Electronics · Free Calculator
Transistor Base Resistor Calculator
Find the base resistor needed to switch a load fully on with an NPN or PNP transistor driven from a logic or PLC output.
BJT switchSaturationRelay driverOverdrive factor
V
V
×
Base current needed5 mA
Calculated base resistor860 Ω
Use standard value820 Ω (E24, rounded down)
Actual base current5.244 mA
Forced beta19.1
How this result was calculated
- IB = IC ÷ hFE × k = 100 mA ÷ 100 × 5 = 5 mA
- RB = (Vin − VBE) ÷ IB = 860 Ω
Advertisement
Base Resistor Formula
IB = (IC ÷ hFEmin) × k
RB = (Vin − VBE) ÷ IB
RB = (Vin − VBE) ÷ IB
k is the overdrive factor (2 to 10) that makes sure the transistor saturates even with the lowest hFE and at low temperature. A forced beta of 10 to 20 is a common target.
Worked example
A 5 V output switches a 100 mA relay with a transistor of hFE 100. IB = 1 mA × 5 = 5 mA, RB = 4.3 ÷ 0.005 = 860 Ω, so fit 820 Ω.
Key insight: always add a flyback diode across a relay coil; the base resistor sets the switch, but the diode is what keeps the transistor alive at turn off.
Advertisement
Frequently Asked Questions
How do I calculate a transistor base resistor?
R = (Vin − 0.7) ÷ IB, with IB = IC ÷ hFE times an overdrive factor of about 5.
Why use an overdrive factor?
hFE varies widely between parts and falls at low temperature and high current. Extra base current guarantees saturation.
Should I round the base resistor up or down?
Down, so the base current is at least the calculated value.
You May Also Like
Learn more on the blog
Electromagnetic relay working and coil suppression
A practical, field focused guide on instrumentationblog.in that explains the theory behind this calculator and how engineers apply it on real plants.
Read Full ArticleMore Electronics Calculators
SMD Resistor Code Calculator3 digit (472), 4 digit (1002)Capacitor Code Calculator3 digit code, Tolerance letterStandard Resistor Values Calculator (E Series)E6 and E12, E24 (5 %)Inductors in Series and Parallel CalculatorSeries, ParallelAir Core Coil Inductance CalculatorWheeler formula, Single layer coilResistor Power Rating CalculatorP = V²/R, P = I²RCurrent Divider CalculatorTwo or three branches, Branch currentWheatstone Bridge CalculatorBalance condition, Output voltageRC and RL Filter Cutoff Frequency CalculatorLow pass, High passRLC Circuit Impedance CalculatorSeries RLC, Parallel RLCLED Series and Parallel Array CalculatorLEDs per string, String resistorInverting Op Amp Amplifier CalculatorGain = −Rf/Rin, Output voltageNon Inverting Op Amp Amplifier CalculatorGain = 1 + Rf/Rg, Output voltageOp Amp Differential Amplifier CalculatorVout = (R2/R1)(V2 − V1), Four resistorsOp Amp Summing Amplifier CalculatorOp amp adder, Weighted sumInstrumentation Amplifier Gain Resistor CalculatorAD620, AD8221, INA128, INA333Op Amp Gain Bandwidth and Slew Rate CalculatorGBW ÷ gain, Slew rate555 Timer Astable CalculatorFrequency, Duty cycle555 Timer Monostable Calculatort = 1.1 RC, One shotCrystal Load Capacitor CalculatorC1 = C2, Load capacitanceFrequency to Period CalculatorHz to seconds, Period to HzPWM Duty Cycle CalculatorDuty cycle %, Average voltageLM317 Voltage Regulator CalculatorVout = 1.25(1 + R2/R1), R2 for targetZener Diode Series Resistor CalculatorSeries resistor, Zener powerLinear Regulator Power Dissipation CalculatorPower dissipation, Junction temperatureHeat Sink Thermal Resistance CalculatorθSA in °C/W, Junction temperatureBuck Converter CalculatorDuty cycle, Inductor valueBoost Converter CalculatorDuty cycle, Inductor valueRectifier Filter Capacitor CalculatorSmoothing capacitor, Ripple voltagePCB Trace Width Calculator (IPC 2221)IPC 2221, External and internalPCB Trace Resistance CalculatorCopper resistance, Voltage dropBinary, Decimal, Hex and Octal ConverterBinary, HexadecimalBattery Life Calculator for Electronic DevicesmAh capacity, Sleep currentdBm to Watts CalculatordBm to mW, Watts to dBmResistor Color Code Calculator4 band, 5 bandLED Resistor CalculatorSingle LEDVoltage Divider CalculatorOutput and resistor valuesResistors in Series and ParallelTotal resistanceOhm's Law CalculatorV, I, R and PRC Time Constant CalculatorCharge and dischargeReactance CalculatorXL and XCLC Resonant Frequency CalculatorTank circuitsCapacitors in Series and ParallelTotal capacitanceCapacitor Energy CalculatorJoules and chargeDecibel CalculatorPower and voltage ratiosADC Resolution CalculatorBits and LSB size
More Calculator Categories
Temperature Measurement15 live toolsInstrumentation Signal4 live toolsPLC Digital Signal3 live toolsFlow Measurement19 live toolsLevel Measurement12 live toolsPressure Measurement13 live toolsControl Valve12 live toolsCalibration10 live toolsSafety and SIL7 live toolsAnalytical8 live toolsElectrical37 live toolsUnit Converters76 converters, 700+ units