29-12-2012, 11:13 AM
SCR Applications
1SCR Applications.ppt (Size: 449.5 KB / Downloads: 25)
Series Static Switch
A half-wave series static switch is shown in Fig. 20.11a.
If the switch is closed as shown in the Fig. 20.11b, a gate current will flow during the positive portion of the input signal, turning the SCR on.
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.
The Combination of the resistors R and R1 will limit the gate current during the positive portion of the input signal.
If R1 is set to its maximum value, the gate current may never reach turn on magnitude.
As R1 is decreased from the maximum. The gate current will increase from the same input voltage.
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.
D1 and D2 establish a full-wave-rectifier signal across SCR1 and the 12-V battery to be charged.
At low battery voltages, SCR2 is in the “off” state.
With SCR2 open, the SCR1 controlling circuit is exactly the same as the series static switch control.
Temperature Controller
The schematic diagram of a 100-W heater control using an SCR appears in Fig. 20.14 It is designed such that the 100-W heater will turn on and off as determined by thermostat.
Mercury-in-glass thermostats are very sensitive to temperature change.
In fact, they can sense changes as small as 0.1°C.
It is limited in applications, however, in that it can handle only very low levels of current – below 1mA.