Guide to the 2N2646 UJT and Its Applications

Catalog

2N2646 Transistor Basic

The 2N2646 Unijunction Transistor (UJT) is a silicon switching device designed mainly for timing and triggering applications. Unlike standard bipolar transistors, it has a single PN junction and three terminals: emitter, base 1, and base 2. It is best known for its negative resistance characteristic, which allows it to switch quickly once a specific emitter voltage is reached.

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2N2646 Transistor CAD Models

2N2646 Transistor Pinout Details

Alternatives & Equivalent Model

• 2N2647

• 2N4870

• 2N4871

• 2N1671

• 2N1670

• 2N6027  

• 2N6028  

• NTE6401

2N2646 Transistor Specifications

Electrical Characteristics

2N2646 Transistor Features

Negative Resistance Characteristic

The 2N2646 exhibits a negative resistance region once the emitter voltage reaches its triggering threshold. This unique behavior allows the device to switch rapidly from a non-conductive to a conductive state, making it ideal for pulse generation and timing circuits.

Simple Triggering Operation

This UJT requires only a small increase in emitter voltage to turn on, eliminating the need for complex biasing. Its predictable triggering point makes it easy to design stable and repeatable oscillator and control circuits.

Reliable Timing Performance

The intrinsic stand-off ratio of the 2N2646 enables consistent timing when used with basic resistor-capacitor (RC) networks. This makes it well suited for relaxation oscillators and delay circuits with minimal component count.

Wide Operating Temperature Range

Designed for dependable operation across a broad temperature range, the 2N2646 performs reliably in both industrial and educational environments, even under varying thermal conditions.

High Pulse Current Capability

Although it operates at low average current, the device can handle high peak emitter currents for short durations. This feature is especially useful for triggering SCRs and TRIACs in power control applications.

Low Power Consumption

The 2N2646 consumes very little power during normal operation. This efficiency helps reduce heat generation and improves reliability in continuous-duty and low-frequency switching applications.

2N2646 Working in Circuit

2N2646 as a Relaxation Oscillator (LED Flasher Circuit)

In this circuit, the 2N2646 works as a relaxation oscillator that produces periodic pulses. The capacitor slowly charges through the resistor until the emitter voltage reaches the UJT’s intrinsic stand-off threshold. At that point, the 2N2646 suddenly turns on and rapidly discharges the capacitor through Base 1. This quick discharge creates a current pulse that lights the LED. Once the capacitor voltage drops, the UJT turns off again and the cycle repeats, resulting in a blinking LED. The flash rate depends mainly on the resistor and capacitor values.

2N2646 as a Triggering Device (Pulse Control Circuit)

Here, the 2N2646 is used to generate sharp trigger pulses for other components, such as SCRs or pulse transformers. The RC network sets the timing, while the UJT ensures a fast and clean transition when the threshold is reached. When the emitter fires, the sudden current change at Base 1 produces a narrow pulse. This makes the 2N2646 ideal for precise and repeatable triggering in control and power electronics.

2N2646 as a Pulse and Timing Generator

In this application, the 2N2646 combines with inductors, diodes, and capacitors to shape pulses and delays. The UJT controls when energy stored in the timing components is released, producing controlled output pulses. Its negative resistance characteristic allows stable operation with very few parts, making it suitable for timing, testing, and signal-conditioning circuits.

2N2646 Transistor Applications

• Relaxation oscillators

• LED flasher circuits

• Pulse generators

• Timing and delay circuits

• Sawtooth waveform generators

• SCR triggering circuits

• TRIAC triggering circuits

• Phase control circuits

• Pulse shaping circuits

• Test and measurement signal sources

2N2646 Safely Operation

The 2N2646 Unijunction Transistor (UJT) should always be operated within its rated voltage, current, and power limits to ensure reliable and safe performance. Exceeding the maximum interbase voltage or emitter voltage can permanently damage the device, so supply voltages and biasing components must be carefully selected. Using proper resistor and capacitor values helps control the emitter current during triggering and prevents excessive surge currents.

Thermal management is also important for safe operation. Although the 2N2646 typically works at low average power, it can experience high peak currents during switching. Adequate spacing, good airflow, and correct mounting help dissipate heat and improve long-term stability. Operating the device within its specified temperature range further reduces the risk of failure.

Finally, stable circuit design plays a key role in safety. Proper grounding, correct pin connections, and noise suppression components help prevent false triggering and erratic behavior. By following datasheet recommendations and using quality components, the 2N2646 can operate safely and consistently in timing, triggering, and control applications.

Comparison: 2N2646 VS 2N6027

2N2646 Mechanical Dimensions

Manufacturer

NTE Electronics supports the 2N2646 Unijunction Transistor through a replacement-focused manufacturing model rather than in-house semiconductor fabrication. The company sources equivalent devices from qualified manufacturers and applies quality screening and performance validation to ensure they meet standard specifications. This approach allows NTE to reliably supply legacy components like the 2N2646 for timing, triggering, and control applications across repair and industrial markets.

Datasheet PDF

2N2646 Datasheet:

Frequently Asked Questions [FAQ]

1. What frequency range does the 2N2646 typically operate in?

The 2N2646 is best suited for low-frequency applications, typically from fractions of a hertz up to a few kilohertz.

2. How do you test a 2N2646 to check if it is working?

A basic test involves measuring interbase resistance and confirming that the emitter triggers conduction at the expected voltage threshold.

3. Why does the 2N2646 use a metal can package instead of plastic?

The metal TO-39 package provides better heat dissipation, mechanical durability, and electrical shielding compared to plastic packages.

4. Can a 2N2646 be replaced directly with a 2N6027?

No, the 2N6027 is not a drop-in replacement because it has a different pin configuration and requires external resistors to set its trigger point.

5. What causes false triggering in 2N2646 circuits?

False triggering is usually caused by electrical noise, poor grounding, or unstable supply voltages in the circuit.

6. How can you identify counterfeit or low-quality 2N2646 devices?

Counterfeit parts often show inconsistent markings, unusual trigger behavior, or incorrect interbase resistance compared to datasheet values.