The transistor as a switch is one of the most fundamental circuits in electronics. Not spectacular, but indispensable. With a small control signal — from a microcontroller, a sensor, or another low-power circuit — you switch a higher current or voltage.
How does an NPN transistor work as a switch?
An NPN transistor has three terminals: base (B), collector (C), and emitter (E). As soon as the voltage on the base is high enough — typically 0.6 to 0.7 V above the emitter — the transistor begins to conduct.
- Cut-off: no base current, no collector current.
- Saturation: sufficient base current, maximum collector current.
BC547: the everyday choice
The BC547B is one of the most widely used NPN transistors in hobby projects.
| Parameter | Value |
|---|---|
| Maximum collector current | 100 mA |
| Maximum collector-emitter voltage | 45 V |
| hFE at datasheet conditions | minimum 200 at VCE = 5 V and IC = 2 mA |
| Package | TO-92 |
KSC1845: higher voltage, but a lower current limit
The KSC1845 is primarily specified as a low-noise audio amplifier. Its 120 V VCEO is higher than the BC547B rating, but its absolute collector-current limit is lower at 50 mA. It is not automatically the higher-current or relay-driving choice.
| Parameter | Value |
|---|---|
| Maximum collector current | 50 mA (absolute maximum) |
| Maximum collector-emitter voltage | 120 V |
| hFE2 | 300–600 at VCE = 6 V and IC = 1 mA |
| Package and pinout | TO-92; 1 Emitter, 2 Collector, 3 Base |
Comparison
| Comparison | BC547B | KSC1845 |
|---|---|---|
| Max. collector current | 100 mA | 50 mA (absolute maximum) |
| Max. voltage (VCEO) | 45 V | 120 V |
| hFE at datasheet conditions | minimum 200 at 5 V / 2 mA | hFE2: 300–600 at 6 V / 1 mA |
| Primary application | low-voltage, general signal switching | low-noise audio, higher voltage |
| Pinout | C-B-E | E-C-B |
BC547B or KSC1845 as a switch? Do not start with maximum voltage alone. Check collector current, collector-emitter voltage and dissipation at the real load; guaranteed values at the operating point; required base current and driver limits; VCE(sat) at the datasheet test conditions; temperature, pulse duration and duty cycle; and the exact manufacturer pinout. For an inductive load, select a flyback or clamp solution for its current, stored energy, voltage and switching frequency.
Practical points to watch out for
Do not forget the base resistor
Never drive the base of a transistor directly from a microcontroller pin without a series resistor. Without a resistor, a large current flows through the base-emitter junction, which will destroy your transistor and likely your microcontroller pin as well. Calculate the base resistor from the required collector current, a guaranteed drive condition, the control voltage and what the driver can safely source.
Flyback diode for inductive loads
Are you switching a relay or a motor? Use a suitable flyback or clamp solution selected for coil current, stored energy, voltage and switching frequency.
