Choosing a transistor- selection criterion

Choosing a suitable NPN transistor

The circuit diagram shows how to connect an NPN transistor, this will switch on the load when the chip output is high. If you need the opposite action, with the load switched on when the chip output is low (0V)
The procedure below explains how to choose a suitable switching transistor.

  1. The transistor's maximum collector current Ic(max) must be greater than the load current Ic.
    load current Ic =   supply voltage Vs
    load resistance RL
  2. The transistor's minimum current gain hFE(min) must be at least five times the load current Ic divided by the maximum output current from the chip.
    hFE(min)  >   5 ×     load current Ic  
    max. chip current
  3. Choose a transistor which meets these requirements and make a note of its properties: Ic(max) and hFE(min).

  4. Calculate an approximate value for the base resistor:
    RB =   Vc × hFE    where Vc = chip supply voltage
      (in a simple circuit with one supply this is Vs)
    5 × Ic
    For a simple circuit where the chip and the load share the same power supply (Vc = Vs) you may prefer to use: RB = 0.2 × RL × hFE Then choose the nearest standard value for the base resistor.
  5. Finally, remember that if the load is a motor or relay coil a protection diode is required.
The output from a 4000 series CMOS chip is required to operate a relay with a 100ohm coil.
The supply voltage is 6V for both the chip and load. The chip can supply a maximum current of 5mA.

  1. Load current = Vs/RL = 6/100 = 0.06A = 60mA, so transistor must have Ic(max) > 60mA.
  2. The maximum current from the chip is 5mA, so transistor must have hFE(min) > 60 (5 × 60mA/5mA).
  3. Choose general purpose low power transistor BC182 with Ic(max) = 100mA and hFE(min) = 100.
  4. RB = 0.2 × RL × hFE = 0.2 × 100 × 100 = 2000ohm. so choose RB = 1k8 or 2k2.
  5. The relay coil requires a protection diode.

Choosing a suitable PNP transistor

The circuit diagram shows how to connect a PNP transistor, this will switch on the load when the chip output is low (0V). If you need the opposite action, with the load switched on when the chip output is high
The procedure for choosing a suitable PNP transistor is exactly the same as that for an NPN transistor described above.


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