I'm a beginner to electronics. I just started to test transistors, but I've some doubts.

I've used this schematic to test a NPN 2N2222:

enter image description here

My questions:

1 - How much Voltage or Amps should the base and the collector receive to pass current to the emitter? I can't figure out the math.

2 - With this setup, the LED when I don't have the button pressed have 1V current. I'm wasting energy? I can tunning this schematic to not wast any voltage when the button is not pressed?

Please give me some clues.

Best Regards,

  • \$\begingroup\$ possible duplicate of How does the collector emitter relationship work with a transistor? \$\endgroup\$ Apr 25, 2013 at 21:26
  • \$\begingroup\$ On your point (2) check you haven't got base and collector swapped over in your circuit - this would possibly explain your 1V across the LED. \$\endgroup\$
    – Andy aka
    Apr 25, 2013 at 22:09
  • \$\begingroup\$ Thanks for your answer. The transistor was actually swapped and I was measuring the voltage incorrectly. I've read that the voltage should be reade allways across the resistors. This is correct? \$\endgroup\$
    – André
    Apr 25, 2013 at 22:48
  • \$\begingroup\$ Your 10k resistors will probably result in a very dim led, even with the transistor entirely shorted. \$\endgroup\$
    – pjc50
    Apr 25, 2013 at 22:53

2 Answers 2


This may be a common emitter config but it seems that he's trying to test whether the npn is good or not, sorta like a go - no-go test, which is more like a comparator (saturates the npn). May I suggest a simpler circuit? You dont need two supplies, just a +9V battery. Assuming the LED lights up with ~5mA, you'll get 1.8V to 2V drop across the LED. So the emitter resistor should be 1K, and tie the collector directly to +9V. When the button is pushed, the 100uA or so of base current will be sufficient to make 5mA or so of emitter current. You'll get 5V drop across the 1k, 2v across the LED, and the npn will be in sat. You might want to add a 1M resistor on the base to gnd (to ensure npn is off when button is not pushed).


The configuration it appears that you are attempting to construct is called a "common emitter amplifier". That should point you in the right direction for finding examples and explanations.

Here is another good explanation (thanks @LeonHeller!)


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