Fundamentals of Electrical Practice: Identification and Measurement of Thyristors

Fundamental Skills Training in Marine Electronics and Electrical Systems

I. Basic Concepts of Thyristors

A thyristor is short for "crystal-controlled rectifier" and is also known as a silicon-controlled rectifier; it was previously referred to simply as a "silicon-controlled rectifier." During operation, the thyristor’sAnode (A) 和Cathode (K) Connected to the power source and the load, respectively, to form the main circuit of the thyristor;Gate (G) It is connected to the cathode (K) and the control unit to form a control circuit.

Figure 12-7 Thyristor Symbols and Package Types

Thyristor Operating Conditions (Semi-controlled Characteristics)

(1) Reverse blocking:When a reverse anode voltage is applied, the device remains in the cutoff state regardless of the gate condition.

(2) Forward conduction:subject to a positive anode voltage andThere is a forward voltage across the gateConducts only when a current flows (thyristor characteristics).

(3) Maintain the conduction path:Once the gate is turned on, the back gate becomes inactive and is maintained solely by the main circuit voltage. The gate serves only as a trigger.

(4) Turn-off conditions:When the main-circuit voltage (or current) decreases to near zero, the thyristor turns off.

[Image of a thyristor characteristic curve showing the forward and reverse blocking regions and the conduction state]

II. Practical Thyristor Measurement Techniques

1. Pin Identification (G, K, A)

Principle:Only the region between the gate (G) and the cathode (K) is a PN junction, which conducts current in only one direction.

Procedure:Set the multimeter to R×1k Block, measure any pin. When a very low resistance is measured:

● The black probe is connected to: the control terminal (G)
● The red probe is connected to: the cathode (K)
● The remaining pin is: Anode (A)

2. Criteria for Determining Quality

(1) A-K Test (R×100 range):

Under normal conditions, the forward and reverse resistance should both beSeveral hundred thousand euros. If the resistance is only a few ohms or a few dozen ohms, it indicates that the thyristor hasShort-Circuit Damage。

(2) G-K Test (R×10 or R×1 gear):

● Forward Resistance:It should be very small (a few dozen euros).

● Reverse Resistance:It should be quite large (from tens of kiloohms to hundreds of kiloohms).

Note: Due to variations in PN junction characteristics, a reverse resistance measurement of several thousand ohms does not necessarily indicate poor performance. However, if both the forward and reverse resistances are zero or extremely high, the thyristor is definitely damaged.