Transistor base resistor calculator

Here is a calculator for transistor base resistor values. Its IMPORTANT that you read the following.

Calculate the current you need to pass through the transistor when its on, that is your collector current.

The current gain, beta, Hfe, is a value you can find in the datasheet for the specific transistor. To calculate for the worst case use the minimum given Hfe value or the correct value for the collector current you need.

Vce voltage is the voltage over collector and emitter. A design guide not included in calculations for now.

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 insure to turn the transistor fully on, you can double the value for collector current, which will result in a base resistor value half of what this calculator gives you.

The examples are 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.

Choose transistor
Max rating Input Calculated
Collector current A A
Beta (Hfe)
Vce voltage V
Base voltage V V
Voltage drop V
Base resistor value Ω

Examples are given for transistors 2N2222, 2N3055, 2N3904, BC547, TIP31, TIP31A, TIP31C, TIP41, TIP41A, TIP41C.

21 Responses to Transistor base resistor calculator

  1. Alex says:

    Hi great little calculator. I am having trouble finding Vbe(sat) for a BD244 in the data-sheet

    And it states two different Hfe values, would it be ok if you took a look?

    I want to drive the transistor using an SG3525 chip and I want to ideally have it passing about 2 amps (for a gate drive transformer).


  2. Mads Barnkob says:

    Hey Alex

    If you look at “Figure 8. DC Current Gain” in the datasheet you can see a DC current vs Hfe curve, at 2 Ampere its 50 gain.

    Kind regards

  3. Kenneth says:

    With every Arduino project that activates a relay which is connected via a TIP120 transistor to a 12 volts psu, the base resistor suggested is 1K.

    Using your calc:
    collector current=5 amp
    supply voltage=12v

    then base resistor=1.9k

    Why? Am I doing something wrong? Should I use a 2k instead?

    Best regards

  4. Mads Barnkob says:

    Hey Kenneth

    As I wrote in the bottom of the important text above the calculator

    “To insure to turn the transistor fully on, you can double the value for collector current, which will result in a base resistor value half of what this calculator gives you.”

    Kind regards

  5. Greg says:

    Any chance of getting the formulas behind this calulator so that I can put them in an excel spread sheet and better understand how to do them manually?

  6. Mads Barnkob says:

    (Supply voltage – voltage drop) / (Collector current / Hfe)

  7. Pingback: BJT Base Resistor Value | Electric Curiosity

  8. Danilo Tostes says:

    Drop Voltage meaning Vebo (Collector-Emitter Voltage), right?

  9. Mads Barnkob says:

    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.

  10. Pete says:

    “TIP32C”: PNP-transistor; Collector current=3A
    – hfe:
    Ic=-1A Uce=-4V min=25
    Ic=-3A Uce=-4V min=10
    Both max=50

    Power supply: batteries about 12V (3×18650)
    Motor: 12V stall current as high as 2.5A(3A safe)

    How to count base resistor ? (Someone suggested pull-up resistor 10kΩ why ?!?)

  11. Hello, I am a semi-beginner (intermediate, I guess), but have little experience with bjt transistors. I tend to stick to things with MOSFETS and relays. I was wondering if I could use a resistor value lower than if I gave double collecter current (.16A), but higher than if I used normal collecter current (.08A).

  12. Mads Barnkob says:

    Pete: I am unsure about your application, a motor driver will not draw a continues current, but rather pulses with a higher current. I think you should use the average current, lets say 0.5A and Hfe 50, supply voltage 12V, voltage drop 1.2V, you get around 1K base resistor. If your logic that switches the base of the transistor can not deliver enough current, a pull-up resistor can be used.

    Constantin Bushofsky: Yes you can use other values outside of these ranges, but you risk to be operating in liniar mode if it is too large and thus will slow turn on or if the resistance is too small you will allow more current to be drawn from your driver and it might not be able to do so.

  13. Pete says:

    PWM from here: (This is were I start building. “Skeleton body PWM”)

    They suggested I should add transistor(TIP32 or something) to make it able to use more amperes because now only 0.2A: 555′s IC is the bottle neck.

    Tuned same PWM:(It is an amateur drawing so be gentle judging it. My first pretty much. Don’t know if it even works but so far it looks at least somewhat like a pwm.)

    To use basic 12V geared DC motor(60rpm) which uses a lot more amperes(more than 555′s 0.2A) I have to add transistor and I don’t know what kind of and what resistor to use ?

  14. Mads Barnkob says:

    Hey Pete

    You have quite a few mistakes in your schematic, take a look at this example:

    If you compared your circuit and the above, you can see that you have only put a transistor in parallel over the 555 output, this will not work. You want the 555 to drive the transistor and the transistor to drive the motor.

    A base resistor is used to limit the amount of current the 555 have to supply, a 555 that delivers 200 mA is stressed a lot btw. I would aim for somewhere around a maximum of 100 mA, with a 100R base resistor and a 10K pull-up you should have a sturdy driver for your TIP32C.

    Kind regards

  15. Pete says:

    Ok got it :) :) +++++
    Do I need all components PCB above 3A or P=UI=12V*3A=36W ?
    - 40W resistors.
    - 3A diodes.
    - Capacitors 12V(x4 safe)= about 50V.
    - Potentiometer 40W.
    - Transistor 3A(TIP32C)

    => Shopping list ready to the cave I go… :)

  16. Mads Barnkob says:

    Hey Pete

    Only your transistor needs the rating it has. Your capacitors will be more than happy with 4x rating. Resistors, diodes and potentiometer should just be 1/4W resistors, 1A diodes and potmeter just a regular one, no need for high power ratings in the control circuit. The heavy current only flows from your battery through transistor and into motor.

    Kind regards

  17. aku says:

    in your example BC547 transistor has hfe Ic=2mA Vce=5V Min value is 200 how then you put Beta(hfe) 100?

  18. Mads Barnkob says:

    Hey Aku

    In my example Ic is 100 mA.

    Kind regards

  19. Hello, I was wondering something. I have a NPN transistor set up with a battery pack at the collector, another battery pack at the base, and an LED at the emitter. I was wondering where I should connect the ground for the battery pack at the base.

  20. Mads Barnkob says:

    Hello Constantin

    Use a common ground for you batteries and circuit. Base current flows from base to emitter in a transitor.

    Kind regards

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