calculator
Transistor base resistor calculator
An NPN transistor turns a small signal at the base into a bigger current through the collector. This works out the base resistor that drives the transistor into saturation — the state where it behaves like a closed switch — for your supply voltage, load current and transistor gain.
The answer
3.3 kΩ
- Base current the collector needs (Ic ÷ hFE): 10 mA ÷ 100 = 0.100 mA
- Design for 10× that so the transistor stays saturated: 10 × 0.100 mA = 1.00 mA
- Voltage left for the resistor (Vin − Vbe): 4.5 V − 0.7 V = 3.8 V
- Ohm's law, Rb = V ÷ Ib: 3.8 V ÷ 0.0010 A = 3800 Ω
- Nearest standard value, rounding down: 3.3 kΩ
- Real base current with that resistor: 3.8 V ÷ 3300 Ω ≈ 1.15 mA
- Overdrive factor you actually got: 11.5× — anything above 5× is saturated.
The breadboard below runs the exact base resistor through a 220 Ω + red LED collector lane, so you can watch the transistor turn on when the signal switch closes.
simulating…
What a base resistor is for
An NPN transistor has three legs: base, collector, emitter. The emitter sits at ground, the collector drives the load (an LED, a buzzer, a small motor), and the base is the control input. Push a small current into the base and the transistor lets a much bigger current flow through the collector — about hFE times bigger. For a 2N2222, hFE is around 100 at modest currents.
The base itself only drops about 0.7 V (the base–emitter junction, Vbe), so a signal source at 4.5 V would push an unlimited current into the base and burn the transistor out. The base resistor is there to limit that current to the amount the collector needs, times a safety factor.
How to read the answer
The calculator rounds down to a standard E12 value. For a base resistor that is the safe direction: smaller resistor → more base current → deeper saturation. Compare that to the LED resistor calculator, which rounds up because less current is the safe side for an LED. Same Ohm's law, opposite rounding, because the thing you are protecting is different.
The overdrive factor is how many times the strict minimum base current you ended up with. 5× is a comfortable floor; 10× is the usual aim; 20× is fine too. Below 5× the transistor slips out of saturation when hFE drops at the wrong moment and starts dissipating heat.
Common questions
Why multiply by 10? A transistor only saturates — collector voltage near zero, acting like a closed switch — when the base is driven harder than it strictly needs. If you aim for exactly Ic ÷ hFE, any drop in gain, temperature change, or load wobble will push the transistor out of saturation and heat it up. 5× is the textbook minimum; 10× is comfortable.
Why round the resistor down? The base-resistor direction is opposite to the LED-resistor direction. A smaller base resistor means more base current, which saturates the transistor harder — the safe side. A smaller LED resistor means more LED current, which cooks the LED — the unsafe side. Same formula, opposite rounding.
What is hFE? The transistor's current gain: the ratio of collector current to base current when the transistor is active. A 2N2222 is spec'd for hFE ≥ 100 at Ic = 150 mA, but drops to about 50 at Ic = 10 mA and under 20 near its 800 mA ceiling. Design for the worst case your load lands in, not the typical.
Try it for real in Automatic Night Light (LDR to base through a 1 kΩ resistor, LED on the collector), or read the transistor-switch explainer.