Transistor Base Resistor Calculator (NPN Switch Saturation, Drive Current & Power)

How it works

I_B(min) = I_C ÷ hFE(min) · I_B = I_B(min) × overdrive (3–5×) · R_B = (V_drive − V_BE) ÷ I_B → nearest E24 · P_transistor ≈ I_C × V_CE(sat) + I_B × V_BE; P_RB = I_B² × R_B

To use a bipolar transistor as an on/off switch you drive enough base current that the collector cannot supply the load any faster — saturation — so the collector-emitter voltage collapses to a couple of tenths of a volt and the device stays cool. The minimum base current is the load current divided by the transistor's gain, but gain is a loose, temperature-dependent minimum, so designers feed three to five times that. The base resistor is then just the drive voltage minus the base-emitter drop, divided by that current — checked against what a microcontroller pin can actually supply.

Worked example

Switching a 100 mA, 12 V load from a 5 V pin with a transistor whose minimum hFE is 100: the minimum base current is 1 mA, ×4 overdrive is 4 mA, so Rb = (5 − 0.8)/0.004 = 1,050 Ω → 1.1 kΩ (nearest E24), giving 3.8 mA from the pin — fine for a 20 mA pin. The transistor dissipates about 23 mW at Vce(sat) 0.2 V, so a 2N2222 or BC337 in TO-92 is comfortable.

Frequently asked questions