Family Devices: Photo Sensitive SCR, GTO, SCS, TRIAC & DIAC: -
Power electronics plays a vital role in modern electrical
engineering, enabling efficient control of power in industrial, commercial, and
household applications. Among the wide range of semiconductor devices, a
special family of thyristor-based components has been developed to handle
switching, control, and protection in AC and DC circuits. Some of the most
widely used devices in this family are the Photo-Sensitive SCR (PSCR), Gate
Turn-Off Thyristor (GTO), Silicon Controlled Switch (SCS), TRIAC, and DIAC.
In this blog, we will explore these devices in
detail—looking at their structure, working principles, advantages, and
applications.
1. Photo Sensitive SCR (PSCR)
A Photo-Sensitive SCR is a variant of the traditional
Silicon Controlled Rectifier (SCR) that can be triggered not only by a gate
pulse but also by incident light. When photons strike the sensitive area of the
device, they generate charge carriers, which initiate conduction.
Structure & Symbol:
It resembles an SCR but has a transparent window or light-sensitive junction. |
| SCR symbol and structure |
Working Principle:
Normally, an SCR requires a gate current to switch ON. In a PSCR, shining light on the junction acts as the trigger, eliminating the need for a gate signal.Applications:
- Optical isolation in high-voltage circuits
- Light-activated switches
- Industrial automation using optical control
- Safety circuits where electrical isolation is critical
The PSCR is particularly useful in environments where
electrical triggering is inconvenient or where interference-free operation is
required.
2. Gate Turn-Off Thyristor (GTO)
A Gate Turn-Off Thyristor (GTO) is a specially designed
thyristor that can be both turned ON by a positive gate pulse and turned OFF by
a negative gate pulse. This makes it far more versatile than a conventional
SCR, which can only be turned off by external circuit conditions.
Structure & Symbol:
The symbol resembles an SCR but has two arrows at the gate terminal (one for turn-on and one for turn-off). |
| layer structure of GTO |
Working Principle:
In a forward-biased condition, a small positive gate pulse initiates conduction. To turn it off, a large negative gate current extracts the charge carriers from the junction, forcing the device into the OFF state.Applications:
- Variable speed motor drives
- Inverters and choppers
- High-voltage DC transmission (HVDC)
- Traction systems in railways
Although GTOs require complex gate drive circuits and
protective snubbers, their controllability makes them valuable in modern power
systems.
3. Silicon Controlled Switch (SCS)
A silicon-controlled switch (SCS) is similar to an SCR but
has two gate terminals—one for turning it ON and another for turning it OFF.
This makes it a versatile switch for low- and medium-power applications.
Structure & Symbol:
The symbol has an additional gate terminal compared to SCR.
Working Principle:
A small current at the anode gate can switch it ON.
A current at the cathode gate can turn it OFF.
This dual gate control makes the SCS act like a transistor-thyristor hybrid.
Applications:
- Pulse circuits
- Timing and control circuits
- Logic circuits in early digital electronics
- Overvoltage protection systems
Although not as widely used today as SCRs and TRIACs, the
SCS played an important role in the development of semiconductor switching
technology.
4. TRIAC (Triode for Alternating Current)
A TRIAC is a bidirectional device that can conduct current
in both directions when triggered. It is often described as two SCRs connected
in inverse parallel with a common gate terminal. This makes it an ideal device
for controlling AC power.
Structure & Symbol:
The TRIAC symbol looks like two SCRs facing opposite directions with a single gate. |
| layer structure of TRIAC |
Working Principle:
When a gate pulse is applied, the TRIAC can conduct during both the positive
and negative half cycles of the AC input. It automatically switches off at the
zero-crossing point when the current goes below the holding current.
Applications:
- Light dimmers
- Fan speed controllers
- AC motor control
- Household appliances like heaters and mixers
TRIACs are very popular in domestic and industrial AC
control because they reduce circuit complexity and cost.
5. DIAC (Diode for Alternating Current)
A DIAC is a two-terminal device that conducts current only
after the applied voltage exceeds its breakover voltage. Unlike an SCR or
TRIAC, it has no gate terminal and is purely voltage-triggered.
Structure & Symbol:
The DIAC symbol resembles two diodes connected in series but in opposite directions.
Working Principle:
At low voltages, the DIAC remains in the OFF state. Once the applied AC voltage exceeds the breakover point, it conducts sharply in either direction until the current drops below the holding value.Applications:
- Triggering circuits for TRIACs
- AC phase control
- Dimmers and speed regulators
- Protection against voltage transients
DIACs are almost always used in combination with TRIACs for
smooth and stable switching in AC power applications.
Conclusion
The family of thyristor-based devices—Photo Sensitive SCR,
GTO, SCS, TRIAC, and DIAC—forms the backbone of modern power control systems.
While each device has unique features, together they cover a wide spectrum of
applications ranging from light dimmers in our homes to high-voltage industrial
drives.
- PSCR offers light-based triggering.
- GTO provides full gate-controlled ON/OFF switching.
- SCS allows dual gate operation.
- TRIAC simplifies AC control.
- DIAC provides reliable triggering and protection.
Understanding these devices is essential for anyone studying
or working in power electronics, as they continue to find new roles in
energy-efficient systems, renewable energy, and automation technologies.
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