Fluorescent tube firing circuit - has triac fired to draw DC heating current through tube electrodes and ceases operation once tube fires or fails

摘要

A firing circuit (7) for a fluorescent tube comprises a triac (TR) connected in parallel with the tube. The gate circuit of the triac (TR) includes a diac (d) controlled by a capacitor (C) which charges on each mains half cycle to the threshold voltage of the diac (d) and fires the triac (TR) to draw d.c. heating current through the tube electrodes. The charging resistor (R1) for the capacitor (C1) determines the firing angle of the triac (TR) which preferably is nonconductive over an effective angle of 90 deg.. Once the tube fires, a resistive divider (R1, R2) including the charging resistor prevents the capacitor charging to the triac firing voltage. A thermistor (10) may be connected between the tube and the triac (TR) such that if the tube fails to fire, the thermistor (10) heats up increasing its resistance and eventually rendering the firing circuit (7) inoperative.