I started learning Transistors with this tutorial from the Sparkfun website. The tutorial is good and pretty easy to understand. So I decided to put my learning to the test and assembled a simple NPN transistor as switch. Here is what the tutorial states as the condition to toggle it between Saturation and Cut-off states:
Saturation: Ve < Vb and Vc < Vb
Cut-off: Ve > Vb and Vc > Vb
Below is the circuit I assembled on a simple breadboard to test these conditions in each state.
It works as expected. The LEDs turn on in the Saturation state and turn off in the Cut-off state. But when I measured the voltages to see if the conditions are satisfied as per the tutorial, I can see some voltage readings don't obey the above conditions. I have noted down my readings below:
- Vbe or Vb = 0.62
- Vbc or Vc = -4.92
- Ve = 0
In the saturation state, the conditions are satisfied Ve < Vb and Vc < Vb
However in the Cut-off state, the readings are:
- Vbe or Vb = 0.02
- Vbc or Vc = -7.24
- Ve = 0
In this case the readings doesn't hold the conditions (Ve > Vb and Vc > Vb). Actual Vb is still greater than Ve and Vc. Why is this happening? Did I miss anything here? Why do the actual measured readings seem to not be obeying the above conditions?
Besides these, in the Cut-off state where there is no collector current, I can still observe voltage drops between those LEDs connected to the collector terminal of transistor. Why is this happening? When current is zero, voltage drop shouldn't exist, isn't that true?