TERMS

TERMS In INDUSTRIAL ELECTRONICS

· The term 'silicon-controlled rectifier' is a trade name used by General Electric in 1957 to refer to this type of thyristor.
· Examples of applications for SCR's include: 1) power switching; 2) phase control; 3) battery charging; 4) power inverters; 5) motor switching and control; 6) high-voltage DC conversion
· This extra terminal is called the gate, and it is used to trigger the device into conduction (latch it) by the application of a small voltage.
· To trigger, or fire, an SCR, voltage must be applied between the gate and cathode, positive to the gate and negative to the cathode. When testing an SCR, a momentary connection between the gate and anode is sufficient in polarity, intensity, and duration to trigger it.
· SCRs may be fired by intentional triggering of the gate terminal, excessive voltage (breakdown) between anode and cathode, or excessive rate of voltage rise between anode and cathode. SCRs may be turned off by anode current falling below the holding current value (low-current dropout), or by "reverse-firing" the gate (applying a negative voltage to the gate). Reverse-firing is only sometimes effective, and always involves high gate current.
· A variant of the SCR, called a Gate-Turn-Off thyristor (GTO), is specifically designed to be turned off by means of reverse triggering. Even then, reverse triggering requires fairly high current: typically 20% of the anode current.
· SCR terminals may be identified by a continuity meter: the only two terminals showing any continuity between them at all should be the gate and cathode. Gate and cathode terminals connect to a PN junction inside the SCR, so a continuity meter should obtain a diode-like reading between these two terminals with the red (+) lead on the gate and the black (-) lead on the cathode. Beware, though, that some large SCRs have an internal resistor connected between gate and cathode, which will affect any continuity readings taken by a meter.
· SCRs are true rectifiers: they only allow current through them in one direction. This means they cannot be used alone for full-wave AC power control.
· If the diodes in a rectifier circuit are replaced by SCRs, you have the makings of a controlled rectifier circuit, whereby DC power to a load may be time-proportioned by triggering the SCRs at different points along the AC power waveform.
· A TRIAC acts much like two SCRs connected back-to-back for bidirectional (AC) operation.
· TRIAC controls are more often seen in simple, low-power circuits than complex, high-power circuits. In large power control circuits, multiple SCRs tend to be favored.
· When used to control AC power to a load, TRIACs are often accompanied by DIACs connected in series with their gate terminals. The DIAC helps the TRIAC fire more symmetrically (more consistently from one polarity to another).
· Main terminals 1 and 2 on a TRIAC are not interchangeable.
· To successfully trigger a TRIAC, gate current must come from the main terminal 2 (MT2) side of the circuit!
· A MOS-gated thyristor uses an N-channel MOSFET to trigger a thyristor, resulting in an extremely low gate current requirement.
· A MOS Controlled Thyristor, or MCT, uses two MOSFETS to exert full control over the thyristor. A positive gate voltage triggers the device; a negative gate voltage forces it to turn off. Zero gate voltage allows the thyristor to remain in whatever state it was previously in (off, or latched on).
· A silicon-controlled switch, or SCS, is essentially an SCR with an extra gate terminal.
· Typically, the load current through an SCS is carried by the anode gate and cathode terminals, with the cathode gate and anode terminals sufficing as control leads.
· An SCS is turned on by applying a positive voltage between the cathode gate and cathode terminals. It may be turned off (forced commutation) by applying a negative voltage between the anode and cathode terminals, or simply by shorting those two terminals together. The anode terminal must be kept positive with respect to the cathode in order for the SCS to latch.
· A unijunction transistor consists of two bases (B1, B2) attached to a resistive bar of silicon, and an emitter in the center. The E-B1 junction has negative resistance properties; it can switch between high and low resistance.
· A PUT (programmable unijunction transistor) is a 3-terminal 4-layer thyristor acting like a unijunction transistor. An external resistor network “programs” η.
· The intrinsic standoff ratio is η=R1/(R1+R2) for a PUT; substitute RB1 and RB2, respectively, for a unijunction transistor. The trigger voltage is determined by η.
· Unijunction transistors and programmable unijunction transistors are applied to oscillators, timing circuits, and thyristor triggering.
· A TRIAC acts much like two SCRs connected back-to-back for bidirectional (AC) operation.
· TRIAC controls are more often seen in simple, low-power circuits than complex, high-power circuits. In large power control circuits, multiple SCRs tend to be favored.
· When used to control AC power to a load, TRIACs are often accompanied by DIACs connected in series with their gate terminals. The DIAC helps the TRIAC fire more symmetrically (more consistently from one polarity to another).
· Main terminals 1 and 2 on a TRIAC are not interchangeable.
· To successfully trigger a TRIAC, gate current must come from the main terminal 2 (MT2) side of the circuit!
· A Silicon-Controlled Rectifier, or SCR, is essentially a Shockley diode with an extra terminal added. This extra terminal is called the gate, and it is used to trigger the device into conduction (latch it) by the application of a small voltage.
· To trigger, or fire, an SCR, voltage must be applied between the gate and cathode, positive to the gate and negative to the cathode. When testing an SCR, a momentary connection between the gate and anode is sufficient in polarity, intensity, and duration to trigger it.
· SCRs may be fired by intentional triggering of the gate terminal, excessive voltage (breakdown) between anode and cathode, or excessive rate of voltage rise between anode and cathode. SCRs may be turned off by anode current falling below the holding current value (low-current dropout), or by "reverse-firing" the gate (applying a negative voltage to the gate). Reverse-firing is only sometimes effective, and always involves high gate current.
· A variant of the SCR, called a Gate-Turn-Off thyristor (GTO), is specifically designed to be turned off by means of reverse triggering. Even then, reverse triggering requires fairly high current: typically 20% of the anode current.
· SCR terminals may be identified by a continuity meter: the only two terminals showing any continuity between them at all should be the gate and cathode. Gate and cathode terminals connect to a PN junction inside the SCR, so a continuity meter should obtain a diode-like reading between these two terminals with the red (+) lead on the gate and the black (-) lead on the cathode. Beware, though, that some large SCRs have an internal resistor connected between gate and cathode, which will affect any continuity readings taken by a meter.
· SCRs are true rectifiers: they only allow current through them in one direction. This means they cannot be used alone for full-wave AC power control.
· If the diodes in a rectifier circuit are replaced by SCRs, you have the makings of a controlled rectifier circuit, whereby DC power to a load may be time-proportioned by triggering the SCRs at different points along the AC power waveform.
· Shockley diodes are four-layer PNPN semiconductor devices. These behave as a pair of interconnected PNP and NPN transistors.
· Like all thyristors, Shockley diodes tend to stay on once turned on (latched), and stay off once turned off.
· To latch a Shockley diode exceed the anode-to-cathode breakover voltage, or exceed the anode-to-cathode critical rate of voltage rise.
· To cause a Shockley diode to stop conducting, reduce the current going through it to a level below its low-current dropout threshold
· Electrical hysteresis, the tendency for a component to remain "on" (conducting) after it begins to conduct and to remain "off" (nonconducting) after it ceases to conduct, helps to explain why lightning bolts exist as momentary surges of current rather than continuous discharges through the air.
· Simple gas-discharge tubes such as neon lamps exhibit electrical hysteresis.
· More advanced gas-discharge tubes have been made with control elements so that their "turn-on" voltage could be adjusted by an external signal. The most common of these tubes was called the thyratron.
· Simple oscillator circuits called relaxation oscillators may be created with nothing more than a resistor-capacitor charging network and a hysteretic device connected across the capacitor.