UJT Schematic Symbols & Standard Variants
Before tracing a circuit or designing a trigger board, you must correctly identify the UJT symbol on the schematic. The visual representation changes depending on whether your documentation follows North American or international standards.
| Component Type | IEEE 315 / ANSI (US Standard) | IEC 60617 (EU / Global Standard) | Key Visual Identifier |
|---|---|---|---|
| N-Channel UJT | Arrow pointing IN toward a diagonal bar | Arrow pointing IN toward a diagonal bar, enclosed in a circle | Emitter arrow touches the base bar; Base 1 and Base 2 extend from the bar ends. |
| P-Channel UJT | Arrow pointing OUT away from a diagonal bar | Arrow pointing OUT away from a diagonal bar, enclosed in a circle | Emitter arrow originates on the base bar and points outward. |
| Programmable UJT (PUT) | Resembles an SCR symbol with the gate arrow on the anode side | Similar to IEEE, often with a rectangular enclosure | Three terminals: Anode, Cathode, and Gate. NOT a true UJT internally. |
| Standard NPN BJT (Contrast) | Arrow pointing OUT from a vertical bar | Arrow pointing OUT from a vertical bar, no circle | Collector and Emitter are on the same side of the base bar; UJT bases are on opposite ends. |
What Each Symbol Element Means in Practice
The diagonal or vertical bar in the UJT symbol represents the lightly doped N-type (or P-type) silicon bar that forms the base region. The two connections at the ends of this bar are Base 1 (B1) and Base 2 (B2). The arrow represents the Emitter (E), which is a heavily doped P-type (or N-type) region alloyed into the base bar closer to B2. When the emitter voltage exceeds the intrinsic standoff ratio voltage plus the diode forward drop, the UJT fires, dropping the resistance between E and B1 to near zero.
Which Standard Applies to Your Region
If you are reading schematics from US-based legacy manufacturers (like Motorola, GE, or RCA) or working with North American industrial equipment, you will encounter the IEEE 315 (formerly ANSI Y32.2) symbols without the enclosing circle. If you are working with modern European designs, IEC-compliant PLC schematics, or global academic textbooks, the IEC 60617 standard mandates a circle around the transistor symbol to denote the physical device envelope. According to Electronics Tutorials, the underlying circuit behavior remains identical regardless of the circle's presence.
Rows People Get Wrong
- Confusing the PUT with the UJT: A Programmable Unijunction Transistor (like the 2N6027 or BRY39) is actually a four-layer PNPN thyristor. Its symbol looks like an SCR with the gate moved to the anode side. You cannot swap a PUT into a circuit designed for a true UJT (like the 2N2646) without redesigning the biasing network.
- Misreading the N-UJT as a PNP BJT: An N-channel UJT has an arrow pointing into the base bar. A standard PNP BJT also has an arrow pointing inward (on the emitter). The giveaway is the base connections: a BJT has one base connection and two connections on the opposite side (Collector/Emitter). A UJT has the emitter on one side, and two base connections on the opposite ends of the bar.
Physical Pinouts & Testing Faded Markings
Most classic true UJTs, such as the widely used 2N2646 or 2N6114, come in the standard TO-92 plastic package. However, when salvaging components or working with vintage equipment, the flat-side silkscreen marking is often rubbed off or obscured by conformal coating.
Standard TO-92 Pinout (Bottom View)
Hold the TO-92 package with the flat side facing you and the leads pointing downward. For the vast majority of Motorola/ON Semi and Fairchild 2N2646 variants, the pins from left to right are:
- Emitter (E)
- Base 1 (B1)
- Base 2 (B2)
Note: Always verify with a datasheet for specific manufacturers, as some Japanese-market UJTs (e.g., 2SH21) occasionally reverse the B1 and B2 positions.
Safe Interpretation When Markings Are Faded
If the text is illegible, use a digital multimeter (DMM) in Diode Test mode to definitively map the pins. This method relies on the internal physics of the UJT and works 100% of the time for true N-channel UJTs.
- Find the Emitter (E): Probe all three pins. You are looking for the pin that shows a forward diode voltage drop (typically 0.45V to 0.75V) to both of the other two pins when the red lead is on the Emitter and the black lead is on the Bases. The reverse polarity (black on Emitter) should read 'OL' (Over Limit) for both. This pin is the Emitter.
- Identify B1 and B2: Switch your DMM to Resistance mode (Ohms). Measure the resistance between the two remaining Base pins. You should read the interbase resistance ($R_{BB}$), typically between 4kΩ and 10kΩ for a 2N2646. To distinguish B1 from B2, measure the resistance from the Emitter to Base A, and then Emitter to Base B. Because the emitter junction is physically located closer to B2 inside the silicon die, the resistance from E to B2 will be slightly higher than E to B1 when measured with a low-voltage DMM (which doesn't trigger the junction). The pin with the lower resistance to the Emitter is Base 1.
For deeper analysis of UJT relaxation oscillator math and intrinsic standoff ratio ($\eta$) calculations, All About Circuits provides excellent worked examples of timing capacitor selection based on these physical pin characteristics.
Frequently Asked Questions
What is the difference between a UJT and a PUT symbol?
The unijunction transistor (UJT) symbol features an emitter arrow pointing at a simple base bar with two base connections. A Programmable Unijunction Transistor (PUT) symbol looks like a Silicon Controlled Rectifier (SCR) where the gate arrow is attached to the anode side rather than the cathode side. Internally, a UJT is a three-layer device with a single PN junction, while a PUT is a four-layer PNPN thyristor. You can configure a PUT's firing voltage using external resistors, whereas a true UJT's firing voltage is fixed by its internal manufacturing geometry (the intrinsic standoff ratio).
Can I substitute a 2N2646 with a standard NPN transistor?
No. A standard NPN BJT (like the 2N2222 or 2N3904) requires continuous base current to maintain conduction between the collector and emitter. A UJT operates as a negative-resistance switch; it remains non-conductive until the emitter voltage reaches the peak point ($V_P$), at which point it snaps into conduction and stays on until the emitter current drops below the valley point ($I_V$). Substituting a BJT into a UJT relaxation oscillator circuit will result in a circuit that either fails to oscillate or simply shorts the supply rail.
Why does my schematic show a UJT symbol with a circle around it?
The circle indicates that the schematic was drawn to the IEC 60617 standard, which is prevalent in Europe, the UK, and international industrial documentation. The circle represents the physical envelope or package of the semiconductor device. The IEEE 315 (ANSI) standard, common in North America, omits the circle. The presence or absence of the circle does not change the electrical function or pinout of the component.
How do I read the intrinsic standoff ratio from the symbol?
You cannot read the exact intrinsic standoff ratio ($\eta$) directly from the schematic symbol, as it is a manufacturing parameter, not a visual one. However, the symbol gives you a structural clue: the emitter arrow is drawn physically closer to the Base 2 terminal than to Base 1. This visual asymmetry represents the internal doping geometry that creates the voltage divider effect inside the silicon bar. To find the exact $\eta$ value (typically 0.5 to 0.8 for a 2N2646), you must consult the specific manufacturer's datasheet or measure it on the bench using a variable power supply and an oscilloscope to capture the firing threshold.
