Programmable Unijunction Transistors (PUT) are versatile and cost-effective semiconductor devices primarily used in timing, triggering, and pulse generation applications. While the name may suggest similarity to the conventional Unijunction Transistor (UJT), a PUT is actually more flexible and programmable in nature, making it a preferred component in many modern electronics applications.
In this article, we will explore the internal structure, working principles, characteristics, and real-world uses of PUTs to understand why they remain relevant in today’s analog and mixed-signal circuit designs.
1. What Is a Programmable Unijunction Transistor (PUT)?
A Programmable Unijunction Transistor (PUT) is a four-layer, three-terminal device that belongs to the thyristor family. It consists of an anode (A), a cathode (K), and a gate (G). Structurally, it resembles a Silicon-Controlled Rectifier (SCR) but is configured to behave like a UJT.
The main feature that distinguishes the PUT is its programmable nature. By selecting the appropriate values of external resistors connected to the gate terminal, the peak point voltage of the PUT can be modified, which allows the designer to “program” the triggering point of the device.
2. Programmable Unijunction Transistors (PUT) Structure and Symbol
2.1 Internal Structure
The PUT is typically made using an NPNP or PNPN semiconductor structure. Its operation depends heavily on the gate biasing provided through external resistors.
Basic structure overview:
- The gate terminal is connected to the base of the equivalent transistor inside the PUT.
- A voltage divider (often two resistors) is connected between the anode and cathode to set the gate bias.
2.2 Symbol and Pin Configuration
The symbol of a PUT is quite similar to a UJT or SCR but with emphasis on the programmable gate terminal. The three terminals are:
- Anode (A)
- Cathode (K)
- Gate (G)
3. Working Principle of Programmable Unijunction Transistors (PUT)
The PUT remains off as long as the anode voltage is less than the gate voltage plus the forward voltage drop of the internal junction. When the anode voltage exceeds the gate voltage by approximately 0.7V (a silicon diode drop), the PUT turns on, conducting from anode to cathode.
This negative resistance behavior makes it suitable for triggering and oscillating circuits.
Key Steps:
- The anode voltage increases.
- When VA > VG + 0.7V, the junction becomes forward-biased.
- The PUT switches on and conducts.
- Once the current drops below the holding current, it switches off again.
4. Programmable Unijunction Transistors (PUT) Characteristics and Parameters
Important Electrical Characteristics:
- Peak Voltage (Vp): Voltage at which the PUT turns on.
- Valley Voltage (Vv): Voltage at which the PUT returns to the off state.
- Holding Current (IH): Minimum current to keep the PUT on.
- Intrinsic Standoff Ratio (η): Not directly specified but controlled externally.
Programmability Advantage:
Unlike UJTs, PUTs allow you to define Vp by choosing the external resistor divider connected to the gate terminal.
5. Comparison Between PUT and UJT
| Feature | PUT | UJT |
|---|---|---|
| Construction | Similar to SCR | Unique intrinsic base construction |
| Programmability | Yes, via external resistors | No |
| Peak Point Voltage | Adjustable | Fixed by design |
| Flexibility in Design | High | Limited |
| Usage in Modern Design | Still used | Becoming obsolete |
6. Typical Programmable Unijunction Transistors (PUT) Circuits and Applications
6.1 Relaxation Oscillator
A PUT can be used in a relaxation oscillator circuit that generates periodic waveforms. A capacitor charges through a resistor until the PUT fires, then discharges rapidly.
Applications:
- Sawtooth wave generators
- Clock pulses for digital circuits
6.2 Timer Circuits
PUTs are used in timing circuits where precision control over the triggering point is required. The timing interval depends on the RC time constant.
6.3 Triggering Circuits
In power electronics, PUTs are often used to trigger SCRs by providing the required gate pulse at the right moment.
Example:
- Light dimmers
- Motor speed controls
6.4 Pulse Generation
PUTs are highly suitable for narrow pulse generation due to their sharp switching behavior.
7. Designing with a Programmable Unijunction Transistors (PUT): Example Circuit
PUT Relaxation Oscillator Example:
diffSalinEditComponents:
- R1 = 100 kΩ
- R2 = 10 kΩ
- C = 0.1 µF
- PUT (2N6027 or equivalent)
Operation:
- R1 and R2 form the voltage divider to set gate bias
- C charges through a resistor until PUT fires
- Capacitor discharges through PUT rapidly
- Generates repetitive pulses
This circuit can be connected to an SCR gate for controlled switching of AC loads.
8. Advantages of Programmable Unijunction Transistors (PUT)
- Simple and cost-effective
- Programmable triggering point
- Sharp switching behavior
- Works well in analog pulse circuits
- Can replace UJT in many circuits
9. Limitations of Programmable Unijunction Transistors (PUT)
- Limited to low and medium power applications
- Sensitive to temperature variations
- Not suitable for high-speed switching applications
10. Popular Programmable Unijunction Transistors (PUT) Part Numbers
Some commonly available PUTs:
- 2N6027
- 2N6028
These can be found easily from electronics suppliers and are often used in educational kits and hobby projects.
11. Conclusion of Programmable Unijunction Transistors (PUT)
The Programmable Unijunction Transistor (PUT) remains a valuable component in analog electronics, offering an adjustable, reliable, and efficient way to manage triggering and pulse-generation tasks. Its programmability makes it more flexible than traditional UJTs, and its simplicity allows for quick prototyping and integration into timing circuits.
Although it may not be suited for high-frequency or high-power applications, the PUT continues to play a role in both educational and practical electronic systems, especially where control and timing are critical.
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