Thyristor Circuits

In this article we will examine the thyristor circuits. This time we will look at how we can use thyristor and thyristor switching circuits to control much larger loads, such as lamps, motors or heaters.

We said that we had to inject a small trigger current pulse (not a continuous current) into the Gate(G) Terminal to make the ductor "ON".

Thyristor Circuits
Typical Thyristor

In general, the duration of this trigger pulse should be only a few microseconds. However, the longer the door impact is applied, the faster the internal avalanche breakage occurs, and the faster the "ON" rotation time of the estrifier, but the maximum Door current should not be exceeded. Once triggered and fully transmitted, the voltage drop from Anottan To Cathode along the thyristor is reasonably constant at approximately 1.0V for all Anod current values up to the rated value.

However, keep in mind that once a Tristor starts transmitting, even if there is no Door signal, the Anode current continues to transmit until it falls below the devices (IH) holding the current, and below this value it is automatically "OFF". Unlike bipolar transistors and FET's, thyristors cannot be used for amplification or controlled switching.

Thyristors are semiconductor devices designed specifically for use in high-power switching applications and without amplifier capability. Tristors can only work in a switching mode that acts as an open or closed switch. After the transmission is triggered by the door terminal, a thyristor will always remain conductive (current passing). Therefore, in DC circuits and some high inductive AC circuits, the current must be artificially reduced with a separate switch or shut-off circuit.

DC Thyristor Circuit

When the direct current is connected to the DC source, the tristor can be used as a DC switch to control larger DC currents and loads. The tristor acts like an electronic latch when used as a switch because once activated it remains in the "ON" state until it is manually reset. Consider the DC thyristor circuit below.

DC Tristor Switching Circuit

Thyristor Circuits
DC Tristor Switching Circuit

This simple "on-off" thyristor ignition circuit uses a thyristor as a switch to control a lamp, but can also be used as an on-off control circuit for an engine, heater or other DC loads. The thyristor is tilted forward and the normally on "ON" press button is triggered by a short-term shutdown of the S1, which connects the S1 Door terminal to the DC source through door resistance, allowing the current to flow to the Door. If the RG value is set too high according to the supply voltage, the thyristor may not be triggered.

After the circuit is made "ON", it locks on itself even when the push button is released, provided that the load current is greater than the thyristor locking current, and remains "ON". Additional operations of the push button will have no effect on the circuit state of the S1. Because once the door is "triggered", it loses all control. The thyristor is now fully "ON" (conductive) and allows the full load circuit current to flow forward from the device and back to the battery source.

One of the main advantages of using a thyristor as the switch in the DC circuit is that it has a very high current gain. The thyristor is a current-powered device because a small Door current can control a much larger Anode current.

Gate-cathode resistance RGK is usually included to reduce the sensitivity of the Gate and increase the dv/dt capacity, thus preventing the device from being triggered incorrectly.

Since the thyristor is self-locking to the "ON" state, the circuit can only be reset by cutting the power supply and reducing the Anode current to below the minimum retention current (IH) of the thyristors.

Opening the normally closed "OFF" button, the S2 cuts the circuit and lowers the circuit current passing through the Tristor to zero. This forces it to become "OFF" until another Door signal is reapplied.

Alternative DC Tristor Circuit

Thyristor Circuits
Alternative DC Tristor Circuit

Here the thyristor switch receives the required terminal voltage and door pulse signal as before, but the larger normally closed switch of the previous circuit is replaced with a smaller normally open switch in parallel with the thyristor. Activation of the S2 switch instantly short-circuits between the thyristors Anode and Cathode, reducing the holding current to below its minimum value, stopping the transmission of the device.

AC Thyristor Circuit

When the alternating current is connected to the AC source, the thyristor behaves differently from the previous DC-connected circuit. This is due to the fact that AC power periodically reverses polarity, and therefore any thyristor used in an AC circuit will automatically reverse and cause it to become "OFF" halfway through each cycle. Consider the AC thyristor circuit below.

AC Thyristor Circuit

Thyristor Circuits
AC Thyristor Circuit

The tristor ignition circuit above is similar by design to the DC SCR circuit, except for the absence of an additional "OFF" switch and the inclusion of the D1 diode, which prevents reverse pre-charge to the door. During the positive semi-cycle of the sinusoidal waveform, the device is forward-oriented. However, when the S1 switch is on, zero gate current is applied to the thyristor and remains "OFF". In negative semi-cycle, the device must be inverted and will remain "OFF" regardless of the status of the S1 switch.

If the S1 switch is turned off, at the beginning of each positive semi-cycle the thyristor will be completely "OFF", but after a short time there will be sufficient positive trigger voltage, and therefore, there will be current available at the Door to "TURN ON" the triciror and lamp.

The thyristor is now locked for the positive half-cycle period – "ON" and automatically becomes "OFF" again when the positive half cycle ends and the Anode current drops below the holding current value.

During the next negative half-cycle, when the process repeats itself and the thyristor transmits again as long as the switch is turned off, the device is already completely "OFF" until the next positive half-loop.

Then in this case, the lamp will receive only half the available power from the AC source, since the thyristor acts as a rectifier diode and transmits current only during positive half-cycles when polarized forward. The thyristor continues to provide half power to the lamp until the switch is turned on.

If it was possible to quickly turn the S1 switch ON and OFF, so that if the thyristor receives the Door signal at the "peak" (90 degrees) point of each positive semi-cycle, the device would transmit only for half of the positive half. Cycle. In other words, transmission will occur only halfway through a sinus wave, which will cause the lamp to receive "a quarter" or a quarter of the total power from the AC source.

By accurately changing the timing relationship between the door impact and the positive semi-cycle, the Tristor can be made to provide the load with any desired percentage of power between 0% and 50%. Obviously, it cannot provide more than 50% power to the lamp using this circuit configuration, because it cannot transmit during negative half-cycles when inverted. Consider the circuit below.

Half Wave Phase Control

Thyristor Circuits
Half Wave Phase Control

Phase control is the most common form of thyristor AC power control and a basic AC phase control circuit can be created, as shown above. Here the Door voltage of the thyristors is derived from the RC charging circuit via the trigger diode D1.

During positive semi-cycle, when the thyristor is polarized forward, the capacitor charges through resistance R1 following the C, AC supply voltage. The door is activated only when the voltage at point A rises high enough to cause the trigger diode D1 to transmit, and the capacitor empties the door of the thyristor by making it "ON". The time in the positive half of the cycle in which transmission begins is controlled by the RC time constant set by variable resistance R1.

Increasing the R1 value has the effect of delaying the trigger voltage and current provided to the thyristor door, which causes a delay in the transmission time of the devices. As a result, the half-cycle rate transmitted by the device can be controlled between 0 and 180o. This means that the average power spent by the lamp can be adjusted. However, the thyristor is a one-way device. Therefore, only a maximum of 50% power can be provided during each positive half cycle.

There are several ways to achieve 100% full wave AC control using "thyristors". One way is to add a single thyristor to a diode bridge rectifier circuit that converts AC into a one-way current via the thyristor, while the more common method is to use two tristors connected in reverse parallel. A more practical approach is to use a single Triage. Because this device can be triggered in both directions. Therefore, it makes them suitable for AC switching applications.