Home > Circuit Bricks > Drivers (Transistor)
UPDATED:.21:14 21 October 2013
FUNCTION BLOCK
DESCRIPTION


  • A transistor driver is genrally used as a current amplifying device.
  • The input is usually from a low current source such as a logic gate or a sensor,
  • It usually drives higher current devices such as bulbs and motors.
CIRCUIT DETAILS
A transistor driver uses the transistor as a switch rather than an amplifier by saturating the inputs and outputs.

This means that any device is switched via its negative terminal (sinks) rather than the positive (sources), which is the conventional method

This won't matter in most cases but take care when connecting any polarised device to ensure the supply to it is the correct way round.

The three most common transistor drivers are shown below and are suitable for most applications.

Equivalent circuit using a switch. The transistor acts as an electronic switch.
NPN Transistor
Darlington Pair

MOSFET
Input
Switches on when input >0.7volts

Output
Current up to 300mA

Current (Ic) = Ib x hfe
Input
Switches on when input >1.4volts

Output
Current up to 1 Amp

Current (Ic) = Ib x hfe1 x hfe2
Input
Switches when input > 2 volts

Output
Current up to 4A
COMPONENT TYPES

By changing the transistor types you can increase the current capability of the drivers. The IRF540 MOSFET will drive up to 28A!!

The Darlington Pair can be combined into one device. Some type available are:

  • TIP122 - 5A
  • TIP141 - 10A
  • TIP142 - 8A
Transistor Case Styles
TO92
(BC549C)
TO39
(BFY51)
TO220
(IRF510 / TIP122)
If you are driving high current devices you may find the transistor gets hot. You can attach a heatsink to the tab of the transistor to help cool it.

For more transistor options, go to the
NPN Transistor Specifications page

Heat transfer can be aided by the use of a 'thermal transfer compund' paste and can improve the efficiency of the heatsink.

TO220 heatsinkThermal Transfer Paste
Written by Phil Townshend - 2008
www.edutek.ltd.uk - Working Electronics For Students & Teachers