Transistor Base Resistor Calculator — BJT as a switch

Size the base resistor to drive a BJT into hard saturation, using forced beta rather than the datasheet's optimistic hFE.

Example: Switching 100 mA from a 3.3 V control pin at a forced beta of 15 needs a 375 Ω base resistor — use 360 Ω.

Inputs

Control voltage(V)

Load current(A)

Forced beta

Design ratio of collector to base current. Lower means harder saturation.

V_BE saturated(V)

0.7 V is the classic figure; 0.8–1.0 V is realistic in hard saturation.

Datasheet hFE (optional)

Only used as a sanity check. Do not size the resistor from it.

Result

Base resistor

375 Ω

Use (E24): 360 Ω
Base current: 6.944 mA
Actual forced beta: 14.4
Saturation loss: 20 mW
Resistor dissipation: 17.36 mW

Size the base resistor from a forced beta, never from the datasheet hFE. hFE is a typical value in the active region; using it directly gives just enough base current to leave the transistor half-on and dissipating heat.
In saturation V_CE drops to roughly 0.2 V, so the transistor dissipates about 20 mW. If yours runs hotter than that, it is not saturating — lower the base resistor.

Base resistor against forced beta

Forced beta	Base current	R_b ideal	R_b (E24)
5	20 mA	125 Ω	120 Ω
10	10 mA	250 Ω	240 Ω
15	6.667 mA	375 Ω	360 Ω
20	5 mA	500 Ω	510 Ω
30	3.333 mA	750 Ω	750 Ω
50	2 mA	1.25 kΩ	1.2 kΩ

* BJT low-side switch — generated by urdb.io/tools/transistor-base-resistor
Vctrl ctrl 0 DC 3.3
Rb ctrl base 360.000
Q1 collector base 0 QGENERIC
.model QGENERIC NPN(BF=100)
.op
.end

Formula

I_b = I_c / β_forced    R_b = (V_control − V_be) / I_b

Worked example

Switching 100 mA from a 3.3 V control pin at a forced beta of 15 needs a 375 Ω base resistor — use 360 Ω.

I_b = I_c / β_forced
100 mA / 15
6.667 mA
Forced beta, deliberately far below the datasheet hFE, to guarantee saturation.
R_b = (V_control − V_BE) / I_b
(3.3 V − 800 mV) / 6.667 mA
375 Ω

Frequently asked questions

How do I calculate a base resistor for a transistor switch?

Pick a forced beta of 10 to 20, divide the collector current by it to get base current, then R_b = (V_control − V_BE) / I_b. Switching 100 mA from a 3.3 V pin at a forced beta of 15 gives about 375 Ω, so 360 Ω from E24.

Why not use the hFE from the datasheet?

hFE is a typical active-region figure with enormous part-to-part spread, and it falls at higher currents. Designing to it gives just enough base current to bring the transistor to the edge of saturation, where V_CE stays high and it dissipates far more than it should.

What is forced beta?

The ratio of collector to base current you deliberately design for, well below the transistor's actual capability. Forcing extra base current in guarantees saturation across temperature and part variation. 10 to 20 is the usual range.

My transistor gets hot — what did I get wrong?

Almost certainly too little base current. A saturated BJT drops about 0.2 V, so 100 mA should dissipate around 20 mW. If it is hot, V_CE is high, which means it is in the active region rather than saturated. Lower the base resistor.

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