Transistor Base Resistor Calculator (BJT Switch)
By Santaji GadepatilinUpdated October 28, 2026
To use a bipolar transistor as a switch for a relay, LED strip or buzzer, you must drive enough base current to saturate it fully. This transistor base resistor calculator works out the resistor from the load current and gain — see the basics of transistors and BJT currents and voltages.
How to use the transistor base resistor calculator
Enter the drive voltage, the transistor’s VBE, the load current, the minimum hFE from the datasheet and an overdrive factor. You get the base current and the base resistor value — round down to the nearest standard value.
Base resistor formula
IB = (IC ÷ hFE) × k · RB = (Vin − VBE) ÷ IB
Example
Switching a 100 mA relay from a 5 V microcontroller pin with a BC547-class transistor (hFE ≥ 100), 5× overdrive:
- IB = 100 ÷ 100 × 5 = 5 mA
- RB = 4.3 ÷ 0.005 = 860 Ω → use 820 Ω
Common small-signal transistors
| Transistor | Type | Max IC | Typical hFE |
|---|---|---|---|
| BC547 | NPN | 100 mA | 110–800 |
| 2N3904 | NPN | 200 mA | 100–300 |
| 2N2222A | NPN | 600 mA | 100–300 |
| BC337 | NPN | 800 mA | 100–630 |
| BD139 | NPN | 1.5 A | 40–250 |
| TIP120 | NPN Darlington | 5 A | ≥ 1,000 |
Practical tips
- Check that the microcontroller pin can supply the base current (often 10–20 mA max).
- Add a 10 kΩ resistor from base to emitter to keep the transistor off while the pin floats at power-up.
- Saturation voltage VCE(sat) is typically 0.1–0.3 V; power dissipated ≈ IC × VCE(sat).
Frequently asked questions
How do I calculate a transistor base resistor?
Find the base current I_B = I_C ÷ hFE × overdrive, then R_B = (V_in − V_BE) ÷ I_B. For 100 mA with hFE 100, 5× overdrive and 5 V drive, R_B ≈ 860 Ω.
Why use an overdrive factor?
hFE varies widely between parts and with temperature. Driving 2–10 times the minimum base current guarantees saturation and a low V_CE.
Do I need a diode with a relay?
Yes. Put a flyback diode across the relay coil to absorb the voltage spike when the transistor turns off.
When should I use a MOSFET instead?
For loads above a few hundred milliamps, a logic-level MOSFET needs no continuous drive current and wastes less power.