Triac, Diac, SCR Tutorial Includes:
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When selecting a Triac for a particular circuit application, it is essential to choose one that will be able to perform in the manner required and be able to withstanding the voltages and currents that it needs to accommodate.
They key to selecting the right Triac is to be able to understand the specifications and parameters found in the datasheets. In this way it is possible to know what figures indicate and hence select the best device for the circuit.
Triac datasheet specification highlights
Triac specifications and those of thyristors have many similarities as would be expected, but one of the major differences is that Triacs operate on both halves of an AC waveform cycle.
As a result the Triac specifications need to take account of this and often Triac specifications include the number of quadrants in which they operate.
|Common Triac Specifications & Datasheet Parameters|
|Spec||Triac Specification / Parameter Details|
|VDRM / VRRM||Repetitive peak off-state voltage||This parameter is the maximum peak voltage that is allowed across the TRIAC. This parameter should not be exceeded even instantaneously otherwise the device may fail. It is also always good to leave sufficient margin to allow for transients. This parameter is specified for conditions up to the maximum junction temperature. Also leakage currents (IDRM / IRRM) are also normally defined under this datasheet specification.|
|IT(RMS)||RMS on-state current||This Triac specification is the maximum allowed RMS current through the device. It is specified for a given temperature. Different datasheet specifications may quote ambient temperature, Ta, case temperature, Tc, or even the lead temperature, Tl. The method used for quoting the temperature normally depends upon the type of case used for the triac.|
|dI/dt||Maximum rise of on-state current||There is a maximum rate of rise specification for the on-state current during turn on for the triac. If this rate is exceeded then the device can be damaged.|
|I2t||Overcurrent protection||The I2t parameter indicates the fuse that is required for protection of the triac and its circuit. It is typically for the 10ms overcurrent duration, but this will be stated in the datasheet.|
|IT(AV)||Average on-state current||This parameter is different to the RMS current as it defines the average current rather then the RMS. RMS will give the true heating effect of the current.|
|ITSM||Non-repetitive surge on-state current||As the name implies this datasheet parameter for thyristors defines the maximum peak current in the device under pulse conditions. It is necessary to look at the exact conditions for the manufacturer in question, but it is often defined for a half sine wave. The duration is specified for 50 Hz (10ms duration) and 60Hz (8.3ms duration). It is required because a surge current exceeding the maximum may cause failure of the device.|
|IGT||Gate trigger current||This parameter is the current required in the triac gate circuit to enables the triac to be triggered and to latch in its on-state provided there is sufficient anode cathode current to maintain the current flow.|
|IGM||Peak gate current||This specification parameter is the maximum level of gate current for the triac.|
|VGT||Gate trigger voltage||This specification reflects the voltage that needs to be applied to the Triac gate to enable the gate trigger current to be reached and the device to fire.|
These are some of the main specifications or parameters that are included within the triac datasheets. They help enable the best triac to be chosen for any given circuit application.
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