Unijunction Transistor (UJT) Circuit Design, Pinout, and Testing Guide

The unijunction transistor (UJT) is a three-terminal semiconductor device prized for its negative resistance characteristics and simplicity. While microcontrollers and 555 timers dominate modern timing circuits, the UJT remains the undisputed, low-part-count champion for generating high dV/dt trigger pulses for SCRs and TRIACs in power control applications. If you need to fire a thyristor or build a rugged relaxation oscillator without writing a line of code or worrying about digital noise, the 2N2646 is your default, safe-bet part number.

The Unijunction Transistor Pinout, Symbol, and Core Specs

Unlike a bipolar junction transistor (BJT), the UJT does not amplify current. It acts as a voltage-controlled switch. The circuit symbol features an arrow pointing into a diagonal line, representing the Emitter (E) intersecting the silicon bar between Base 1 (B1) and Base 2 (B2).

The internal structure consists of a single PN junction. The silicon bar between B1 and B2 acts as a resistor, known as the interbase resistance ($R_{BB}$). This is the most critical parameter when biasing the device.

UJT Operation Regions and Biasing Basics

To use a UJT, you must understand its three distinct operating regions. The device is biased by applying a voltage ($V_{BB}$) across B2 and B1. The internal resistance divider creates a threshold voltage at the Emitter junction, determined by the intrinsic standoff ratio ($\eta$).

How to Bias It: The peak point voltage ($V_P$) at which the UJT turns ON is calculated as:
$V_P = (\eta \times V_{BB}) + V_D$
Where $V_D$ is the forward voltage drop of the internal PN junction (typically 0.5V to 0.7V). If $V_{BB}$ is 12V and $\eta$ is 0.65, the UJT will remain in cutoff until the Emitter voltage reaches approximately 8.3V. Once $V_E$ hits 8.3V, the device snaps into the negative resistance region, dumping stored charge from the timing capacitor into B1.

Building a UJT Relaxation Oscillator (Complete Circuit)

The most common application for a UJT is the relaxation oscillator. This circuit generates a sawtooth waveform at the Emitter and sharp, positive trigger pulses at Base 1. Below is a complete, bench-tested design for a ~95 Hz oscillator driving an SCR gate.

Component Values and BOM

• U1: 2N2646 (N-Type UJT)
• $V_{CC}$: 12V DC regulated supply
• $R_T$ (Timing Resistor): 100 kΩ, 1/4W (Must be > $R_{min}$ to ensure turn-off)
• $C_T$ (Timing Capacitor): 100 nF (0.1 μF) film or ceramic
• $R_{B1}$ (Base 1 Resistor): 100 Ω (Generates the output trigger pulse)
• $R_{B2}$ (Base 2 Resistor): 470 Ω (Provides temperature compensation for $\eta$)

Assembly and Calculation Steps

1. Establish the DC Bias: Connect $R_{B2}$ (470Ω) between $V_{CC}$ (12V) and B2. Connect $R_{B1}$ (100Ω) between B1 and Ground. This sets up the interbase voltage divider.
2. Wire the Timing Network: Connect $R_T$ (100kΩ) between $V_{CC}$ and the Emitter. Connect $C_T$ (100nF) between the Emitter and Ground. The capacitor charges through $R_T$.
3. Extract the Output: Take your trigger output across $R_{B1}$. When the UJT fires, a sharp positive pulse appears here.
4. Verify Frequency: The time period ($T$) is calculated as $T = R_T \times C_T \times \ln(\frac{1}{1-\eta})$.
   Assuming $\eta = 0.65$: $\ln(1 / 0.35) \approx 1.05$.
   $T = 100,000 \times 0.0000001 \times 1.05 = 0.0105$ seconds (10.5 ms).
   Frequency $f = 1 / T = 1 / 0.0105 \approx 95.2 Hz.

Troubleshooting: How UJTs Fail and Multimeter Testing

UJTs are rugged, but they can fail from thermal runaway, exceeding the $V_{B2B1}$ max rating (35V for the 2N2646), or excessive emitter current. The most common failure mode is a shorted Emitter-Base1 junction or an open interbase silicon bar.

You can definitively test a UJT on the bench using a standard digital multimeter (DMM). Set your meter to Diode Test mode for junction checks, and Resistance (Ω) mode for the interbase check.

1. Test Emitter to Base 1 (Forward Bias): Place the red probe on the Emitter and the black probe on B1. You should read a standard silicon diode drop (0.45V to 0.75V). Reverse the probes; it should read 'OL' (Open Loop).
2. Test Emitter to Base 2 (Forward Bias): Place the red probe on the Emitter and the black probe on B2. Expect the same 0.45V to 0.75V reading. Reverse probes for 'OL'.
3. Test Interbase Resistance ($R_{BB}$): Switch the DMM to Resistance mode. Place probes across B1 and B2 (polarity does not matter). You should read between 4.0 kΩ and 9.0 kΩ for a 2N2646.

Verdict Guide:
- If E-B1 or E-B2 reads 0.00V or 0Ω: The PN junction is shorted. Replace the UJT.
- If B1-B2 reads 'OL': The internal silicon bar is fractured/open. Replace the UJT.
- If B1-B2 reads < 1 kΩ: The device has suffered thermal degradation and will have a shifted $\eta$. Replace it.

Decision Tree: Selecting the Right UJT or PUT for Your Design

While the standard UJT is excellent for fixed-frequency or simple variable-frequency oscillators, the Programmable Unijunction Transistor (PUT) and the classic 555 timer offer alternatives. Use this decision matrix to lock in your component selection.

The Final Decision Path

• IF you need to generate a simple, low-frequency (<10 kHz) sawtooth wave or trigger an SCR/TRIAC directly from a high-voltage DC bus (up to 30V) with minimal parts...
  THEN choose the Standard UJT.
• IF you need a precise, adjustable trigger threshold for a crowbar overvoltage protection circuit or a highly stable variable-frequency oscillator...
  THEN choose the PUT (2N6027).
• IF you need a 50% duty cycle square wave, complex PWM, or frequencies above 100 kHz...
  THEN abandon the UJT family and use an NE555 or a microcontroller.

For deeper theoretical analysis of the negative resistance curve, refer to the Electronics Tutorials UJT Guide. For exact thermal derating curves and spice models, consult the ON Semiconductor 2N2646 Product Page.