3V, 3.3V or 5V logic supply from AC power. It uses the MAX6330 shunt
regulator which can supply up to 50mA as well as offering a reset output
(power-on reset, POR).
"white goods" products such as industrial controllers and sensors. Those applications usually
include 24VAC, 115VAC, or higher levels of AC power for conversion to 3.3V or 5.0V. (The
following circuit suits applications that include 24VAC. With proper safety precautions, it can also
be applied to double-insulated white goods and other products that require a logic supply for
control or monitoring functions.)
rectified and filtered AC input to a high-input-voltage linear regulator. But, power dissipation in
the regulator can be considerable, even for modest load currents. A standard shunt regulator
also dissipates notable power in the limiting resistor. A switching regulator minimizes power
dissipation, but that type may not be practical for cost-sensitive designs.
shunt-regulator designs. To transfer energy to the regulator with negligible power loss, use a
coupling capacitor in conjunction with an IC containing a shunt regulator and power-on reset
(Figure 1). Available with 50mA maximum output-current capability in 3.0V, 3.3V, and 5.0V
shunt-voltage versions, IC1 also includes a power-on-reset (POR) function. Because IC1 is an
active shunt (vs. a passive zener diode) one must rectify the AC voltage before applying it.
Typically, a capacitor follows the rectifier to hold the charge during off cycles. As shown, a
simple half wave rectifier is used to save cost.
power-on reset and efficient logic display.
discharges the transfer capacitor during negative cycles. R1 limits surge current during the
discharge of C1, and (if applicable) during high-voltage transient testing.
drops in the diodes, and a zero shunt voltage in the IC regulator. With (for example) a 60Hz
sinusoidal input of 24VRMS amplitude (Vpeak = 33.94V), you can then calculate as follows:
shunt current (50mA), while achieving an output of 20mA or so. The voltage rating of C1 should
be able to withstand the maximum input voltage.
half-wave rectifier (D1). D2 discharges C1 during the negative portion of the cycle. The current
rating of D2 depends on the value of Vpeak and the selected value (50 ) of surge-limiting
resistor R1. The maximum reverse voltage at D1 and D2 is (Vshunt + Vdiode).
cycle. To calculate its value, use the following approximation based on the allowable level of
ripple voltage (Vripple):