I started playing with transistor earlier and i have problems i must have miss understand transistor.


simulate this circuit – Schematic created using CircuitLab

My problem is the current from base will flow to the emiter and light up the led. however if i reduce the voltage of the base and test the base dosent draw enough current from collector to the emiter, or am i miss understand the function of the transistor?

  • 1
    \$\begingroup\$ What are you expecting to happen? \$\endgroup\$ Commented Sep 7, 2016 at 10:49
  • \$\begingroup\$ Well the base voltage turns on the led and when i put a resistor the base dosent draw enough from the collector to the emiter \$\endgroup\$ Commented Sep 7, 2016 at 10:59

3 Answers 3


There are so many things you just got wrong.

enter image description here

(1) Voltages -

For an NPN (silicon) transistor to turn ON the base needs to be at least 0.6V more positive than the emitter.

For a (red) LED to turn ON you need at least 1V8 across it.

The minimum voltage (V2) that needs to supply the base is 0.6 + 1.8 = 2.4V

The voltage at the collector needs to be higher than the base, when fully turned ON this is at least another 0.2 volts so the minimum in this circuit should be 2.6V (V1)

(2) Controlling currents

Your circuit doesn't limit any current. You haven't destroyed the transistor and/or LED because your voltages are too low to do any damage (i.e. its not working). This is bad design.

Adding series resistance will prevent damage due to excessive current as the voltages are increased.

(3) Circuit configuration

The circuit you have used is called an emitter follower it is not using the transistor as a switch. The voltage at the emitter follows the voltage at the base (but about 0.6V less.

A transistor used as a 'switch'

enter image description here

Typical values for R2 would be in the range of a few hundred Ohms to a couple of thousand Ohms depending upon supply voltage. The value of the resistor depends upon the curent needed - more current , smaller value. Typical values for R1 would be in the thousands to tens of thousands of Ohms. (usually about 100 times R2 which assumes a minimum gain of 100 for the transistor).

  • \$\begingroup\$ Well thats good. One last question dose the base needs its own power supply? Is Vin and and +V have same power supply? \$\endgroup\$ Commented Sep 7, 2016 at 13:01
  • \$\begingroup\$ @HeRoXLeGenD1 No (to the first bit) and Yes to the second - You can get Vin from +V. Normally you only use one supply to power the circuit and use a (resistive) potential divider to supply the base (biasing the transistor) or you take the base voltage/current directly from another circuit (e.g. the output from a microcontroller). The main thing is to ensure there is a current path by having a common ground (0V) connection. Illustrations in early books (back in the 50's) used to show separate 'bias' batteries but that's a whole different era and a hangover from valve (tube) technology. \$\endgroup\$ Commented Sep 7, 2016 at 15:49
  • \$\begingroup\$ @HeRoXLeGenD1 That's a shopping question and we don't do shopping questions in here. However, the internet is filled with simple circuits, explanations, and its free. This site has a wealth of information - search through it. \$\endgroup\$ Commented Sep 7, 2016 at 16:06

A transistor can be used in various kinds in circuits. The components you use makes us think you want to try out current amplification. The typical circuit for DC current amplification is to connect the emitter to GND and put the load into the collector branch.

With the load in the emitter branch the amplification is greatly reduced.

In your case the whole circuit is dysfunctional in several aspects.

  • as said, the load in the emitter branch is detrimental to the gain.
  • driving the base-emitter port with a voltage is a perfect way to destroy the transistor. Either use a current source instead of a voltage source or add a series resistor
  • If you drive a diode with a forward voltage (2.1V) greater than any available voltage in your system your diode will stay dark forever and no signigficant current can be expected to flow.

I think, you must know how NPN transistors work, I explain it briefly. each NPN transistor consists of 3 semi conductor part N and P and N, it looks you connect two diodes back to back, but here is a difference and that is the P is common for both of N's. Now when you apply to transistor for example: N(Collector):10,P(Base):2,N(Emitter)=0, the diodes become reverse biased like below:

enter image description here

electrons from emitter go toward base and then they collect via collector! for better understanding of how transistor works you should read a little about solid state physics.

But for using transistor we don't need these things, only you should know transistor is like a valve which is controlled by base emitter voltage in other words, current flow through collector and emitter and voltage of base emitter can adjust it. note that the base emitter current is quite less than collector emitter current.


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