What is a snubber circuit?

It consists of a series combination of a resistor and a capacitor in parallel with the thyristors. It is mainly used for dv / dt protection.

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The snubber circuit is used for the dv/dt protection of the SCR. It is a series combination of a resistor and a capacitor in parallel with the SCR.

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  1. The snubber Circuit is a series combination of resistor ‘R’ and capacitor ‘C’.
  2. They are connected across the thyristor to be protected.
  3. The capacitor ‘C’ is used to limit the dv/dt across the SCR.
  4. The resistor ‘R’ is used to limit high discharging current through the SCR.
  5. When switch S is closed, the capacitor ‘C’ behaves as a short-circuit.
  6. Therefore voltage across SCR is zero.
  7. As time increases, voltage across ‘C’ increases at a slow rate.
  8. Therefore dv/dt across ‘C’ and SCR is less than maximum dv/dt rating of the device.
  9. The capacitor charges to full voltage Vs; after which the gate is triggered, and SCR is turned ON and high current flows through SCR.
  10. As di/dt is high, it may damage the SCR. To avoid this, the resistor R in series with ‘C’ will limit the magnitude of di/dt.
  11. The technique of ‘snubbing’ can apply to any switching circuit, not only to thyristor/triac circuits.
  12. The rate of rise of turn-off voltage is determined by the time constant
  13. RLC. Where RL is the circuit minimum load resistance, for instance the cold resistance of a heater or lamp, the winding resistance of a motor or the primary resistance of a transformer.
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Due to overheating, over voltage, over current or excessive change in voltage or current switching devices and circuit components may fail.

From over current they can be protected by placing fuses at suitable locations. Heat sinks and fans can be used to take the excess heat away from switching devices and other components. Snubber circuits are needed to limit the rate of change in voltage or current ( di/dt or dv/dt ) and over voltage during turn-on and turn-off.

These are placed across the semiconductor devices for protection as well as to improve the performance. Static dv/dt is a measure of the ability of a thyristor to retain a blocking state under the influence of a voltage transient.

These are also used across the relays and switches to prevent arcing.

RC_snubber.pdf (101.9 KB)

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