistor, it can handle up to 10 A and a maximum operating voltage of 70 V in a TO-220 casing. This plastic power transistor is overkill and not a logical choice in terms of cost, but this is the only transistor on hand, a left-over from my last school project. Also, I already have its datasheet, downloaded from the Motorola website. According to the datasheet, its ß is a minimum of 20 and a maximum of 100 if the collector current is below 4 A. In computations, use the minimum value.

Step 2: Select a resistor value. The resistor will set the level of the output current. From the given values, we formt he following conclusions:

To computer the value of the resistor, we need three equations:

where:

using equation 1, we derive the input current.

note that the input current is smaller than the output curent.

Next we determine the voltage that flows in the resistor using equation 2

Finally, we compute the value of the resistor using Ohm's law (equation 3).

If for some reason, you get a resistor value that does not have a commercial equivalent, you can connect resistors in series or even in series-parallel to get the value.

But the most feasible is to select the nereast resistor value not only to minimize component count but because it is easier to use a different value.

For example, the required current is 1.91 A and the computation will yield a value of 45 ohms. Looking at the availability list (from Alexan), the two nearest would be 43 ohms and 47 ohms. By using the 3 equations in reverse order we'll see that if we use 43 ohms, the output curent will be 2 A but if we use 47 ohms, the output current will be 1.83 A.

Clearly 47 ohms will not satisfy the required current so we will use 43 ohms.

Step 3: Select a general purpose diode. Almost any diode in the 1N54XX series will do.

revised 07-05-2001

orig 03-20-2001