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Hi, I would like to use an npn transistor as a high side switch, but I have seen multiple websites stating that a pnp transistor should be used for a high side switch. Could someone tell me if it matters whether the npn transistor is used as a high side switch vs a low side switch? I don't understand why it matters whether the load is on the collector vs the emitter.



    • An NPN transistor can indeed be used as a high side switch in conjunction with an optocoupler.
    • When you use an NPN transistor on the high side, it means that the transistor is placed between the load and the positive supply voltage (Vcc).
    • The optocoupler provides electrical isolation between the low-voltage control circuit (such as a microcontroller) and the high-voltage load.
    • The optocoupler’s output transistor (usually an NPN) is controlled by an LED inside the optocoupler.
    • By driving the optocoupler’s LED with a low current from the microcontroller, you can switch the external NPN transistor, which in turn controls the load.
    • This configuration ensures that the base-emitter voltage of the NPN transistor remains constant, avoiding voltage drops due to the diode characteristics.
  1. Voltage Levels and Diode Drop:
    • When using an NPN transistor as a high side switch, the base-emitter voltage (Vbe) is critical.
    • The base voltage (Vb) should be high enough to turn on the transistor (typically around 0.7V for silicon transistors).
    • If you drive the optocoupler’s LED with the microcontroller, the base voltage will be sufficient to activate the NPN transistor.
    • However, keep in mind that the diode drop (0.7V) across the transistor affects the input-to-output voltage relationship.
    • The output voltage (Vout) will be slightly lower than the input voltage (Vin) due to this diode drop.
  2. Switching Logic Levels:
    • Using the optocoupler to switch the base of the external NPN transistor allows you to maintain the base voltage.
  3. Why Does It Matter?
    • The choice between low-side and high-side switching depends on the specific application and circuit requirements.
    • High-side switching can be more complex because:
      • The transistor needs to handle the load voltage directly.
      • The gate voltage for PNP BJTs or P-channel MOSFETs must be higher than the load voltage.
      • Level shifting or specialized drivers may be necessary to interface with microcontrollers.
    • Low-side switching is often preferred for simplicity, especially when the load voltage is lower than the control signal voltage.

Here are a option to use the NPN as high side switch control by microcontroller.