a low power high efficiency switching power supply comprises a switching transducer having primary and secondary windings . the primary winding is supplied with a primary voltage over a series connected constant current source and zener diode . the primary voltage is chopped by an fet which has a gate connected to an oscillator operating at relatively low frequency . the secondary windings of the transformer are connected to full - wave rectifiers which convert the a . c . signal produced by the chopping of the primary voltage , int a d . c . signal . efficiency is maximized by using fast switching fet &# 39 ; s and low flux densities in the transformer . the power supply is particularly useful in 4 - 20 ma current loops .