A voltage transformer circuit is described for supplying a spark plug with ignition energy and for ionic current measurement. A transformer of the circuit generates a secondary voltage from a primary voltage and applies the secondary voltage to the spark plug to ignite an electric arc. Using a current effected by the secondary voltage, a capacitor is charged to a breakdown voltage of a Zener diode bridging the same, via a first branch, which leads from the transformer to a first of two sides of the capacitor. The primary voltage is disconnected from the transformer in order to extinguish the electric arc. The capacitor delivers charges for an ionic current via a second branch, which connects the second side of the capacitor to the transformer. A measuring signal of the ionic current is obtained by measuring a voltage drop at a measuring resistor.