SCR Full Form is Silicon Controlled Rectifier or Semiconductor Controlled Rectifier. An SCR or Silicon Controlled Rectifier is a four-layer solid-state current-controlling device. Silicon controlled rectifier is the trade term for thyristor types used by General Electric.

The silicon controlled rectifier is most useful in electrical devices that need high voltage and power regulation. As a consequence, they are suitable for applications needing medium to high Alternating Current power. When gate pulses are applied to an SCR, it conducts like a diode. It is made up of four layers of semiconductors that form two distinct structures: NPNP and PNPN. It also features three intersections and three terminals.

The many phases of SCR

When a Silicon controlled rectifier is turned off, the anode receives positive voltages, the gate receives zero voltages, and the cathode receives negative voltages. As a result, the first and third junctions are forward biassed, but junction two is reverse biassed. J2 reaches its avalanche breakdown value and starts running. J1’s resistance is quite high below this amount, therefore many believe it to be faulty.

The ON state is obtained by either increasing the possible difference between the cathode and anode voltages above the avalanche voltages or delivering positive signals to the gate. As soon as the SCR starts to conduct, the gate voltage is no longer required to keep the SCR in the ON state, and the SCR switches off.

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Triggering/Firing

This way of assuring Silicon controlled rectifier conduction is known as triggering or firing, and it is one of the most often used methods of latching SCRs in practise. In actuality, users often choose SCRs with a break overvoltage greater than the greatest voltage expected from the power supply, enabling them to be turned on only by an intentional voltage pulse applied to the gates.

Reverse activation

It’s worth noting that Silicon-controlled rectifiers may be turned off directly by connecting their cathode and gate terminals, or by “reverse-triggering” them using negative voltages (around the cathode) to force the transistor at the lower end into cutoff. I think this is “occasionally” doable since it includes shunting the collector current of the upper transistor through the base of the transistor at the lower end. This current might be substantial, making a triggered SCR shut-off difficult at best. A thyristor with Gate-Turn-Off is a variation of the SCR that simplifies this function. Even with Gate-Turn-Off, the current required at the gate to turn it off might be as much as 20% of the load current!

SCR Function Testing using an Ohmmeter

An ohmmeter may be used to perform a rudimentary test of Silicon-controlled rectifier performance or, at the most basic level, terminal identification. Because the connection between the gate and the cathode on the inside is a PN junction (single), a metre with the gate (red test lead) and the cathode (black test lead) should show continuity between these terminals. Any further continuity tests performed on a Silicon-controlled rectifier will provide a “open” result. It is critical to realise that this is a rather simple test that does not offer an accurate picture of the SCR. Even if an SCR delivers good ohmmeter readings, it might still be defective.

The gate-to-cathode junction voltage signal produced by a multimeter equipped with a “diode check” function may or may not be related to silicon PN junctions. In certain cases, the junction voltage will be substantially lower: a few hundredths of a volt. This is caused by an internal resistor connected between the cathode and gate in certain SCRs. This resistor protects the SCR against erroneous triggering caused by voltage spikes, noise, or static discharge of electric pulses.

Conclusion

SCRs, like true rectifiers, allow only one direction of current to travel through them. As a consequence, you can’t use them to handle full-wave AC power on their own. When diodes in a rectifier circuit are replaced with SCRs, a controlled rectifier circuit is constructed, with DC power to a load time-proportioned by activating the SCRs at various points along the AC power waveform.