A thermostat or other in-line, two-conductor control device employs a power-stealing technique to obtain DC power for its electronic controls where the thermostat wire run has an R wire but no C (common) wire. The controlled switch for a reactive load, such as the gas valve relay or compressor contactor, is formed of a pair of power capacitors and a pair of switching transistors or other electronic controlled switches. The conductors for feeding DC power to control electronics are connected with the junctions of the power capacitors and their respective switching transistors. The power capacitors and switching transistors can be configured in respective series combinations, with the series combinations forming a parallel combination. The control device can be considered to have an energy storage and switching arrangement that forms with the load an L-C circuit and an electronic switch in circuit between the energy storage arrangement and the load and can be switched on and off to provide an input to a DC power supply.