# Unijunction transistors

#### luis_valderrama

Joined Mar 23, 2006
1
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.

#### Papabravo

Joined Feb 24, 2006
12,775
Originally posted by luis_valderrama@Mar 23 2006, 10:31 PM
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.
[post=15372]Quoted post[/post]​
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.

#### beenthere

Joined Apr 20, 2004
15,819
Hi,

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.

#### Mazaag

Joined Oct 23, 2004
255
Originally posted by Papabravo@Mar 23 2006, 09:46 PM

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.
[post=15373]Quoted post[/post]​

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 ?

#### Papabravo

Joined Feb 24, 2006
12,775
Originally posted by Mazaag@Mar 30 2006, 01:19 AM
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 ?
[post=15612]Quoted post[/post]​
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.
Rich (BB code):
R = (Vdd - Vf)/I_led
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....