BS170 vs TIP122: Choosing a Transistor for Your Project
By Official Hobby Components Online Store | Buy Electronic Kits, Components, Boards, Shields, Modules | Published: 2026-10-19
Category: Comparisons
Compare the BS170 MOSFET and TIP122 Darlington transistor for your electronics project. Learn their strengths, trade-offs, and which one fits your circuit.
Quick answer
Choose the BS170 MOSFET for fast, low‑voltage switching up to 500 mA, and pick the TIP122 Darlington when you need to drive loads up to 5 A with very high current gain from a microcontroller pin.
Switching speed vs. current capacity
The BS170 is an N‑channel enhancement‑mode MOSFET designed for fast switching in low‑voltage, low‑current circuits. Its low gate charge makes it ideal for high‑frequency PWM or small servo motor control. The TIP122, on the other hand, is a Darlington pair that trades switching speed for enormous current gain (1000 minimum). It can handle up to 5 A continuous (8 A peak) at 100 V, but its slower turn‑off time limits it to lower frequency applications like relay drivers or DC motor speed control where speed isn't critical.
Gate drive and voltage compatibility
Both transistors can be driven directly from a 5 V microcontroller output, but for different reasons. The BS170's gate threshold voltage is low enough to turn on fully with 5 V, while the TIP122's Darlington configuration provides enough gain to saturate with a base current of just a few milliamps. However, the BS170 is a voltage‑controlled device—it requires almost no steady gate current—whereas the TIP122 needs a continuous base current (typically 5–10 mA) to stay on, which can matter in battery‑powered designs.
Package and physical size
The BS170 comes in a small TO‑92 package, making it suitable for compact, low‑power designs. The TIP122 uses the larger TO‑220 package, which can be bolted to a heatsink for high‑current operation. If your project is space‑constrained and runs on low power, the BS170 is the obvious choice. If you need to dissipate heat from a 5 A load, the TIP122's package gives you that option.
Trade‑offs at a glance
- BS170 MOSFET: Fast switching, low gate drive current, small TO‑92 package, limited to 500 mA and low voltage. Use for PWM, servo control, or any application where switching speed matters.
- TIP122 Darlington: Very high current gain (1000), handles up to 5 A at 100 V, but requires continuous base current and switches slowly. Use for high‑current loads like motors, relays, or solenoids where speed is not critical.
- Alternative: low power transistor kit: If your project needs general‑purpose small‑signal transistors for amplification or low‑current switching, a 110‑piece assortment covers 11 common values. Good for prototyping or when you need a variety of small‑signal transistors on hand.
Product facts
- BS170 MOSFET Transistor in TO-92 Package: N‑Channel enhancement mode MOSFET designed for low voltage, low current applications up to 500 mA DC. Its fast switching and low on‑state resistance make it a good fit for small servo motor control or power MOSFET gate drivers in 5 V or 3.3 V systems.
BS170 MOSFET Transistor in TO-92 Package - TIP122 NPN Darlington transistor: Complementary power Darlington transistor with a very high current gain of 1000, collector‑emitter voltage up to 100 V, and continuous current of 5 A (8 A peak). Its high gain lets a microcontroller pin switch large loads like motors or solenoids without an extra driver stage, but its slower speed limits use to lower‑frequency switching.
TIP122 NPN Darlington transistor
Which one should you choose?
Best for
- Projects requiring fast PWM or high‑frequency switching (BS170)
- Driving high‑current loads (5 A) with minimal control pin current (TIP122)
Not for
- High‑frequency switching with the TIP122 – its Darlington structure slows turn‑off
- Driving loads above 500 mA with the BS170 – it may overheat or fail
