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

To calculate for a PNP transistor, enter negative numbers in collector current, base voltage and voltage drop.

Examples are given for NPN transistors 2N2222, 2N3055, 2N3904, BC547, TIP31, TIP31A, TIP31C, TIP41, TIP41A, TIP41C and PNP transistors 2N3906.

38 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

  21. Rajesh says:

    Sir, kindly tell me what is the base resistor value and watt. Is it really 1 ohm 5 watt as predicted? And is there any other resistor required for collecter also ? (my in put is 24 volt 6 amps maximum ) If so what is the value and watt for uploaded figure. What about the heat sink? Only for 2N6292 or 7815 also. required ? Please tell me

    2n6292 Suply

  22. Mads Barnkob says:

    Hello rajesh

    The 2N6292 transistor can only conduct uninterrupted DC current of 2.3A at 15VDC, look at the Safe Operating Area graph in the datasheet. If you try to pass a higher current than this, even with a large heat sink on the transistor, it will dissipate more than its maximum rating of 40W and will be destroy from excessive heat.

    With a Icm of 2.3A, Hfe 30, base voltage 15 and Vbe(sat) at 3.5V I calculated the base resistor to be 150 Ohm, so you could use between 75 to 100 Ohm 3W base resistor to ensure fast switch on.

    You need a heat sink on the 2N6292 transistor, but not in the 7815 regulator as it only conducts 0.15A.

    Kind regards

  23. Rajesh says:

    Sir. Thank you for your quick reply sir thanks ..
    But According to data sheet maximum collector current is 7 amps for 2N6292. How you say it gives only 2.3 AMPS maximum ? Please describe me sir. I am new to transistor circuits
    And also you had calculated the care resistor as 150 ohm and telling of to use 75 to 100 ohm resistor . Why ? Please explain me sir.

  24. Mads Barnkob says:

    Hey Rajesh

    The maximum collector current is determined by the maximum power dissipation of 40 Watt. So the 7 A is only at 5 VDC, take a look at page 4, figure 5 in the datasheet:

    150 Ohm is the calculated value, it is calculated on lossless and absolute values assuming zero resistance in all wires, so to ensure full turn on, we lower the calculated resistance down to 50% lower to account for these parameters or if we want the transistor to turn on faster.

    Kind regards

  25. Rajesh says:

    Thank you sir,
    if I use 56 ohm base resistor what is the wattage of the resistor?

  26. Mads Barnkob says:

    Hey Rajesh

    If we assume the worst amplification, Hfe = 30 and it is conducting 1.5 A C-E, the base would need 50 mA. That is 0.75 W at 15 VDC base voltage. So use a 1 Watt resistor.

    Kind regards

  27. my name says:

    Why there are only NPN trnsistors in the dropdown list? How do I get base voltage for PNP when I drive it to 0V to turn it on?

  28. Mads Barnkob says:

    To calculate for a PNP transistor, enter negative numbers in collector current, base voltage and voltage drop.

  29. jose says:

    for some reason the base resistor value is not updating for me. tested in windows and linux, firefox, msie and chromium

  30. Mads Barnkob says:

    Hey jose

    Thank you for giving me a notice, javascript and wordpress editor is not the best friends and it strips some code without asking, I corrected the errors and it should be working again.

    Kind regards

  31. Amdani Wijaya says:

    Dear Admin,

    Thanks for posting.
    Now I want to calculate the wattage of the resistor to the transistor base (Rb).
    Do you think the formula that I use it correctly?
    P-Rb = Vin x Ib.
    Please confirm and correction if I am wrong.


  32. Mads Barnkob says:

    Hi Amdani Wijaya

    You are correct about the formula for base resistor wattage calculation.

    Kind regards

  33. Amdani Wijaya says:

    Hi Mads,

    thank you so much for your quick response.
    very useful

    Kind Regards,

  34. Steven says:

    I’m an engineering (but not electrical) student and need to design a psu (no ICs). Input ranges from 70Vpk 40A 200hz to 22Vpk 5A 50hz (from a turbine alternator). Output must be 12V, < or =5A. Using multisim I tried a PWM and switch but couldn't get it to work.
    I put in a darlington (although I don't know much about them yet) and it works well enough but the power across it must be a problem at 70Vpk 40A, is this even possible?
    And must I have a resistor if I'm trying to use it as a switch with a PWM controller? Sorry if its a stupid question, I'm an absolute beginner and its super confusing.


  35. Mads Barnkob says:

    Hi Steven

    You do not have a specific input current, the current drawn from your turbine alternator will be dependent on the load you present to it.

    You always need a base resistor to control the amount of current you want to conduct through the transistor.

    I think you should read some more about power supply design or find a proven design. With a wide input voltage range it can be quite complex to build without a IC.

    Kind regards

  36. Steven says:

    Hi Mads
    Thanks for your reply. I think the complexity is the aim of the project and I’m struggling to find something similar to compare with, also I’m not sure why input current was specified. I thought a buck regulator would be best but under simulation the voltage increased under decreasing load whereas it worked well with the darlington. There must be too much power dissipation across a transistor with 68Vdc at collector and 12Vdc at emitter? Can a PWM switch a transistor on and off, but maybe not a darlington? I thought I had seen it.
    Thanks for your time,

  37. Steven says:

    And could I please just clarify, the collector current would be same as load current? Ice= Iout e.g. 5A and so power rating would be Vin-Vout (68-12) x Ice = 280W? Completely impractical even with a heat sink right?
    Thank you again, I’m having trouble understanding transistors as you can tell.
    Cheers, Steven.

Leave a Reply

Your email address will not be published. Required fields are marked *