Transistor Base Resistor Calculator (Switching)
Base resistor that drives a bipolar transistor fully on as a switch, from the drive voltage, the load current and the transistor's gain.
Results
Base resistor
860.0
Ω
Base current
5.0000
mA
Forced gain (Ic ÷ Ib)
20.00
Power in the resistor
0.02150
W
What this tool does
To use an NPN transistor as a switch it must be saturated, not merely conducting, or it heats up and the load gets less than full voltage. The base current that would just barely do it is Ic ÷ hFE, so designers multiply it by an overdrive factor — commonly somewhere between 2 and 10 — to cover the gain's spread and temperature. Use the minimum hFE from the datasheet at your current, check the drive pin can supply the base current, and round the resistor down to the nearest standard value.
Formula
I_B = I_C ÷ hFE × k · R_B = (V_in − V_BE) ÷ I_B
Variables
| Symbol | Meaning | Unit |
|---|---|---|
vi | Driving voltage (e.g. microcontroller pin) | V |
vb | Base–emitter voltage | V |
ic | Load (collector) current | mA |
hf | Current gain hFE (minimum from the datasheet) | — |
od | Overdrive factor for saturation | × |
RB | Base resistor | Ω |
IB | Base current | mA |
FB | Forced gain (Ic ÷ Ib) | — |
PB | Power in the resistor | W |
Worked example
- Driving voltage (e.g. microcontroller pin)5 V
- Base–emitter voltage0.7 V
- Load (collector) current100 mA
- Current gain hFE (minimum from the datasheet)100
- Overdrive factor for saturation5 ×
- Base resistor860.0 Ω
- Base current5.0000 mA
- Forced gain (Ic ÷ Ib)20.00
- Power in the resistor0.02150 W
Limitations
- Coverage and consumption figures vary between products. Take the values from the manufacturer's datasheet for the product you are actually using.
- The result is an estimate based only on the values you type. Real situations often include factors this calculator does not know about.