What is an  SCR?

The SCR (Silicon Controlled Rectifier) is a 4-layer semiconductor device that works as an almost ideal switch. Its symbol and structure are shown in the first image. A = anode, G = gate and K = cathode.

Basic Operation

To better understand its basic operation, an equivalent circuit is shown in the second image below.

A = anode, G = gate and C = K = cathode

• When an IG current is applied to the G terminal (base of Q2 and collector of Q1), two currents are produced: IC2 = IB1.
• IB1 is the base current of transistor Q1 and causes a collector current of Q1 (IC1) to exist.
• IC1 in turn feeds the base of transistor Q2 (IB2) which causes more current in IC2, which is the same current IB1 at the base of Q1, and ……
• This regenerative process is repeated until transistors Q1 and Q2 become saturated, causing the SCR to conduct and operate like a short circuit.

Parameters are:

• VRDM: Maximum reverse voltage (VG = 0)
• VFOM: Maximum direct voltage (VG = 0)
• IF: Maximum allowed direct current
• PG: Maximum dissipation of power between gate and cathode.
• TGV-IGT: Maximum voltage – current required in the gate (G) to trigger the SCR
• IH: Minimum anode current required to keep the thyristor on its ON state
• dv/dt: Maximum voltage variation without triggering.
• di/dt: Maximum accepted current variation before destroying the thyristor.

Note: dv/dt, di/dt: See parameters of SCR in DC.

Characteristic curve of the SCR

The picture below shows the relation between the switching voltage and the gate current.

• When it is reversed biased; it operates like a common diode (see the leakage current feature).
• When it is in the direct bias region, it also operates like a common diode but it has been already activated (On). See points D and E. (high conduction).

For high gate current (IG) values (see point C), the anode-cathode voltage is lower (VC).

If the IG decreases, the anode-cathode voltage increases. (See points B and A, and anode-cathode voltage VA and VB). In conclusion, if we reduce the gate IG current, the anode-cathode voltage will tend to increase before the SCR is ON.

Below are two scr of different power. The one with more power is on the left.

Thuringius [CC BY-SA 3.0], via Wikimedia Commons

Applications of Silicon Controlled Rectifier

• As AC voltage stabilizer
• In battery chargers
• As switch
• For power control
• In inverters (to switch from DC voltage to AC voltage)
• etc.