24 volt power source as contrasted to the 100 volt requirement of the prior art systems this permits manufacture of a less expensive unit with smaller components the transformer t3 provides a 2 to 1 voltage stepup for the flame sensor with no other loading and therefore can be quite small and inexpensive the rectification efficiency of a flame is very low and therefore prior art systems have required a 100 volt supply on the flame sensor also while the present system provides the same or improved sensitivity with one half the voltage while reference is made herein to a 24 volt ac supply is should be realized that this value is given by way of example and that higher or lower voltages may be utilized a typical specification for this type of equipment is that it operate with supply voltages over the range of 18 to 32 volts ac all of the circuits are designed for failsafe operation that is any failure in a circuit component either shorting or open circuiting will not result in the main burner valve remaining open in an unsafe condition the combination of the field effect transistor q1 the storage capacitor c3 the diodes d2 and d3 and the controlled rectifier q2 is similar to that shown in u s pat no 3 619 097 however the circuit of fig1 of the invention utilizes a unilateral trigger diode as the diode d3 conventional diodes such as the.