Electrical Engineering ⇒ Topic : Controlled Rectifiers

Controlled Rectifiers

The supply of average current to a load or motor may be controlled by the use of silicon controlled rectifier (SCR), which performs most the duties of a rheostat. The characteristics/working of an SCR *given below

• An SCR is a three-terminal device used to control rather large currents to a load An SCR or any power thyristor does not have the drawbacks of high power rheostats.
• SCRs are small, inexpensive and energy efficient.
• An SCR acts very much like a switch.

When it is turned-on there is a low resistance current flow path from anode to cathode; then it acts like a "closed switch".

When it turned-off, no current can flow from anode to cathode, then it acts like an "open-switch".

Because an SCR is a solid-state device, its switching action is very fast.

The switching action of gate takes place only when :

• SCR is forward biased (i.e., anode positive with respect to cathode);
• A suitable positive voltage is applied between the gate and cathode.

Once the SCR has' switched on it has no control on the magnitude of current flowing through it. The current through the SCR is entirely controlled by the external impedance connected in the circuit and the applied voltage.

The forward current through the SCR increasing the circuit can be reduced by reducing the votlage or by must be maintained impedance. There is, however, a minimum forward current that to keep the SCR in conducting state. This is called the holding current rating of SCR.

The SCR can be switched off by reducing the forward current below the level of holding current which may be done either by reducing the applied voltage or by increasing the circuit impedance.

The gate can only trigger or switch on the SCR, it cannot switch off.

In SCRs output voltage or current can be varied by controlling the point in the input A.C. cycle at which thyristor is turned-on with the application of á suitable low-power gate pulse. Once triggered or fired into conduction, the thyristor remains in the conducting state for the rest of the half cycle, i.e., upto 180°. The firing angle a can be adjusted with the help of a control circuit. Firing delay angle and Conduction angle always total 1800.

Controlled rectifiers may be either half-wave or full wave