To use the calculator for transistor base resistor values, Its IMPORTANT that you read the following.
Transistor datasheet values
- First, calculate the current you need to pass through the transistor when its on, that is your collector current.
- Secondly, find the current gain, beta, Hfe. These values is in the datasheet for the specific transistor. To calculate for the worst case, use the minimum Hfe value or the correct value, for the collector current you need.
- Vce voltage is the voltage across collector and emitter.
- Base voltage is the voltage that you use to drive the base of the transistor with.
- Voltage drop is the Vbe(sat) value, you will get this from the datasheet looking at the graph for Vbe and Vce(sat) vs. collector current.
To get the best result, consider the following
- To insure to turn the transistor fully on, you can double the value for collector current. As a result you get a base resistor value half of what this calculator gives you.
- For example, it is here given for the highest collector current and worst case amplification factor, Hfe. Values for your circuit might vary, be sure to enter values for your own needs.
- To calculate for a PNP transistor, enter negative numbers in collector current, base voltage and voltage drop.
To help you remember the pinout of the NPN and PNP transitors:
Formulas and math used for the transistor base resistor calculator
Ohms law: Base resistor = (Supply voltage – Voltage drop) / (Collector current / Beta)
The electronic components needed are a regular color banded 5-band resistor, the bands represent the Ohm value, resistor tolerance and in some cases temperature coefficient.