The SCR symbol on a schematic represents a Silicon Controlled Rectifier, a four-layer (PNPN) solid-state switching device. Unlike a standard diode that conducts whenever forward-biased, an SCR remains blocking until a small current pulse is applied to its Gate terminal. Once triggered, it latches on and continues conducting until the Anode-to-Cathode current drops below the device's holding threshold. Because SCRs are foundational in motor controls, phase-angle dimmers, and crowbar overvoltage circuits, correctly reading their symbols and physical pinouts is mandatory for safe bench work and repair.
Complete SCR Symbol and Pinout Reference
The table below maps the schematic representations to their most common physical packages. Pinouts are listed reading left-to-right with the component facing you (text side forward) and pins pointing downward.
| Component Type | Schematic Symbol Description | Governing Standard | Common Package | Pinout (L to R) / Tab |
|---|---|---|---|---|
| Standard SCR | Diode triangle + bar + inward gate arrow | IEC 60617 / IEEE 315 | TO-220 (e.g., BT151, TIC106D) | K, A, G / Tab = Anode |
| Standard SCR (Small Signal) | Diode triangle + bar + inward gate arrow | IEC 60617 / IEEE 315 | TO-92 (e.g., 2N5060, C106) | K, G, A (Varies by mfg) |
| Light-Activated SCR (LASCR) | Standard SCR + two inward-pointing light arrows | IEC 60617-5 | TO-18 / Epoxy Dome | K, A, G (or optically isolated) |
| Gate Turn-Off Thyristor (GTO) | Diode triangle + bar + gate arrow with perpendicular bar | IEC 60617 | TO-220 / Puck | K, A, G / Tab = Anode |
Standard Variants: IEC 60617 vs. IEEE/ANSI
While wire color codes vary wildly between NEC (North America) and IEC (Europe/UK), schematic symbols for semiconductors are largely harmonized, though subtle drafting differences remain between international and legacy US standards.
- IEC 60617 (International): The dominant global standard. The SCR symbol features a standard diode triangle pointing toward a cathode bar, with the gate lead entering at a 90-degree angle to the cathode junction. The gate arrow points inward, indicating that conventional current flows into the gate to trigger the PNPN junction.
- IEEE Std 315 / ANSI Y32.2 (North America): Historically, US schematics sometimes drew the gate lead originating from the center of the cathode bar rather than the junction point, or used a slightly different enclosure box for the semiconductor. Modern CAD tools (Altium, KiCad, Eagle) default to the IEC-style geometry even in US-based libraries, making the distinction mostly relevant when reading legacy industrial prints from the 1980s or older.
Symbols and Rows People Get Wrong
Misidentifying a thyristor family member on a schematic or at the bench leads to blown fuses and destroyed driver circuits. Watch out for these common traps:
1. Confusing the SCR with a TRIAC
An SCR symbol only has one gate and conducts in one direction (Anode to Cathode). A TRIAC symbol looks like two SCRs drawn in inverse-parallel sharing a single gate terminal. If you replace a TRIAC (like a BTA16) with an SCR (like a BT151) in an AC phase-control circuit, you will only rectify half the AC waveform, resulting in severe DC offset that can saturate and destroy downstream transformers or motors.
2. The Gate Arrow Direction (GTO vs. Standard)
The standard SCR gate arrow points into the cathode region. If you see an arrow pointing outward, or an inward arrow intersected by a small perpendicular bar, you are looking at a Gate Turn-Off (GTO) thyristor. GTOs can be turned off by applying a negative gate pulse. Standard SCRs cannot; they require the main current to drop to zero (commutation). Driving a standard SCR with a GTO driver circuit will result in a short circuit when the driver attempts to pull the gate negative to turn it off.
3. TO-92 Pinout Ambiguity
The table above notes that TO-92 SCRs (like the 2N5060) have variable pinouts. Unlike the highly standardized TO-220 package where the Anode is almost universally tied to the metal tab, small-signal TO-92 SCRs swap Gate and Anode positions depending on the manufacturer (Motorola vs. ON Semi vs. STMicro). Never trust a generic footprint pinout for a TO-92 SCR without checking the specific manufacturer datasheet.
Identifying Pins on Faded or Unmarked SCRs
On the bench, you will frequently encounter TO-220 SCRs with laser etching burned off by heat, or salvaged parts with completely faded markings. You can definitively identify the Anode, Cathode, and Gate using a standard digital multimeter (DMM) in Diode Test mode.
- Set your DMM to Diode Mode. Ensure the open-circuit voltage of your probes is at least 2V (most standard meters output 2.5V to 3V on this setting).
- Find the Gate and Cathode. The Gate-to-Cathode junction behaves exactly like a standard silicon PN diode. Probe the pins until you find a pair that reads between 0.600V and 0.800V in one direction, and OL (Open Loop) when you swap the probes. The pin connected to the red (positive) probe during the 0.7V reading is the Gate. The pin connected to the black (negative) probe is the Cathode.
- Identify the Anode. The remaining third pin is the Anode. To confirm, measure between the Anode and Cathode, and the Anode and Gate. Both must read OL in both directions. The Anode is separated from the Gate and Cathode by reverse-biased junctions that will not conduct at multimeter test voltages.
- Verify the Tab. On a TO-220 package, place the black probe on the Cathode and the red probe on the metal mounting tab. It should read OL. Now place the red probe on the Anode pin; the tab should also read continuity or a forward diode drop to the Anode pin, confirming the tab is internally bonded to the Anode.
Note: Small signal SCRs (like the 2N5060) have very low holding currents (under 5mA). If your multimeter supplies enough current during the Gate-Cathode test, the SCR may accidentally latch on, causing the Anode-Cathode reading to suddenly drop from OL to ~0.8V. If this happens, simply short the Anode to the Cathode momentarily with a piece of wire to force the device to commutate off.
Frequently Asked Questions
How do I read an SCR symbol on a schematic?
Locate the standard diode triangle pointing toward a vertical cathode bar. Look for a third terminal (the Gate) intersecting the junction where the triangle meets the bar. The arrow on the gate lead will point inward toward the cathode. The terminal at the flat base of the triangle is the Anode (positive/main current entry), and the terminal at the cathode bar is the Cathode (current exit). Current flows from Anode to Cathode only after the Gate receives a positive trigger pulse relative to the Cathode.
What is the difference between an SCR symbol and a TRIAC symbol?
An SCR symbol represents a unidirectional device with one Anode, one Cathode, and one Gate; it looks like a single diode with a gate trigger. A TRIAC symbol represents a bidirectional device capable of conducting AC in both directions. Visually, the TRIAC symbol is drawn as two diodes pointing in opposite directions (inverse-parallel), sharing a single gate terminal that branches off the main current path. While an SCR has Main Terminal 1 (MT1) and Main Terminal 2 (MT2), a TRIAC's symbol reflects its ability to be triggered by either positive or negative gate currents.
How to identify SCR pins when the part number is faded?
Use a multimeter in diode-test mode. The Gate and Cathode form a single PN junction, so they will show a standard silicon forward voltage drop (0.6V to 0.8V) when the red probe is on the Gate and the black probe is on the Cathode. The Anode will read open (OL) against both the Gate and the Cathode in either probe direction. For TO-220 packages, the metal heatsink tab is almost universally connected to the Anode, which you can verify by checking for continuity between the tab and the identified Anode pin.
