Darlington Pair Speed Control
By John Hewes

A normal potentiometer (variable resistor) cannot directly control the speed of a motor since motors draw large amounts of current which would burn out the potentiometer.  Instead, the small amount of current that the potentiometer can pass can be amplified into order to run the motor.  This amplification can be achieved using transistors.

The job of a transistor is to allow the small amount of current that enters its 'base' terminal to control the amount of current flowing from its 'collector' terminal to its 'emitter' terminal.  This allows a low power circuit to control a higher power circuit, either in an on/off fashion or linearly.

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A Simple Darlington Pair Circuit

The circuit in figure 1 shows a linear potentiometer connected between Vs and 0V such that the voltage at its wiper terminal will always be somewhere at or between these two voltages.

The small amount of current flowing out of the potentiometer's wiper is amplified by two transistors, connected together in a configuration known as a 'Darlington pair'.  The current from the potentiometer is amplified by the first transistor, and then again by the second transistor, greatly increasing the amount of current that can be controlled by the potentiometer.

There are, however, a couple of disadvantages of this simple circuit.  Firstly, about 0.7V is lost in each transistor, so the maximum voltage that can ever be applied to the motor is Vs - 1.4V.  Secondly, the transistors are not absolutely linear so the change in motor speed for a given rotation of the potentiometer will be more subtle in the middle of its range.

Because a motor is an inductive load, it will produce a 'back-emf' which could damage the second transistor.  The 1N4148 signal diode prevents this damage by shorting out the back-emf.

This circuit can also be used to control other devices - it could be used as a lamp dimmer for example.

Figure 1: Darlington pair speed control

Building the Circuit

A stripboard layout for this circuit is given in figure 2.  The layout includes space for an optional 6-way screw terminal block to make connecting up the circuit easier.

Figure 2: A stripboard layout for the darlington pair speed control

The power supply for this circuit should preferably be un-smoothed (i.e. directly from the power supply rectifier).  This helps prevent the motor 'sticking' at low speeds.  With the TIP31C transistor given, the maximum power supply voltage may be 60V and the maximum motor current consumption may be 3A.

Pin-out diagrams of the two transistors used in this circuit are given in figure 3.  The TIP31C transistor will get hot in use because the high current supplying the motor flows through it.  You should therefore fit a heat sink to it to dissipate the heat - a suggested heat sink is given in the Shopping List, or you could make one out of Meccano!  Space is provided in the stripboard layout for a heat sink, but the positions and sizes of the holes are not shown for they will depend on the nature of the heat sink you choose.

Figure 3: Pin-out diagrams of the BFY51 and TIP31C transistors

  1. Cut a piece of stripboard to 11 tracks x 18 columns + extra columns if the heat sink you use is larger.
  2. If you wish to fit a 6-way screw terminal block, use a 1.5mm diameter drill bit to enlarge the six holes in the stripboard shown within the left-hand rectangle.  Somewhere within the right-hand rectangle, you will also need to drill holes of the appropriate size and in the appropriate position for your chosen heat sink.
  3. Fit the 5 wire links.
  4. Fit the 1kΩ resistor and 1N4148 diode, taking care to place the diode the correct way around.
  5. Fit the 6-way screw terminal block.
  6. Fit the BFY51 transistor.  Place a crocodile clip around each leg of the transistor before soldering.  This 'heat shunt' will prevent the heat from your soldering iron damaging the transistor.  Try to keep the soldering time to a minimum for this component.
  7. Fit your chosen heat sink - this must be done before you fit the TIP31C transistor to it!
  8. Fit the TIP31C transistor with the B leg at the top, such that the legend on it points to the left.  Bolt the TIP31C to the heat sink.
  9. Cut the copper tracks where an X is shown.  If the heat sink shorts out any of the tracks, cut them at an appropriate place to avoid this.
  10. Connect a potentiometer to the circuit as shown.  The 'wiper' terminal on a potentiometer (shown in figure 2 by the line with the arrow) is normally the centre terminal.  The other two terminals may be connected either way around.
  11. Connect a motor between the Output and 0V.
  12. Connect up a power power supply and test the circuit.

Another Darlington Pair Circuit

The article below describes another version of the Darlington pair circuit which reduces the voltage loss to 0.7V.

Related Articles Another Darlington Pair Speed Control


Article Information
Source: Electronics in Meccano - www.eleinmec.com | First published in EiM: Issue 8 (July 2000)
Topic: Controlling Motors | Created: 3/26/2002 | Last modified: 2/8/2007

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