Thyristors and Raage numbers.

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Thyristors

Some confusion exists over the use of the term thyristor. Here it is taken as a generic title, the silicon controlled rectifier (s.c.r.) being one particular type. An s.c.r. with its equivalent circuit and symbol is shown below. With G open-circuit 01 is cut off, so the device will not conduct unless VAK is made sufficiently positive for it to avalanche. If the gate is made positive /CJ flows. Now /CJ is base current for 02 and fez is base current for 01 • Therefore /CJ brings on 02 and the cumulative action makes both transistors saturate, i.e. the voltage across the device falls to a minimum and it is in the condition of a closed switch. Once the s.c.r. is on, the gate voltage is not required to sustain it. In fact the gate cannot be used to turn it off, and this must be done by reducing VAK to a very small voltage.

To switch a thyristor on without any gate voltage the device must be made to avalanche. Such a device, which will avalanche in either direction in a controlled manner, is the bi-directional diode thyristor or diac. A gated version of this is the bi-directional triode thyristor or triac, which will conduct in either direction when gated with a pulse of the appropriate pt>larity. In this way it performs as a pair of thyristors connected in inverse parallel.

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Pro Electron system of semiconductor type labelling

The Pro Electron system of semiconductor labelling, used by most European manufacturers, describes a device by means of a code comprising two letters followed by a serial number. The letters define

the semiconductor material used and the device's general function, as listed below:

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The serial number defines the device's particular application, and will consist of either: three numbers (which shows the device is intended for use primarily in consumer applications), or; a letter followed by two numbers (which shows the device is intended for use primarily in industrial or professional environments).

Raage numbers

Where variants of a device exist, the above code is addended with a further code (separated by a hyphen) to identify the specific device type within the range. Two classes of device are affected:

(a) Rectifer diodes and thyristors; the group of figure indicate either the repetitive peak inverse voltage, V..... or the repetitive peak off-state voltage, V00.., whichever is the lowest.

(b) Voltage regulator diodes and transient suppression diodes; a first letter (voltage regulator diodes only) indicates operating voltage tolerance, where:

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and a group of figures indicate the typical operating voltage (or the maximum recommended stand-off voltage, in the case of transient suppressor diodes).

In all cases, a final letter (R) may be used, to indicate a reverse polarity version (i.e., one with a stud anode).

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