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 P2
Darlington
Pair

How the circuit works…..


A simple way to understand this circuit is to think of the two transistors as a single transistor - as represented by the dotted box. When a voltage greater than 1.2 volts (2 x 0.6 volts) is applied to the Darlington Pair, it "switches" the transistors on. This allows current to enter the "collectors", flow through the device and leave through the "emitters". Because of the direction of current flow, this is how the emitter gets its name and the arrow on the transistor symbol shows the direction of the current flowing through the device. Again the output device (eg buzzer, lamp) is connected in the emitter/collector circuit as shown by the output voltage arrow. It will be sinked by the transistors (see OP1). When the transistors are switched on, they are said to be "biased" by the voltage applied to the base. The 1K resistor is needed to prevent too much current entering the transistor's base, when the input subsystem voltage is applied. (See IP1 and IP2)



Circuit Diagram


The overall amplification
(Gain)

For the Darlington Pair circuit:

Gain = hFE for BC108 x hFE for BFY51

= 200 x 40 = 8000 times (typically)

Other useful Information: Please see P1 and P5 also.

The BFY51 transistor can power output devices with current up to 1 amp (Ic = 1 amp). This output will be sufficient to drive small motors and relays and outputs where devices are connected in parallel.

The BCX38B is a Darlington Pair in a single package. It can take the place of the two transistors shown in the dotted box in the circuit diagram above. It provides about 0.8 amps output current (Ic). Being a low-cost item, it makes a good alternative, but please note the leg connections shown above when connecting up.