A Silicon controlled Rectifier (SCR) is a semiconductor device which conducts in only one direction.  It has three terminals.  An anode, a cathode and a gate.  Unlike a diode, however, it needs to be switched on by a pulse applied to the gate.  

The circuit below shows the method of switching on an SCR using a resistor.  The power source is connected across the SCR.  The gate voltage is provided by the voltage divider circuit.  The variable resistor, R4 is used to control the firing angle.  

The Diode D1 prevents negative voltage from reaching the gate during the negative half cycle.  The SCR will be switched off during the negative half cycle by the supply voltage


Resistor switching of an SCR
Circuit Diagram - Resistor switching of an SCR