The Unijunction Transistor (UJT) symbol is defined by an emitter arrow pointing at an angled base bar, flanked by Base 1 (B1) and Base 2 (B2). Unlike a standard Bipolar Junction Transistor (BJT), the UJT has only one PN junction, which the schematic symbol reflects by terminating the arrow directly against the base bar rather than passing through it. If you are troubleshooting a relaxation oscillator or a thyristor trigger circuit, identifying the exact pinout and verifying the intrinsic standoff ratio ($\eta$) is critical before applying power. Below is the complete symbol and physical pinout reference.
UJT Symbol and Pinout Reference Table
| Symbol Element | Schematic Name | Physical Pin (TO-18 Metal Can) | Physical Pin (TO-92 Plastic) | Electrical Function |
|---|---|---|---|---|
| Angled Arrow | Emitter (E) | Pin 1 (Adjacent to tab) | Pin 1 (Left, flat side facing you) | Injects carriers into the base bar; triggers when $V_E > \eta V_{BB} + V_D$ |
| Base Bar (Bottom) | Base 1 (B1) | Pin 2 (Clockwise from E) | Pin 2 (Center) | Reference ground for the emitter; outputs the trigger pulse |
| Base Bar (Top) | Base 2 (B2) | Pin 3 (Clockwise from B1) | Pin 3 (Right) | Positive bias supply connection; sets the interbase voltage ($V_{BB}$) |
| Circle / Box Outline | Package Envelope | N/A (Metal can is grounded/shielded) | N/A | Denotes discrete semiconductor package in IEEE 315 standard |
Standard Variants: IEEE vs. IEC Symbol Conventions
While the UJT is largely considered a legacy component, you will still encounter it in older industrial control boards, vintage synthesizers, and legacy power supply crowbar circuits. The way it is drawn depends on the regional standard your schematic adheres to.
- IEEE 315 / ANSI Y32.2 (North America): The standard representation uses a circle enclosing the symbol. The base bar is drawn as a straight vertical or horizontal line, and the emitter arrow points directly at the bar at a 45-degree angle. The arrowhead touches the bar, distinguishing it from a JFET where the gate arrow is separated by a gap.
- IEC 60617 (Europe / International): The IEC standard generally favors rectangular boxes for semiconductor devices rather than circles. In strict IEC schematics, the UJT is drawn inside a rectangular boundary with the alphanumeric designators (E, B1, B2) placed outside the box. However, because the UJT is rarely designed into new IEC-compliant equipment, most modern European schematics simply borrow the IEEE circle-and-arrow symbol for clarity.
Rows and Markings People Get Wrong
When reading schematics or wiring physical boards, two specific mistakes account for nearly all UJT-related failures on the bench.
- Swapping B1 and B2: Physically, a UJT looks symmetrical. Electrically, it is not. The silicon bar is doped with a gradient. If you swap B1 and B2, the intrinsic standoff ratio ($\eta$) inverts. A circuit designed for an $\eta$ of 0.65 will suddenly see an $\eta$ of 0.35. The relaxation oscillator will either fail to trigger entirely, or it will fire at a drastically higher frequency, potentially destroying the downstream SCR or TRIAC.
- Confusing the UJT with a JFET: A Junction Field Effect Transistor (JFET) also has a gate arrow and a channel bar. The critical visual difference is that a JFET's gate arrow does not touch the channel bar (indicating a reverse-biased depletion region), whereas the UJT's emitter arrow physically touches the base bar (indicating a forward-biased PN junction when triggered).
- Ignoring the TO-18 Tab: On metal-can TO-18 packages, the small metal tab protruding from the rim is not just a mechanical key. It is electrically tied to the Emitter (E) pin on most military-spec and legacy Motorola parts. Grounding the tab inadvertently will short the emitter.
Safe Interpretation: Testing Faded TO-92 and TO-18 Packages
Legacy parts like the 2N2646 or 2N2647 often have faded silk-screening or rusted metal cans where the pinout markings are completely illegible. You can definitively identify the pins using a standard digital multimeter (DMM) without guessing. According to Electronics Tutorials, the internal structure acts as a diode and a fixed resistor.
Step-by-Step Identification Procedure:
- Set your DMM to Diode Test mode.
- Probe all three pins to find the Emitter (E). Place the red lead on a pin and the black lead on the other two. If you read a forward voltage drop of ~0.5V to 0.7V on both combinations, the pin under your red lead is the Emitter. (Reverse the leads for PNP/CUJT variants, though N-type is 95% of use cases).
- Switch your DMM to Resistance (Ohms) mode, preferably the 20k$\Omega$ range.
- Measure the resistance between the two remaining pins (B1 and B2). You should read the interbase resistance ($R_{BB}$), which is typically between 4k$\Omega$ and 9k$\Omega$ for a 2N2646. If it reads OL (open) or near 0$\Omega$ (short), the UJT is dead.
- Note: The multimeter cannot easily distinguish B1 from B2 via resistance alone since $R_{BB}$ is bidirectional. To find B1, look for the physical flat-spot (TO-92) or tab (TO-18). Pin 1 (Left/Tab) is E, Pin 2 (Center/Clockwise) is B1, Pin 3 (Right) is B2.
Decision Path: Sourcing for Repair vs. New Design
The UJT is effectively obsolete in modern manufacturing. While you can still buy them for repairs, designing a new circuit around a 2N2646 is poor engineering practice in 2026. Use the decision tree below to select the exact part number for your scenario.
| Scenario | Condition | Action / Concrete Pick |
|---|---|---|
| Repairing vintage audio, old welders, or legacy industrial timers | Must match original $\eta$ and $R_{BB}$ to maintain factory timing | Buy: 2N2646 (NTE6400 or Central Semiconductor equivalent). Do not substitute without recalculating the RC timing network. |
| Designing a new thyristor/SCR trigger circuit | Need adjustable trigger point without changing physical resistors | Buy: 2N6027 (Programmable UJT / PUT). Allows you to set $\eta$ externally via a resistor divider on the gate. |
| Designing a new relaxation oscillator or PWM generator | Need high stability, adjustable duty cycle, and modern availability | Buy: NE555P (Bipolar 555 Timer) or use an ESP32 GPIO with hardware PWM. Abandon the UJT topology entirely. |
| Need a simple DIAC replacement for AC phase control | Circuit requires a bidirectional breakover device, not a 3-terminal switch | Buy: DB3 DIAC (30V breakover). A UJT cannot be used as a direct drop-in for a DIAC without a bridge rectifier. |
For deeper theoretical analysis of the negative resistance region that makes the UJT oscillate, refer to the semiconductor physics breakdown in the All About Circuits semiconductor textbook. Always verify your specific manufacturer's datasheet for the exact $I_P$ (peak point emitter current) before finalizing your timing capacitor value.
