Simple Thyristor Switching using Snubber Circuit

In this article thyristors are switches used in power electronics to regulate high voltage and current.

They are often referred to as SCRs (Silicon Controlled Rectifiers).

A gate signal activates them, and the only way to turn them off is to stop the current flow.

On the other hand, voltage spikes and abrupt current fluctuations might happen while switching (turning on/off), which could stress the thyristor and perhaps cause damage.

A resistor R and capacitor C coupled in series and positioned in parallel with the thyristor make up a basic snubber circuit.

Taking into consideration, a simple thyristor switching using snubber circuit enhances thyristor switching performance by:

• shielding them from sudden increases in voltage and current.
• extending durability and dependability.

Circuit Working:

Parts List:

The circuit functions as a dual section test bench.

In one area, a common timer chip 555 IC is used to provide a variable frequency signal.

This signal activates and deactivates the thyristor TYN612 via controlling its gate.

The other component allows us to observe the difference in thyristor on/off operation with and without a snubber circuit.

In the absence of the snubber, there are sharp voltage spikes when the thyristor goes on.

The thyristor may be harmed by these spikes.

Here is when the snubber gets useful:

The snubber circuit functions similarly to a miniature electrical shock absorber.

A resistor and a capacitor make up its two constituent pieces.

During turn on the capacitor functions initially essentially as a temporary bridge.

By doing this, unintentional triggering is avoided by preventing the voltage across the thyristor from increasing too rapidly.

When the thyristor is turned off, the circuit it controls typically a motor or another device resists current changes.

The snubber capacitor shields the thyristor from excessive current surges by providing a smoother route for the current to drop.

Essentially, the snubber circuit performs two crucial functions:

• keeps the thyristor safe from damage caused by voltage spikes.
• prevents current spikes that may harm the thyristor as well.

However, there is a cost although the snubber shields the thyristor, it can use a little additional power which lowers the overall efficiency of the circuit.

Here is a detailed look at each component of the snubber in action:

Capacitor (Cs):

When the thyristor initially goes on, this functions as a transient short circuit.

It prevents the voltage across the thyristor from increasing too rapidly by doing this.

The capacitor gradually charges up over time, enabling the voltage to increase at a safe rate.

Resistor (Rs):

The capacitor must discharge once the thyristor is switched on.

This resistor restricts the rate at which the discharge occurs.

The thyristor might be damaged by an excessively high discharge current in the absence of a resistor.

The resistor regulates the current flow in a manner similar to a safety valve.

Formulas:

Above simple circuit design and formulas for a snubber circuit and a thyristor SCR is mentioned below:

Snubber Circuit Time Constant (τ):

During switching operations, the time constant of the snubber circuit, which is usually constructed by a resistor R and capacitor C, is crucial for managing voltage transients, it is computed as follows:

τ = R * C

where,

• R is the ohms Ω of resistance.
• The capacitance in farads F is denoted by C.

Power Dissipation PD:

The power dissipation in the thyristor is given by:

P_D​ = V_AK​ * I_T​

where,

• The voltage drop across the anode and cathode is VAK.
• The thyristor current is represented by IT.

Gate Power PG:

This is the amount of power lost in the gate circuit

P_G​ = V_GT​ * I_GT​

where,

• The gate trigger voltage is VGT.
• IGT stands for gate trigger current.

These formula are essential for comprehending and creating thyristor based circuit designs that provide appropriate triggering, conduction and protection against transients and voltage spikes.

How to Build:

To build a Simple Thyristor Switching using Snubber Circuit follow the below mentioned steps for connections:

• Assemble all the required components parts as shown in above circuit diagram
• Connect pin 1 of IC1 555 to ground.
• Connect pin 2 to pin 6 of IC1 555, through a capacitor C1 to ground.
• Connect pin 3 of IC1 555 to gate of SCR TYN612 through resistor R5.
• Connect pin 4 and pin 8 of IC1 555 to +9V positive supply.
• Connect pin 5 of IC1 555 to ground through capacitor C2.
• Connect a resistor R2 between pin 6 and pin 7 of IC1 555.
• Connect a resistor R1 to positive supply through pin 7 of IC1 555.
• Connect cathode of SCR TYN612 to ground, anode to positive supply through resistor R4, connect gate to pin 3 of IC1 555.
• Connect an oscilloscope output across anode and cathode of SCR TYN612.
• Connect from oscilloscope output a snubber circuit through a resistor R6 and capacitor C4 in series to ground.
• Connect resistor R3 and capacitor C3 in parallel across the positive line and the cathode of D1, and connect anode of D1 to the anode of SCR.

Safety Measures:

• Understand how each part works, including the snubber circuit and the possible risks associated with it.
• If power electronics are not your thing, work with a skilled technician.
• Turn the circuit off right away if you think there could be a problem, and solve the problem before turning it back on.
• When dealing with snubber and thyristor switching circuits you may reduce the chance of harm by adhering to these safety precautions.

Conclusion:

By reducing current surges and mitigating voltage spikes, a snubber circuit safeguards thyristors during switching.

Simple thyristor switching using snubber circuit despite its slight complexity enhances the thyristors longevity and dependability, by shielding it from harm caused by high voltage and abrupt current fluctuations.

References:

Optimum design of snubber circuits for thyristor assemblies using an improved PSPICE thyristor model and computational intelligence

Datasheet SCR TYN612