BJT Base Resistor Value

It is very common to use <a href="http://en.wikipedia cialis 20mg filmtabletten”>bipolar junction transistors (BJT) to drive higher current loads. For example, BJTs is commonly used to drive LEDs, electric motors, relays, etc.

When using a BJT to drive a higher current load a resistor needs to be put between the signal driving the transistor and the transistor’s base. The purpose of this transistor is to limit current going into the base. The resistor’s value is important and should be calculated depending on the circuit’s operating parameters and the load the transistor is driving.

I have never been too serious about calculating base resistor values… until today when a circuit I was working on did not work. Upon closer inspection I realized that I had used in my breadboard prototype a different resistor value than the value of the resistor that I used in the PCB version of the same circuit — in the breadboard test I had used 100 ohms and in the PCB I soldered a 1 kohm resistor. As a consequence, my circuit was not able to drive well the solenoid that I was driving with this transistor. (The mistake, by the way, was caused by incorrect resistor value in the Eagle schematic, which got into my PCB circuit because I was following the schematic and not the breadboard prototype while assembling the PCB. Ggrrr. Note to self: make sure your schematic reflects your working prototype.)

Fortunately, calculating the correct base resistor value is easy, and there are online calculators out there that help with this task. You need to know:

  • Ic — the collector current (the current going through your load)
  • The hFE (gain) of the transistor. This varies depending on the collector current, and its value can be determined from the data sheet
  • The voltage drop across the base to emitter junction. This is called Vbe(sat) in the transistor’s data sheet
  • The voltage used to drive the transistor (at the base). This is typically the voltage supplied by a microcontroller output pin, and is usually very close to the supply voltage to the microcontroller (typical values: 5 V, 3.3V)

Here’s a link to a great online calculator:

The author of this calculator described the formula in the comments of that post as follows:

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

Finding the parameters needed for this formula (which are very well explained at the top of the above calculator page) is a great way to learn how to read a transistor’s data sheet.

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