BSS138 | Datasheet, Level Shifter, SOT23, Circuit & Pinout Yangzhou

BSS138 npn mosfet application

The BSS138 is a popular small N-channel MOSFET commonly used by electronics enthusiasts for switching applications and voltage level shifting. Even though it comes in a tiny SOT-23 package, it’s incredibly convenient to use.

This MOSFET has a relatively low on-resistance of around 3.5Ω, which means it doesn’t heat up easily and maintains good efficiency during operation. Additionally, it has a low gate threshold voltage—typically between 1.5V and 2V—making it a perfect match for low-voltage microcontrollers like Arduino or ESP32.

In everyday projects, the BSS138 is frequently used for converting signals between 3.3V and 5V logic levels, or controlling small loads such as LEDs and relays. It’s also particularly useful in portable or space-constrained electronics.

Overall, if you’re designing a low-voltage, low-current circuit, the BSS138 offers excellent value and is definitely worth considering.

BSS138 pinout

Here’s the pinout for the BSS138 MOSFET in a clear and easy-to-understand table:

The BSS138 MOSFET typically comes in a small SOT-23 package. Always double-check the pin orientation when placing this MOSFET on your PCB to ensure correct circuit functionality and avoid accidental damage.

BSS138 equivalent mosfet

• For direct replacement: The onsemi BSS138LT1G and Rohm BSS138BKT116 are excellent choices, offering specifications nearly identical to the original BSS138.

• For higher current applications: The Nexperia BSS138P,215 provides a higher current rating and is automotive-grade, making it suitable for more demanding environments.

• For low RDS requirements: The UMW AO3400A offers a significantly lower on-state resistance, ideal for applications where efficiency is critical.

BSS138 level shifter circuit

Idle state: Both sides (SDA1 and SDA2) are pulled high by resistors R1 and R2 to their respective voltages (5V and 3.3V). At this point, the MOSFET gate and source have the same potential, keeping the MOSFET off.

Low-voltage side (3.3V) pulled low: When the 3.3V side line is pulled low, the gate of the MOSFET stays at 3.3V while the source drops to 0V. This creates a sufficient gate-to-source voltage (V_GS) to turn on the MOSFET. Once the MOSFET is on, it also pulls the higher-voltage side (5V) line low, effectively translating the signal from the 3.3V side to the 5V side.

High-voltage side (5V) pulled low: When the 5V side line is pulled low, the MOSFET’s internal body diode initially conducts, slightly lowering the voltage on the low-voltage side (3.3V). This change in voltage again satisfies the gate-to-source voltage requirement, causing the MOSFET to turn on. Once turned on, it fully pulls the 3.3V side low, completing the translation from the 5V side to the 3.3V side.