Drivers

Logic devices like PICs are designed to deliver data - signals that carry information. Their outputs are not designed to source high currents. If you need outputs to drive high powered devices like lamps and motors, it is necessary to incorporate a driver. A driver delivers high currents to a device under the control of the data line. An example of a driver is given in the lamp worksheet.

The PICs used for eChip and PICAXE can deliver a maximum 25mA per output providing the total current supplied by the device does not exceed 100mA. This makes it possible to drive LEDs (including high brightness ones) directly from the output pins. Most other devices require a driver.

There are many different ways of implementing drivers. The example used so far, based on a small signal NPN transistor can drive a maximum current of about 100mA which is sufficient for indicator lamps. For higher currents, it is necessary to use a higher power transistor.

Darlington Driver

Set up this circuit again in Livewire.

Transistor driver

Run the simulation again to remind yourself of the current flowing into the transistor base and transistor collector. (For the exam, you need to remember which lead is which from the transistor symbol. You will not be reminded by the letters b, c, e !)

Now add a second transistor to the circuit like this:

Notice that the base limiting resistor has been increased to 100K.

Run the simulation and measure the current entering at the input terminal and the current entering the lamp.

Insert a new slide into your presentation and title it 'Darlington driver'.

Put a clip of your simulation onto your slide.

Record your current measurements.

Calculate the current gain of the circuit. (The lamp current divided by the current entering at the input terminal).

What do you notice about this driver compared with the single transistor driver? In what way is this a better driver? Explain how the added transistor improves the circuit.

This particular arrangement of two transistors is called a Darlington pair. It is especially useful for drivers and can handle over 1A if the appropriate transistor is chosen for Q2. Conveniently, Darlington pairs are available in a single packages. They look like ordinary transistors and are often shown on circuit diagrams as such. (On diagrams you should always show a Darlington pair as two transistors. It makes a difference when reading voltages at the transistor terminals.)

A very useful darlington transistor is the inexpensive MPSA14 which can switch over 1A and is housed in a small transistor package.

MOSFET Driver

The MOSFET transistor is useful for switching high currents of several amps.

MOSFET transistor symbol

Set up the following circuit in Livewire.

MOSFET driver

Run the simulation and measure the current entering the lamp and at the logic input. Calculate the current gain.

Insert a new slide into your presentation and title it 'Mosfet driver'.

Put a screen clip of the simulation onto your slide.

Record the current measurements.

Explain why there is no point in adding another transistor to improve the driver (unlike the NPN transistor).

H-Bridge Driver

The H-bridge driver is used for on/off and direction control of DC motors.

To understand how the H-bridge works, set up this circuit in Livewire.

Run the simulation and experiment with the switches to make the motor rotate clockwise and anticlockwise.

Insert a new slide into your presentation and title it 'H-bridge driver'.

Put screen clips of the simulation for switching on in the forward and in the reverse direction onto your slide. On each clip draw arrows showing the current path from 9V to 0V.

A	B	C	D	Motor
0	0	0	0	Off

Some switch combinations could be damaging and must be avoided. To discover these combinations switch on the 'explosions' option in Livewire.

Run the simulation and try different switch combinations. Which combinations cause problems. Explain the nature of the problem.

A useful H-bridge driver IC is the L293D. Each switch in the bridge consists of a transistor driver. The L293D also incorporates protection diodes which help to simplify circuits. (EN stands for enable. When this connection is made low, the driver disconnects from the power supply allowing the motor to freewheel to a standstill.)

H-bridge symbol

Set up and simulate the following circuit. The L293D can be found in the 'integrated circuits' folder.

Put a screen clip of the simulation into your presentation

Copy and complete the following table.

A	B	Motor
0	0	Off

Explain why there are no damaging input switch combinations.

On a practical note:

For most motors, it is essential to suppress commutation noise by connecting a capacitor across the motor terminals (Without suppression, eChip may not function reliably.)

For some motors that demand a high start-up current it may me useful to include a series resistance of around 6R8 Ohms.