24vac is a very popular power supply in
many industrial systems. It is often used in systems that involve water.
It is also popular with many inside and outside lighting systems. The lower
voltage means the danger of human electrocution is much less. But, many DC
powered sensors and control circuits associated with 24vac systems need to be fully
isolated from the 24vac supply. The circuit below accomplishes this feat using
an off the shelf 24v DC to 5v power module. A charge pump scheme using a series
capacitor limits the current to a safe level. A zener diode keeps the input
voltage to the module at about 24v DC. Two polarized capacitors wired in series
operate as a non-polarized capacitor. A schottky diode in parallel with witch
each capacitor insures that the part is not reversed biased. With the components
selected, the +5v output is rated at about 100ma.