admin January 18, 2011 15 Comments


To design and setup a UJT relaxation oscillator.


1. UJT (2N2446)

2. Resistors

3. Capacitor

4. CRO

4. DC power supply



Take Vp= 10 V and f =1 KHZ

Vp= η VBB+ Vd (Vd can be neglected)

VBB=15V (Assume η = 0.62)

From data sheet valley point specifications are Vv=1.5 V and Iv =4 mA and Ip= 5 mA

Combination of R2 and R4 is indicated as R.

R max= (VBB-Vp)/Ip =1 M

R min = (Vbb-Vv)/Iv =3.2 K

Take the GM of the two values R= 60 K (For this use a 100 K pot in series with 10 K

Now T =RC ln[1/(1-η)]

So C=0.004 uF (use 0.01 uF)

Use 100 Ω resistors at the two bases to provide low discharging path


UJT is the Uni Junction Transistor. It is a three terminal device. They are: a) emitter b) base1 c) base2. The equivalent circuit is shown with the circuit diagram. So there are two resistors. One is a variable resistor and other is a fixed resistor. The ratio of internal resistances is referred as intrinsic standoff ratio (η).It is defined as the ratio of the variable resistance to the total resistance. Due to the existing pn junction, there will be a voltage drop. If we apply a voltage to the emitter, the device will not turn on until the input voltage is less than the drop across the diode plus the drop at the variable resistance R1.When the device is turned on holes moves from emitter to base resulting in a current flow. Due to this sudden increase in charge concentration in base1 region conductivity increases. This causes a drop at base1.This region in the graph is known as negative resistance region. If we further increase the emitter voltage the device undergoes saturation. So a UJT has 3 operating regions:

1. Cut off region

2. Negative resistance region

3. Saturation region

In a relaxation circuit there is an RC timing circuit. When the supply is turned on, the capacitor starts charging. When the voltage across the capacitor reaches the pinch off voltage, the UJT turns on. After discharging of capacitor ,again it starts charging, and this process continues till power supply is turned off.


1. Test the components and identify the leads of UJT

2. Switch on the power supply.

3. Observe the wave forms at bases and emitter of UJT.

4. Plot the graphs



A relaxation oscillator using UJT was designed. Output obtained. Waveforms were plotted.


1. Mention any application of UJT relaxation oscillator

In triggering circuits of SCR

2. What is intrinsic standoff ratio?

See theory


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