Synchronized UJT triggering (or Ramp triggering)

Synchronized UJT triggering, also known as Ramp triggering, is a method used to trigger devices such as silicon-controlled rectifiers (SCRs) or thyristors in electronic circuits. the triggering signal is generated by a Unijunction Transistor (UJT) and is synchronized with an external signal or a specific timing requirement.the circuit ensures synchronization between the trigger circuit and the supply voltage across the SCRs, allowing precise control over the firing angle and output power.Here's a breakdown of the circuit operation:

1. AC voltage is rectified by diodes D₁ to D₄, converting it to a DC voltage. Resistor R1 is used to lower the DC voltage to a suitable level for the zener diode (Z) and the UJT.
2. The zener diode (Z) clips the rectified voltage to a constant level except near the zero voltage crossing. This clipped voltage (Vz) is then applied to the charging circuit, which consists of a resistor (R) and a capacitor (C).
3. The capacitor (C) charges at a rate determined by the resistor (R). The voltage across the capacitor is denoted as (Vc) in the circuit diagrams.
4. When the voltage (Vc) across the capacitor reaches the threshold voltage (ηV_z) of the UJT, the UJT's Emitter-Base (E-B1) junction breaks down. This causes the capacitor (C) to discharge through the primary winding of a pulse transformer, generating a current (I₂) pulse.
5. The pulse transformer has two secondary windings that produce in-phase pulse voltages. These pulse voltages are used to trigger two SCRs in a full-wave circuit. The SCR with a positive anode voltage will turn on.
6. The capacitor (C) then starts to recharge, and the process repeats.

By varying the charging resistor (R), the rate of rise of the capacitor voltage can be controlled. This, in turn, controls the firing angle of the SCRs. The firing angle can be adjusted up to approximately 150°. This method of controlling the output power by varying charging resistor R is called ramp control, open-loop control or manual control.

The circuit achieves synchronization with the supply voltage across the SCRs by utilizing the fact that the zener diode voltage (Vz) goes to zero at the end of each half cycle. This synchronization ensures that the time (t) when the pulse is applied to the SCR for the first time remains constant for the same value of R. Small variations in the supply voltage and frequency do not significantly affect the circuit operation.

In the case where R is reduced so that the voltage (v) reaches the UJT threshold voltage twice in each half cycle, there will be two pulses generated in each half cycle. However, the second pulse in each cycle is redundant because the first pulse is sufficient to turn on the SCR.

synchronized ujt triggering circuit diagram

Synchronized UJT triggering circuit

synchronized ujt triggering circuit waveforms

Generation of output pulse for the circuit of Synchronized UJT triggering

The advantages of synchronized UJT triggering or ramp triggering

1. Include precise and repeatable triggering of circuits or devices, synchronization with external signals, and accurate timing control. 
2. It enables the generation of ramp-like waveforms and facilitates the synchronization of different components or subsystems within an electronic system.

The disadvantage of synchronized UJT triggering or ramp triggering

1. It requires careful calibration and adjustment to ensure accurate synchronization and timing.
2.  Improper calibration or variations in component values can result in timing inaccuracies. 
3. The UJT itself may have limitations in terms of frequency range and power handling capabilities, which can restrict its use in certain applications.

Application of Synchronized UJT triggering or Ramp triggering

The application of synchronized UJT triggering, or ramp triggering, includes its use in timing circuits, voltage-controlled oscillators, and frequency dividers. It allows for precise control over the timing of events and enables synchronization between different components or subsystems within an electronic system.

Summary of Synchronized UJT triggering or Ramp triggering

The synchronized UJT triggering circuit provides a means to control the firing angle and output power of SCRs by utilizing a UJT-triggered pulse generator. The synchronization ensure consistent triggering regardless of small variations in the supply voltage and frequency, while the control resistor allows manual control over the output power.