When the gate voltage is positive enough to turn Q2 on, Q2 will have collector current, Q1 will have base current, and both Q1 and Q2 will be on. The arrows at the equivalent circuit indicate the current flow when Q1 and Q2 are on (SCR is on). Q2 Q2 Gate current flow Q2 I C1 I B2 I B1 = I C2 I E2
Silicon Controlled Rectifier (SCR) • Acts like a switch. – When turned “ on ”, it has low resistance and current could flow from anode to cathode but not from cathode to anode . – When “on”, voltage drop across anode and cathode is approximately zero volt. – When turned “ off ”, it has very high resistance and no current could flow from anode to cathode, or cathode to anode . • Switching action is very fast (no mechanical delay). • Small and relatively inexpensive. • For current to flow from anode to cathode, the following conditions must exist: – Gate must be around + 0.6 to + 0.8 volt with respect to cathode. (Gate- cathode junction must be forward bias) – Gate current must be equal to or greater than gate current required to turn on the SCR (I GT ) – Anode must be positive with respect to cathode, and minimum anode to cathode current to keep the SCR on (I HO ) . must be present. • I GT is typically between 0.1 to 50 mA. • SCR’s can also be turned on by increasing its operating temperature, or by increasing the anode-cathode voltage to its breakover value . However, these are not usually done.
Silicon Controlled Rectifier (SCR) • Once triggered on , SCR will remain on even if I GT is no longer present as long as the anode to cathode current (I AK ) does not fall below some minimum value, called holding current (I HO ) . • SCR’s are normally triggered off when anode to cathode voltage passes through zero into its negative region. • Two ways of turning off an SCR are: – Anode current interruption – anode current is cut off. – Forced commutation – anode current is forced to be in the reverse (cathode to anode) direction. • SCR can control currents of several hundred to 2000 amperes in circuits operating at voltages higher than 1000 to 1800 volts. – Current can be controlled by resistance in series with anode and cathode. • Some SCRs can operate at frequencies as high as 50 Khz, permitting applications involving ultrasonic cleaning and induction heating. • Time during which current could flow from anode to cathode can be controlled using the gate terminal. • Average current from anode to cathode can be controlled by controlling the application of gate to cathode voltage . • Gate current flows from gate to the cathode .
Silicon Controlled Rectifier (SCR) Characteristics • For I G = 0, V F must reach the largest required breakover voltage (V BRF* ) before the collapsing effect results and the SCR can enter the conduction region corresponding to the on state.
- Summer '19
- UJT, SCRs, SCR Circuits