SCR Applications
Resistor R1 limits the magnitude of the gate current.
When the SCR turns on, the anode-to cathode voltage (VF) will drop to the gate circuitry.
For the negative region of the input signal, the SCR will turn off since the anode is negative with respect to the cathode.
The diode D1 is included to prevent a reversal in the gate current.
Variable – Resistance Phase Control
A circuit capable of establishing a conduction angle between 90° and 180° is shown in Fig. 20.12a.
The circuit is similar to that of Fig. 21.11a except for the addition of a variable resistor and the elimination of the switch.
The operation here is normally referred to in technical terms as half-wave variable-resistance phase control.
It is an effective method of controlling the rms current and power to load.
However, if R1 is increased, a larger input voltage (positive) will be required to fire the SCR.
As in the Fig 21.12b, the control cannot be extended past a 90° phase displacement since the input is at its maximum at this point.
If it fails to fire at this lesser values of input voltage on the positive slope of the input, the same response must be expected from the negatively sloped portion of the signal waveform.
Battery-Charging Regulator
A third popular application of the SCR is in a battery-charging regulator.
The fundamental components of the circuit are shown in Fig. 20.13.
Emergency- Lighting System
In Figure 20.15 shows a single source emergency-lighting system that will maintain the charge on a 6-V battery to ensure its availability and also provide dc energy to a bulb if there is a power shortage.