Silicon controlled rectifiers (SCR) are four-layer (PNPN) thyristors with three terminals:
An input control terminal (gate)
An output terminal (anode)
A terminal common to both the input and output (cathode)
SCRs are used mainly with high voltages and currents, often to control alternating current (AC) where the change of sign causes the device to switch off automatically. For example, SCRs are used in dimmer switches where the turn-on point occurs at a specific point on the sine curve of the AC supply. The device then remains on for the remainder of that cycle.
In devices where the gate is left-floating or disconnected, silicon controlled rectifiers behave like Shockley diodes.
Latching is achieved by reaching a breakover voltage or exceeding the critical rate of voltage rise between the anode and cathode.
Dropout is achieved by reducing the current until one or both internal transistors fall into cutoff mode.
Although SCRs offer faster switching speeds than diodes, both types of devices can conduct in only one direction.
Selecting silicon controlled rectifiers (SCRs) requires an analysis of performance specifications.
Peak repetitive reverse voltage is the maximum reverse voltage that may be applied continuously to the anode and cathode.
Root mean square (RMS) on-state current is the maximum RMS value of the principal current when SCRs are turned on.
Peak cycle surge on-current is the maximum on-state current of short-term duration that can be applied for one full cycle of conduction without performance degradation.
Gate trigger current is the minimum current required to switch silicon controlled rectifiers from the off-state to the on-state at the specified off-state voltage and temperature.
Gate trigger voltage is the gate voltage required to produce the gate trigger current.
Holding current is the minimum principal current required to maintain SCRs in the on-state.
Rate-of-rise of off-state voltage is the minimum rate-of-rise value for the principal voltage that causes switching from the off-state to the on-state.
Other performance specifications include:
Repetitive peak controllable on-state voltage
Integrated Circuit (IC) Packages
Silicon controlled rectifiers are available in a variety of integrated circuit (IC) packages and with different number of pins.
Basic IC package types include:
Dual in-line package (DIP)
Diode outline (DO)
Transistor outline (TO)
Small outline diode (SOD)
Small outline transistor (SOT)
Many packaging variants are available:
Discrete packaging (DPAK) - large surface mounted package that includes a heat sink
Integrated packaging (IPAK) - plastic package with three leads
Power packaging (PPAK)
Metal electrode leadless face (MELF)
Thin small outline package (TSOP) - type of DRAM package that uses gull wing shaped leads on both sides, is available with both L-shaped leads (TSSOP) and J-shaped leads (TSOJ)
MIL-PRF-19500/108 - Semiconductor device, thyristors (controlled rectifiers), silicon
DSCC-DWG-03021 - Semiconductor device, diode, silicon, rectifier, modeule, high voltage
Related Products & Services
Diacs are bi-directional diodes that switch AC voltages and trigger silicon controlled rectifiers (SCRs) and triacs. Except for a small leakage current, diacs do not conduct until the breakover voltage is reached.
Sidacs are bidirectional silicon switches that provide greater power handling capabilities than standard diacs. These four-layer (PNPN) semiconductor devices are triggered by thyristors and act as open circuits that are capable of withstanding a specific rated voltage until triggered.
Thyristors are a class of four-layer (PNPN) semiconductor devices that act as switches, rectifiers, or voltage regulators.
Triacs are three-terminal silicon devices that are configured in an inverse parallel arrangement to provide load current during both halves of the AC supply voltage. They are often used to control motor speed.