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MPSA42 NPN Transistor

MPSA42 Pin Configuration:

Pin Number

Pin Name




Current Drains out through emitter



Controls the biasing of transistor



Current flows in through collector



  • Bi-Polar, High Voltage NPN Transistor
  • DC Current Gain (hFE) is 40
  • Continuous Collector current (IC) is 500mA
  • Base- Emitter Voltage (VBE) is 6V
  • Collector-Emitter Voltage (VCE) is 300V
  • Collector-Base Voltage (VCB) is 300V
  • Available in To-92 Package


Note: Complete Technical Details can be found at the MPSA42 datasheet given at the end of this page.


Alternative NPN Transistors:

S9014, SS8550, SS8050, BC547, 2N3904, 2N2369, 2N3055, 2N3904, 2N3906


MPSA42 Equivalent NPN Transistors:



Brief Description on MPSA42:

MPSA42 is a NPN transistor hence the collector and emitter will be left open (Reverse biased) when the base pin is held at ground and will be closed (Forward biased) when a signal is provided to base pin. It has a gain value of 30; this value determines the amplification capacity of the transistor normally MPSA42 will not be used for amplification due to its low gain value. The maximum amount of current that could flow through the Collector pin is 500mA, hence we cannot drive loads that consume more than 500mA using this transistor. To bias a transistor we have to supply current to base pin, this current (IB) should be limited to 5mA.

When this transistor is fully biased then it can allow a maximum of 500mA to flow across the collector and emitter. This stage is called Saturation Region and the typical voltage allowed across the Collector-Emitter (V­CE) or Collector-Base (VCB) could be 300V and 300V respectively. When base current is removed the transistor becomes fully off, this stage is called as the Cut-off Region.


Where to use MPSA42:

The MPSA42 is a high voltage NPN transistor. A transistor can normally be used as a switch or amplifier, but this transistor is commonly is used only for switching purpose since it has a low gain value of 30. The MPSA42 can turn on or turn off high voltage loads (upto 300V) hence they are commonly used for heavy loads or loads that have high peak voltage. Also note that the collector current of the IC is 500mA (0.5A) hence you cannot drive loads which might consume high current. It has a Base-Emitter voltage of 6V hence it can be controlled easily using microcontrollers or other digital electronics.

So if you are looking for an NPN transistor that could switch high voltages with a decent current flow then the MPSA42 transistor might be the right choice.


How to use MPSA42:

MPSA42 transistor is normally used to switch high voltage loads or loads that have high peak voltages. When a transistor is used as a switch it is operated in the Saturation and Cut-Off Region as explained above. As discussed a transistor will act as an Open switch during Forward Bias and as a closed switch during Reverse Bias, this biasing can be achieved by supplying the required amount of current to the base pin. As mentioned the biasing current should maximum of 5mA. Anything more than 5mA will kill the Transistor; hence a resistor is always added in series with base pin. The value of this resistor (RB) can be calculated using below formulae.


In the below circuit we have used the MPSA42 transistor to control a high voltage (48V) motor. The resistance (RB) can be calculated using the above formulae as we know the VBE is 5V in this circuit and IB is 5mA. So RB is (5/0.005) 1000ohms (1K)

 Circuit using MPSA42 NPN Transistor

When +5V is supplied the base current flows through the current limiting 1K resistor to the base pin, the transistor is forward biased and thus the motor is connected to ground. If 0V is supplied to base pin no current flows to base and thus the transistor is reversed biased making the motor to stop. Also note that the operating current of the motor is just 0.4A which is less than the collector continuous current (0.5A) of the transistor.



  • Switching high voltage loads
  • Used to control loads with high peak voltage
  • Speed control of low current motors
  • Drive high voltage stepper motors
  • Drive high power LED


2D model of the component:

If you are designing a PCB or Perf board with this component then the following picture from the MPSA42 transistor Datasheet will be useful to know its package type and dimensions.

 MPSA42 NPN Transistor 2D-model

Download Datasheet:
MPSA42 Datasheet