How to Distinguish SCR Terminals?
In the world of electronics repair and DIY projects, the SCR (also known as a thyristor) is a very common semiconductor device. You can find it in everything from dimmable desk lamps and motor speed controllers to various power supply circuits. However, for many beginners—and even experienced engineers—figuring out the anode (A), cathode (K), and gate (G) of an SCR quickly and accurately can be a major challenge, especially when the device has no markings or is of an unfamiliar model. This article will break down the structure and give you several practical and reliable ways to identify the terminals.
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III. How to Distinguish the Three Terminals?
1. Visual Identification (Preferred Method)
2. Multimeter Resistance Test (Universal Method)
3. Simple Trigger Test (Verification Method)
I. What is an SCR?
An SCR, short for Silicon Controlled Rectifier, is a four-layer (P-N-P-N) high-power semiconductor device with three PN junctions. Its key feature is controllable unidirectional conductivity: not only does it conduct like a diode in one direction, but it can also be triggered to conduct at a specific time by applying a small current to the gate. Once it conducts, it continues to do so even if the trigger signal is removed, until the main circuit current drops below the holding current.
II. Basic Structure of an SCR
A standard SCR typically consists of the semiconductor chip, the package, and the lead terminals. Internally, the core has four alternating semiconductor layers, forming three external terminals:
· Anode (A): The P-type semiconductor region, usually marked as “A.”
· Cathode (K): The N-type semiconductor region, usually marked as “K.”
· Gate (G): The control terminal, usually an N+ region, marked as “G.”
III. How to Distinguish the Three Terminals?
Ideally, the terminals are clearly marked on the device. When markings are unclear or need verification, you can use the following methods.
1. Visual Identification (Preferred Method)
This is the most direct and safest method. Most standard SCRs have terminal information on the package.
· Model & Pinout: First, check the model number printed on the device, such as BT151 or MCR100-6. Look up the datasheet or pinout diagram online—that's the most authoritative way.
· Physical Features: Many TO-220 package SCRs (like BT136) have the metal back connected to the anode (A). The middle pin is often the anode, but this isn't universal—you must confirm with the model.
· Pin Thickness: On some bolt-style SCRs, thicker bolts are usually the anode (A), while thinner pins are the gate (G) and cathode (K).
2. Multimeter Resistance Test (Universal Method)
When visual identification isn't possible, using a digital multimeter in diode mode or an analog meter in the R×1Ω range is effective. Make sure the SCR is fully discharged before testing.
Step 1: Find the Gate (G)
The gate-to-cathode junction is a PN junction, showing different resistances depending on polarity (like a diode). The gate-to-anode path has two reverse-connected PN junctions, so the resistance is usually very high regardless of probe placement.
· Set your multimeter to diode mode or R×100Ω.
· Measure the resistance between all pairs of leads.
If you find a pair showing unidirectional conductivity (one direction low resistance, the other high), the lead where the black probe reads low resistance is the gate (G), and the red probe is the cathode (K).
Step 2: Identify the Anode (A) and Cathode (K)
Once the gate and cathode are identified, the remaining lead is the anode (A). You can also verify:
· Measure resistance between anode and cathode—it should be infinite in both directions.
· Measure resistance between anode and gate—it should also be infinite or extremely high.
3. Simple Trigger Test (Verification Method)
This method should be used cautiously and is suitable for low-voltage, low-power SCRs like the MCR100-6.
· Connect the multimeter red probe to the assumed cathode (K) and black probe to the assumed anode (A); the resistance should be infinite, showing the SCR is off.
· Briefly touch the black probe (anode A) to the assumed gate (G) with a wire (providing a small positive trigger).
· If the meter reading drops from infinite to a low value (a few to tens of ohms) and stays conducting, your assumption is correct: black probe is anode (A), red probe is cathode (K), and the touched lead is gate (G).
IV. Conclusion
Correctly identifying the three terminals of an SCR is essential for proper circuit operation and preventing damage. The recommended workflow is: first, consult the datasheet for the most accurate pin definitions; if unavailable, use the multimeter resistance method to logically infer terminals; for low-power devices, use a trigger test to confirm. Always follow safety precautions, especially when working with high-power or high-voltage SCRs. Mastering these techniques allows you to confidently handle any SCR polarity identification challenge.
