Transistor Behavior (when used as a switch)

Discussion in 'General Electronics Chat' started by mbird, Mar 6, 2012.

  1. mbird

    Thread Starter Active Member

    Jan 29, 2009
    Hi --

    I set up a circuit (attached) to explore the 2N3904 transistor. When I change the 100K Pot I get the expected results (my LED goes from dim to bright as the POT goes from high to low resistance). I see that the current through R1 is changing as expected but what I don't understand is why is the voltage across R1 changing? Is that expected? I was expecting only the current to change as the transistor changed its saturation amount?

    My readings:
    IB 1 mA, IR1 25 mA, VR1 5.6 V, Transistor Saturated, LED bright
    IB 24 uA, IR1 3.7 mA, VR1 0.8V, Transistor Not Saturated, LED dim

    So you see above that the transistor is doing what I expect to the current and also the LED does change brightness as expected -- but why does the VR1 change? I realize this must be expected but I can't see why from looking at the literature so if someone can explain (or point to online source) it would be greatly appreciated!

    Thank you!
  2. Audioguru


    Dec 20, 2007
    Every transistor is different. One 2N3904 transistor can have a high current gain but the next one might have a low current gain. The current gain at some currents is spec'd when the collector to emitter voltage is 10V because the gain drops when the transistor is saturated with a low collector to emitter voltage.

    The maximum saturation voltage loss is spec'd when the base current is 1/10th the collector current regardless of the current gain.

    You see the voltage aross R1 changing because the transistor can turn on hard (high collector current) with a high base current or it can barely turn on (low collector current) with a low base current. The collector current is in R1 so Ohm's Law says that the voltage changes.
    mbird likes this.
  3. crutschow


    Mar 14, 2008
    You can't change the current through a resistor without also changing the voltage across the resistor. You need to study Ohm's law. :rolleyes:
    mbird likes this.