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Transistor Resistor Calculator

A BJT transistor can be used as an electronic switch when it is driven into saturation, or alternatively driven to the cut off region.  Calculating the base resistor is a common engineering task, which this calculator automates. 

Rc (Collector resistor) (K ohms) 
VP (Positive Voltage) (V)
Beta(min) (Current gain)
VBE (Base to emitter drop) (V)
Rb (Base resistor) (K ohms) 





The current through the load at saturations is Ic= VP/Rc.  The base current must be Ib= Ic/Beta. Transistors have a varying gain so we want to use the minimum beta value, to ensure saturation. 
Rb= (VP-VBE)/Ib= (VP-VBE)*Beta/Ic= (VP-VBE)*Beta*Rc/VP


Need Help? Please leave a comment, I'll get back soon with a reply!


  1. i need Calculator mosfet irf3205 and irf1404 resistances gate take voltage from ic cd4047 transformar 12.0.12 100AV I need to convert 24v to 220 1000w mosfet be to hot with out load in do know why

    pleas support me

    1. mosfet gate resistor can be anywhere between 0 and 100 ohm, depending on the switching frequency...higher frequency will require lower value and lower frequency can accommodate higher values

  2. Hi,
    I need to determine the gate resistor for Irfp 450,please assist for 12vdc to 230vac boost converter and also provide the link.

    1. Hi, you cannot calculate a mosfet gate resistance, because it's not required.....for high frequency signals you can try any value between 10 ohm and 33 ohm

  3. Hi sir swagatam.
    Is there a transisitor that switches on at 9v and closes at 8v?if any..
    Im having hard with my project and your the only one i could rely.
    Tnx and good day to you sir.

  4. Good day sir swagatam
    Is there any transistor available that opens in 8.5v and closes at 8v.
    My led project is in 6w.10v. And my input is 12v. With ldr on the middle ang some resistor. The output is only 7.4 volts.
    Need help for this.tnx..

    1. Mark, no single transistor will accurately trigger or switch OFF at a given precise may have to connect 3 or 4 transistors to obtain this feature.

    2. 3 or 4 with the led on the last transisitor?thank you for being so helpful. I salute you.

    3. Good day sir Swagatam.
      Do i need to add a resistor between each transistor?
      Any advice on transistor to be use?
      And would you pls be kind to explain the process happend.(sorry for my bad english)

    4. Mark, you can do it as shown in the following diagram:

      you can improve the tripping accuracy by adding a few more number of transistors in the similar format as the two....

    5. Tnx again sir swagatam...
      You enlightened me.

  5. Can you please help me, I was wondering how to know which type the transistor is. I am having some transistor but the marking (ie. the type number ..BC xxx and so forth) is not visible as I kept it long time ago as I bought them as spare. Is there anyway to find out what type of transistor it belong. Thanks.

    1. without the number it could be difficult to judge the exact specs of the transistor, most probably they could be within the range of 50V, and anywhere from 100mA to 800mA

  6. Sir, can i use ctc 880 transistor instead of 2sd880, I'm making a ark lighter, thanks


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