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Saturday, August 3, 2019

Difference between transistor


Difference between transistor and thyristor?
The main difference between transistor and thyristor is that a transistor has three layers of semiconductors, whereas a thyristor has four layers of semiconductors. Sometimes, thyristors are referred to as silicon controlled rectifiers (SCRs)


A transistor allow current to flow between the Emitter and Collector when a bias current is applied to the Base. The stronger the base current the more the current between the Emitter and Collect is allowed to flow. When the Base current stop so does the current between the Emitter and Collector.

A transistor conducts in two modes. The first mode is linear where the current between the Emitter and Collector is dependent on the base current. The amount of current of gain of the transistor will vary with temperature and the frequency of the change in the bias current.

The second mode is reached when the maximum amount of base current is applied. At this point the transistor is fully on and added more bias current does nothing but heat up the transistor. This mode is called saturation and it has the disadvantage that when the base current is lowered, the transistor will continue to fully conductor for a period of time afterward. For a circuit that use the transistor as a switch this means that the transistor cannot operate as fast due to the turn-off delay. It does however have the advantage that while it is on it will conduct as much as it possible can and be consistent while doing so. Thus when driver by a fast rise and falling base current the transistor will reproduce a fast rising and a delayed but fast falling output signal.

This was the basic for the early logic devices in the 60’s and 70’s. The were reasonably fast and power hungry but they worked reliably. ECL logic was also developed that did not drive the transistor to saturation and enabled much faster operation but it was much harder to ensure the signals were not miss-interrupted and the power consumed was much greater as well.

Thyristor
A SCR acts similarly to a transistor except that when the gate current stop the current between the Anode and Cathode does not stop until the current level falls below a minimum hold value. While a SCR does operate in a linear mode it is a very small area of operation and is not consistent SCRs are design to operation in the saturation mode. An SCR when properly trigger is fully turned on instead of being proportionate to the gate current. An SCR once triggered looks like a diode to the system and thus the power losses are simply a diode voltage drop of around 0.8 volts.

Typically SCR are used in AC application where the reversal of the current turns the SCR off and thus the SCR requires a new trigger to conduct again. SCR offer high over-current capabilities and very large SCRs exist that can handle 100’s of amperes. SCR can be easily falsely triggered due to internal capacitance are fairly low frequencies so they are typically only used at low frequencies like 60 Hz. There are several variants of the SCR available such as the TRIAC which can conduct power in both directions.

A very common use of a SCR is in a photo flash trigger circuit. A high voltage would be pumped into a large capacitor and once fully charged the user would push a button and the SCR would be triggered. This allow the capacitor to quickly discharge into a flash tube and would discharge the capacitor thus resetting the SCR. This application takes advantage of the high breakdown voltage and large current capabilities that SCRs are easily built for. It also provide a very simple circuit design that only used one expensive component the high-voltage capacitor.

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