Unijunction transistors

Discussion in 'The Projects Forum' started by luis_valderrama, Mar 23, 2006.

  1. luis_valderrama

    Thread Starter New Member

    Mar 23, 2006
    I am beginning to experiment with electronics and in the town I live the stores that sell components do not have knowledgeable sales people in the field. Can anyone be so kind as to tell me how can I find out which is the collector, base 1 and base 2 in a unijunction transistor?
    How to test it to see if it is in working order?
    Another topic: Just out of curiosity, why does a white ultrabright led goes out when I connect in paralel another led (mainly red). I can connect in parallel almost any number of white leds.
  2. Papabravo


    Feb 24, 2006
    If you have the part number for the UJT you can go to the manufacturers website and download the datasheet. The datasheet will have the information you are looking for.

    The white LED and the red LED have different forward voltages. The one for the red LED is lower than the one for the white LED. When you connect them in parallel the red LED hogs all the available current at it's Vf. As a result the white LED cannot become forward biased and as a result it can conduct no current. In the LED universe -- no current -- no light.

    I suppose you could market it as a DED, that's a dark emitting diode.
  3. beenthere

    Retired Moderator

    Apr 20, 2004

    Just to be correct, a unijunction transistor has a base and two emitters. I know the tab on the TO-92 can marked the base, but don't know baout the emitters - they might be interchangeable. Look up the specs on a 2N2646 and you will know all about them, unless you have a programmable UJT.
  4. Mazaag

    Senior Member

    Oct 23, 2004

    How is it possible to correct such a situation ?

    Another thing, if LEDs require current to light up , does that mean I have to include a resistor in series with my LED to make it light up ? if so , how can I calculate the voltage drop across the LED to make sure its getting Vf ?
  5. Papabravo


    Feb 24, 2006
    You have to drive the LEDs with different forward voltages separately.

    A resistor, a LED, and a switch(electronic or otherwise) are connected in series across a power supply of some voltage, call it Vdd. With the switch in the ON position we assume that the voltage drop across the switch is approximately zero volts. The voltage drop across the diode will be Vf if the diode is forward biased. At this Vf some amount of current will flow. How much depends on the resistor.
    Either pick the resistor and calulate current, or pick the current and calculate the resistor.
    Code ( (Unknown Language)):
    2. R = (Vdd - Vf)/I_led
    3. I_led = (Vdd - Vf)/R
    The assumes that Vdd >> Vf
    Typical Vf for a RED LED is 2.0 Volts
    Typical Vf for a GREEN LED is 2.2 Volts

    ansd so it goes....