Transistor Base Resistor Calculator
Base resistor to drive a bipolar transistor hard into saturation as a switch. With the formula, a worked example and typical values.
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Estimate only. Not engineering or safety advice. Check the result against the applicable standard and the equipment data, and have a competent engineer review it before it is relied on. Terms of use.
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About this calculator
This page calculates base current, base resistor and next lower E24 value from drive voltage, base-emitter voltage, collector current, minimum current gain hFE and 1 more input. Change any value or its unit and the result updates. The steps below show the working for the default values, so you can check each line against your own method.
Limits of this calculator
Check that the driver pin can supply the base current, and that the transistor dissipation at Vce(sat) is within its rating.
Cross-check: 8 reference cases, calculated from textbook formulas coded separately from the engine by the same author, agree with this calculator. This is not an independent review.
Verification status: Engineering reference
Engineering reference. Formula and method documented and tested against separately coded reference calculations. Not yet Verified: the evidence listed on this page is still needed.
What this calculator checks
- The arithmetic of the published formula for the values you entered, in the units shown.
What remains unverified
- That the inputs suit your application. Take ratings, resistances and constants from the manufacturer data sheet or project specification.
Verification register entry
| Calculation author | Rahul Sharma (author) |
|---|---|
| Technical review | No independent technical review has been carried out |
| Individual professional approval | Not recorded |
| Formula reference | not recorded |
| Standard and edition | Not applicable: The base-current/resistor calculation is a textbook approximation. Transistor gain, saturation voltage, switching and temperature limits must be taken from the device datasheet. |
| Standard review state | Not applicable |
| Scope-screening note | not recorded |
| Cross-check test cases | 8 reference vectors (tests/reference_vectors.json) |
| Review date | not recorded |
| Next review date | not recorded |
| Status | Engineering reference |
Needed before this calculator can be marked Verified: reviewer name, qualification and review date; exact formula reference; formula source (textbook or standard); next review date.
Formula
Worked example
| Input | Value |
|---|---|
| Drive voltage | 5 V |
| Base-emitter voltage | 0.7 V |
| Collector current | 100 mA |
| Minimum current gain hFE | 100 |
| Overdrive factor | 2 |
- Ib = 2 × 100 mA / 100 = 2 mA
- Rb = (5 − 0.7) / 2 mA = 2,150 Ω
- Next lower E24 value = 2,000 Ω
Assumptions
- Transistor used as a switch, not an amplifier
- Vbe about 0.7 V for silicon
Check that the driver pin can supply the base current, and that the transistor dissipation at Vce(sat) is within its rating.
Inputs and results
| Input | Unit | Allowed or shown | Note |
|---|---|---|---|
| Drive voltage | V | greater than 0 V | |
| Base-emitter voltage | V | 0.3 V or more, up to 1.2 V | |
| Collector current | mA | greater than 0 mA | |
| Minimum current gain hFE | (number) | 1 or more | Use the minimum at this collector current, not the typical value |
| Overdrive factor | (number) | 1 or more | 2 to 10 is typical for switching |
| Result | Unit | Allowed or shown | Note |
|---|---|---|---|
| Base current | mA | Shown to 3 decimal places | |
| Base resistor | Ω | Shown to 0 decimal places | |
| Next lower E24 value | Ω | Shown to 0 decimal places |
How it is calculated
Each input is converted to the base unit of its quantity, the formula above is applied in the order shown in the worked example, and each result is shown in the unit you choose. Results are held at full precision and rounded only for display, except where a step selects a standard size, as the worked example shows. A value outside its allowed range is refused with a message instead of giving a result.
How the result changes with drive voltage
| Drive voltage (V) | Base current (mA) | Base resistor (Ω) | Next lower E24 value (Ω) |
|---|---|---|---|
| 1.25 | 2 | 275 | 270 |
| 2.5 | 2 | 900 | 820 |
| 5 | 2 | 2,150 | 2,000 |
| 10 | 2 | 4,650 | 4,300 |
| 20 | 2 | 9,650 | 9,100 |
| 50 | 2 | 24,650 | 24,000 |
Other inputs are the same as in the worked example above.
Standards and further reading
These are textbook circuit relationships. The component data sheet sets the real limits, tolerances and ratings, and takes priority over the calculated value.
Frequently asked questions
What do I need to enter?
Enter drive voltage, base-emitter voltage, collector current, minimum current gain hFE and overdrive factor. Change the unit of a value with its selector and the result stays the same.
What does this calculator assume?
Transistor used as a switch, not an amplifier. Vbe about 0.7 V for silicon.
Which calculators should I use next?
Common next steps are LED Series Resistor and RC Time Constant and Cutoff. The related links at the bottom of this page open them.
How accurate are the results?
Every figure comes from the same tested engine as the Elecmetric calculators and is meant for estimating. 8 reference cases, calculated from textbook formulas coded separately from the engine by the same author, agree with this calculator. This is not an independent review. Check design values against the standard and the equipment data you work to.
Formula guides and reference
Longer explanations with worked examples and tables, written for the same topic.
- BJT Bias Formulas: Voltage Divider Bias: Bipolar transistor voltage divider bias formulas: base voltage, emitter and collector current, collector voltage and VCE
More Electronics and Instrumentation calculatorsAll engineering calculatorsTransistor Base Resistor in Elecmetric