Brief Background on Industrial
Electronics
Silicon Controlled Rectifiers (SCRs), Triacs and
other high –powered transistors are used in many types of circuits to control
large voltages and currents. Many of these use 480 VAC 3 Phase circuits and can
control over 50 A. These devices offer control circuits for general purpose
power supplies, AC and DC Variable speed motor drives, Servo motor controls,
Stepper motor controls, high frequency power supplies, welding power supplies
etc. SCRs, Triacs, and any other solid-state devices used for switching larger
voltages and currents on and off are commonly called thyristors. Thyristors
control switching in an on-off manner similar to a light switch which is
different from a transistor that can vary the amount in its emitter-collector
circuit by changing the bias on its base. The amount of current that flows
through a thyristor must be controlled by adjusting the point in a sine wave
where the device is turned on.
Silicon Controlled Rectifiers (SCRs)
The figure 1 below shows a symbol for the SCR and
identifies the anode, cathode, and gate terminals. The cathode is identified by
the letter C or K. The diagram also shows several types of SCRs.
Figure 1 |
How Silicon Controlled Rectifiers
Work
The SCR acts like a solid-state switch in that the
current will pass through its anode-cathode circuit to a load if a signal is
received at its gate. The SCR is different from a traditional switch in that the
SCR will change ac voltage to dc voltage (rectify) if ac voltage is used as the
power supply. The SCR is also different from a traditional switch in that the
amount of time the SCR conducts can be varied so that the amount of current
provided to the load will be varied from near zero to maximum of the power
supply.
Figure 2 |
The SCR can vary the amount of current that is
allowed to flow to the resistive load by varying the point in the positive
half-cycle where the gate signal is applied. If the SCR is turned on
immediately, it will conduct full voltage and current for the half-cycle
(180°). If the turn-on point is delayed to the 90° point in the half-cycle
waveform, the SCR will conduct approximately half of the voltage and current to
the load. If the turn-on point is delayed to the 175° point in the half cycle,
the SCR will conduct less than 10% of the power supply voltage and current to
the load, since the half-cycle will automatically turn off the SCR at the 180°
point. This implies that the gate of the SCR can be used to control the amount
of voltage and current the SCR will conduct from zero to maximum.
You can also read: How Test Diodes are used to Measure loop Currents
You can also read: How Test Diodes are used to Measure loop Currents
The Operation of SCR explained by the
four-layer (Two-Transistor) Model
The SCR is a four-layer thyristor made of PNPN
material; in fact the proper name for the SCR is the reverse blocking triode
thyristor.
The Figure 3 above shows the PNPN material split
apart as two transistors, a PNP and a NPN. The figure (c) shows the SCR as two
transistors.
The anode is at the emitter of the PNP Transistor
(T2), and the cathode is at the emitter of the NPN transistor (T1). The gate is
connected to the base of the NPN Transistor. Since the anode is the emitter of
the PNP, it must have a positive voltage to operate and since the cathode is
the emitter of the NPN transistor, it must be negative to operate.
When a positive pulse is applied to the gate, it
will cause collector current Ic to flow through the NPN transistor (T1). This
current will provide bias voltage to the base of the PNP transistor (T2). When
the bias voltage is applied to the base of the PNP transistor, it will begin to
conduct Ic which will replace the bias voltage on the base that the gate signal
originally supplied. This allows the gate signal to be a pulse which is then
removed since the current through SCR anode to cathode will flow and replace
the base bias on transistor T1.
Methods of turning on an SCR
Normally the SCR is turned on by a pulse to its
gate but we have 3 other methods you can also use to turn it on.
These methods include:
- Exceeding the forward break over voltage
- By Excessive heat that allows leakage current
- Exceeding the dv/dt level (allowable voltage change per time change) across the junction.
Methods of turning off/Commutating
SCRs
Once an SCR is turned on, it will continue to
conduct until it is turned off (commuted). Commutation will occur in an SCR
only if the overall current gain drops below unity (1). This means that the
current in the anode-cathode circuit must drop below the minimum (near zero) or
a current of reverse polarity must be applied to the anode-cathode. Since the
ac sine wave provides both of these conditions near the 180° point in the wave,
the main method to commutate an SCR is to use ac voltage as the supply voltage.
In an ac circuit, the voltage will drop to zero and across over to the reverse
direction at the 180° point during each sine wave. This means that if the
supply voltage is 60 Hz, this will happen every 16 msec. Each time the SCR is
commutated, it can be triggered at a different point along the firing angle,
which will provide the ability of the SCR to control the ac power between 0° to
180°. The main drawback with using ac voltage to commutate the SCR arises when
higher-frequency voltages are used as the supply voltage. Note that, the SCR
requires approximately 3-4 msec. to turn off; therefore the maximum frequency
is dependent on the turn-off time.
Figure 4 |
Figure 4 (a) A switch is used to
commutate the SCR in a dc circuit by interrupting current flow. This type
of circuit is used to provide control in alarms or emergency dc voltage
lighting circuits, (b) A series of RL resonant circuit circuit used
to commutate an SCR and (c) A parallel RL resonant circuit used to
commutate an SCR.
SCRs are also used in the inverter section where
the dc voltage is turned back into ac voltage. Since the devices must provide
both the positive and the negative half-cycles, a diode is connected in inverse
parallel to provide the hybrid ac switch. This combination of devices is not
frequently used currently as larger Triacs and Power transistors can do better
job in this kind of applications.
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