Thyristor Circuits: how an SCR circuit works

There are a variety of thyristor / SCR circuits that can control both DC and AC - often the AC control circuits use a phase difference on the gate to control the level of current flow.

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Thyristor, SCR circuits are widely used for power control of both DC and AC systems. The circuits use a variety of different methods to control the load current flow, but all require the gate to be fired and the anode cathode voltage to be removed to stop the current flow. Understanding how a thyristor / SCR circuit works enables them to be designed more easily.

Many AC thyristor, SCR circuits use a variable phase difference of a signal created at the gate to control the portion of the waveform over which the thyristor conducts. This type of circuit is relatively easy to design and construct.

DC thyristor / SCR circuit

There are many applications where an SCR circuit is required to control the operation of a DC load. This may be used for DC motors, lamps or any other load requiring switching.

The basic SCR circuit given below is able to control the power to the load using a small switch to initiate the application of power to the load.

Basic DC thyristor / SCR circuit
Basic DC thyristor / SCR circuit

Initially with S1 closed and S2 open, no current will flow. Only when S2 is closed and it triggers the gate by causing gate current to flow, will the SCR circuit turn on and current flow in the load.

Current will continue to flow until the anode circuit is interrupted. This can be done using S1. An alternative method is to place the switch S1 across the SCR and by momentarily closing it, the voltage across the SCR will disappear and the SCR will stop conducting.

As a result of their functions in this SCR circuit S1 may be called the Off switch and S2 the ON switch. In this configuration S1 needs to be able to carry the full load current, while S2 only needs to be able to carry the gate current. Once the SCR is on, the switch can be released and remain open as the action of the SCR sustains the current flow through the device and hence the load.

The resistor R1 connects the gate to the supply via the switch. When the switch S2 is closed, current flows through the resistor, enters the gate and turns SCR on. The resistor R1 has to be calculated to provide sufficient gate current to turn the SCR circuit on.

R2 is included to reduce the sensitivity of the SCR so that it does not fire on any noise that may be picked up.

Basic AC thyristor / SCR circuit

When AC is used with an thyristor circuit, a few changes need to be made as seen below.

The reason for this arises because the AC power reverses polarity over the course of the cycle. This means that the SCR will become be reverse-biased, effectively reducing the anode voltage to zero causing it to turn OFF during one half of each cycle. As a result there is no need to have an off switch as this is achieved as part of the use of an AC supply.

Basic AC thyristor / SCR circuit
Basic AC thyristor / SCR circuit

The operation of the circuit is slightly different to that of the DC SCR circuit. When the switch is turned on, the circuit will need to wait until there is sufficient anode voltage available as the AC waveform progresses along its course. Also the SCR circuit will need to wait until the voltage within the gate section of the circuit can provide sufficient current to trigger the SCR. For this the switch has to be on its closed position.

Once triggered the SCR will remain in its conducting state over the positive half of the cycle. As the voltage falls, there will come a point where the anode cathode voltage is insufficient to support conduction. At this point the SCR will stop conducting.

Then over the negative half of the cycle, the SCR will not conduct. Only when the next positive half of the cycle returns will the process repeat.

As a result this circuit will only conduct when the gate switch is in its closed position.

One of the issues with using an SCR circuit of this nature is that it cannot supply more than 50% power to the load, because it does not conduct during the negative half of the AC cycle because the SCR is reverse biased.

AC SCR circuit with gate phase control

It is possible to control the amount of power reaching the load by altering the proportion of the half cycle over which the SCR conducts. This can be achieved by using an SCR circuit that incorporates phase control of the input gate signal.

AC thyristor / SCR circuit waveforms showing input, output and switching
AC thyristor circuit waveforms

Using the SCR circuit with phase control, it can be seen that the SCR gate signal is derived from an RC circuit consisting of R1, VR1 and C1 before the diode D1.

As with the basic AC SCR circuit, only the positive half cycle of the waveform is of interest because the SCR is forward biased. During this half cycle the capacitor, C1 charges up via the resistor network consisting of R1 and VR1 from the AC supply voltage. The waveform at the positive end of C1 is seen to lag that of the input waveform and the Gate is only triggered when the voltage at the high end of the capacitor has risen sufficiently to trigger the SCR through D1. As a result the turn on point for the SCR is delayed from that which would normally occur had the RC network not been present. Setting the value of the VR1 alters the delay and hence the proportion of the cycle over which the SCR conducts. In this way the power into the load can be adjusted.

AC thyristor / SCR circuit with gate phase control
AC thyristor circuit with gate phase control

The series resistor R1 has been included to limit the minimum value for the resistor network to a value that will give an acceptable gate current level for the SCR.

Typically to give complete control of the 50% of the cycle available for conduction with an SCR, the phase angle of the gate waveform must vary between 0° and 180°.

These circuits give some of the basic concepts behind the design of SCR / thyristor circuits. They demonstrate the basic operation of how they work and how they can be used.

One of the main issues to be aware of when designing thyristor circuits is that of power dissipation. With these circuits often handling high voltages and high power levels, power dissipation can be a major factor in the circuit design and operation.

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