5

A manual fan controller in my PC just 'sploded: fair enough, it was about 10 years old, and driving more fans than appropriate.
I got around to fix it, and figured I needed to change the BJT, because the one employed is not available anymore (D882).

To learn more, I tried to derive the schematics, it's extremely basic and this is the result: enter image description here

I remember being taught in school that there should always be a resistor on the base of a BJT, to avoid pushing too much current through it; afterwards, Ic = hFE * Ib in steady state.

This thing drives an inductive load, a PC cooling fan like this, so it's about 0.2A @ 12V max. When the trimpot is at max, Vb = 12V.
What limits the current through the base of the BJT in this case?
Is it just that the current equation also works the other way around, so Ib = Ic / hFE?

winny
  • 13,064
  • 6
  • 46
  • 63
Jack
  • 352
  • 1
  • 10
  • Potentiometer is the base resistor. As voltage follower that works then the load current via load divided by Hfe transistor smaller than current of voltage divider. Overvise the load voltage depend of it's current. – user263983 Feb 24 '22 at 19:23

2 Answers2

8

What limits the current through the base of the BJT in this case?

In your circuit the emitter is the output and, as you apply more base voltage, the emitter voltage will naturally rise and keep base current to no more than what is needed to drive the load. This type of circuit is called an "emitter-follower" for that reason.

It's quite often called a "common-collector" configuration too.

I remember being taught in school that there should always be a resistor on the base of a BJT, to avoid pushing too much current through it

That does apply when the configuration is "common-emitter" because the emitter can be tied down to ground and inevitably, if you don't use a base resistor, you'll likely end up with problems and failures of the transistor.

Andy aka
  • 434,556
  • 28
  • 351
  • 777
  • Right, but even then, I will add a safety resistor (maybe between the potentiometer high pole and the 12V) tuned to limit the max current/power on the BJT... So that it'll not explode anymore I case of overload or short :-) – Rmano Feb 25 '22 at 13:29
2

The transistor is connected in an emitter follower configuration.

It means that the emitter voltage output Ve is roughly the input base voltage Vb minus one diode drop of 0.7V, and it still amplifies the current by taking it from collector.

So technically, it does not need a base resistor.

Justme
  • 127,425
  • 3
  • 97
  • 261