The standard SCR (Silicon Controlled Rectifier) schematic symbol consists of a diode triangle pointing toward a cathode bar, with a third 'gate' terminal extending from the junction. The three pins are Anode (A), Cathode (K), and Gate (G). Current flows from Anode to Cathode only when the Gate receives a positive trigger pulse relative to the Cathode, and it remains latched until the main current drops below the holding threshold.
SCR Schematic Symbol & Pinout Reference Table
Before wiring any thyristor circuit, you must correctly identify the terminals. The table below breaks down the standard SCR schematic symbol elements, their circuit functions, and how to verify them on the bench. This applies to common discrete power SCRs like the STMicroelectronics BT151 or the Bourns TIC106.
| Symbol Element / Pin | Designation | Function in Circuit | Bench Identification (DMM Diode Mode) |
|---|---|---|---|
| Anode | A (or 1) | Main current entry point. Connected to the positive supply in DC circuits or the phase line in AC circuits. | Reads 'OL' (Open Loop) to both Cathode and Gate in both probe directions. |
| Cathode | K (or 2) | Main current exit point. The reference point for the Gate trigger voltage. Connected to load return or neutral. | Reads 0.5V to 0.8V forward bias when the red probe is on Gate and black probe is on Cathode. |
| Gate | G (or 3) | Control terminal. A brief positive current pulse here latches the SCR into conduction. | Reads 'OL' to Anode. Reads 0.5V-0.8V to Cathode (forward) and 'OL' (reverse). |
| Triangle Arrow | N/A | Indicates the conventional direction of forward current flow (Anode to Cathode). | N/A (Schematic only) |
| Cathode Bar | N/A | The vertical line blocking the triangle; represents the N-type cathode region of the PNPN junction. | N/A (Schematic only) |
Regional Standard Variants (IEEE vs. IEC vs. Old UK)
While the basic physics of an SCR remain identical globally, the way the SCR schematic symbol is drawn on a blueprint depends on the regional drafting standard your engineering team follows. Misinterpreting these subtle drafting differences can lead to incorrect PCB footprint generation.
IEEE/ANSI 315 (North America)
The dominant standard in the US and Canada. The symbol is drawn as a standard diode triangle with a gate line extending from the junction. The gate line is typically drawn at a 45-degree angle to the anode-cathode axis, or perpendicular to the cathode bar. Terminal designations almost exclusively use letters: A, K, and G.
IEC 60617 (International / Europe)
The IEC standard uses the same fundamental triangle-and-bar geometry, but with stricter rules on enclosure and numbering. According to Electronics Tutorials, IEC schematics frequently enclose the SCR symbol in a circle to denote a discrete semiconductor device. More importantly, IEC often replaces letter designations with numeric terminal identifiers: 1 (Anode), 2 (Cathode), and 3 (Gate). If you are reading a schematic from a European manufacturer like Infineon or STMicroelectronics, expect numbers rather than letters.
Old British Standard (BS 3939)
Largely superseded by IEC, but still found in legacy UK industrial equipment manuals (pre-1990s). The BS 3939 SCR symbol often featured a gate line that was strictly parallel to the anode-cathode axis before bending down to the terminal, creating a distinct 'step' shape. If you are troubleshooting vintage British machinery, do not confuse this stepped gate line with a MOSFET or IGBT symbol.
Rows People Get Wrong & Faded Marking Identification
Even experienced technicians make assumptions when reading thyristor schematics or handling unmarked components. Here are the most common pitfalls and how to resolve them.
The 'Rows People Get Wrong' Notes
- Confusing SCR with TRIAC: A TRIAC symbol looks like two SCRs in anti-parallel (often drawn with two opposing triangles or a bidirectional arrow at the gate). An SCR is strictly unidirectional. If your schematic symbol has only one triangle pointing one way, it is an SCR, and it will only control half of an AC waveform unless used in a bridge configuration.
- Confusing Gate (G) with Base (B): A Bipolar Junction Transistor (BJT) has a base terminal, but the emitter leg features an arrow. An SCR gate line has no arrow. It is a simple line touching the cathode junction.
- Anode/Cathode Reversal: The triangle always points toward the cathode bar. Current flows with the triangle. Reversing these in your PCB layout will result in a dead short or a non-functional circuit, as the SCR will be reverse-biased and block current entirely.
Safe Interpretation When Markings are Faded or Missing
When pulling a salvaged TO-220 SCR from an old motor controller, the laser etching is often burned off. Do not guess the pinout based on the package shape. Use a digital multimeter (DMM) in Diode Test mode to safely map the pins. As detailed in All About Circuits, the internal PNPN junction creates a specific diode signature between the Gate and Cathode.
The DMM Pinout Mapping Sequence:
- Test all three pins against each other in both directions (6 tests total).
- Find the only pair that reads a forward voltage drop (0.5V to 0.8V). The Red probe is on the Gate, and the Black probe is on the Cathode.
- The remaining pin, which reads 'OL' to both the Gate and Cathode in both directions, is the Anode.
Frequently Asked Questions
How do I differentiate an SCR schematic symbol from a TRIAC?
Look at the main current path and the gate. An SCR schematic symbol features a single diode triangle pointing in one direction, meaning it only conducts current in one direction (Anode to Cathode). A TRIAC symbol features two overlapping triangles pointing in opposite directions, or a single triangle with a bidirectional arrow, indicating it can conduct current in both directions. Furthermore, a TRIAC gate can be triggered by either a positive or negative pulse, whereas a standard SCR gate requires a positive pulse relative to the cathode.
What does the circle around an SCR symbol mean?
In IEC 60617 drafting standards, a circle enclosing a semiconductor symbol indicates that the component is a discrete, individually packaged device (like a TO-220 or TO-247 through-hole component) rather than part of an integrated circuit or a monolithic silicon die. If you see multiple SCR symbols inside a single large dashed box, it denotes an integrated thyristor module or a monolithic IC containing multiple discrete SCR dies.
Why does my SCR schematic symbol have two gate terminals?
If you see an SCR symbol with a gate terminal on the cathode side and another on the anode side, you are likely looking at a Gate Turn-Off Thyristor (GTO) or a specialized reverse-conducting thyristor. In a GTO, the upper gate (Anode gate) is used to force the device to turn off by pulling current out, while the lower gate (Cathode gate) is used to turn it on. Standard SCRs only have one gate and cannot be turned off via the gate terminal; they only turn off when the main anode current drops to zero.
How do I read the pinout on a TO-220 package SCR?
For the vast majority of modern TO-220 packaged SCRs (such as the widely used STMicroelectronics BT151 series, detailed in the BT151 datasheet), hold the component with the metal tab facing away from you and the text facing you, with the three pins pointing down. From left to right, the pins are: Pin 1 = Cathode (K), Pin 2 = Anode (A), and Pin 3 = Gate (G). Note that the metal tab itself is internally connected to the Anode (Pin 2), which is critical to remember when mounting the SCR to a grounded heatsink to avoid shorting the main supply to ground.
