Typically, ac line sensors use
comparators. But I had to design a power supply to be as low in cost as possible, be self-powered, use through-hole
components, and take up as little space
as possible on a small two-sided pc
board. So I developed the simplest ac
sensor possible. The resulting circuit
senses the high-voltage dc bus (). During normal operation, the
AC_OK signal is low. At about 90 V ac,
the signal goes high.
R1 and D1 create a 5.1-V bias that
allows the design to be self-powered. To
keep dissipation on R1 to a minimum,
the rest of the resistors were chosen to
minimize current. U2, an LM431BCZ, is
employed as a reference and comparator. Q1 and R7 create the hysteresis to
prevent chatter around the trip point.
Because the LM431 still sinks current
when off, using Q1's base current to
drive U1 guarantees that no current
flows through U1. C1 filters noise that
would affect U1. R4 and R6 sense the
power supply's dc bus. Two resistors are
employed to dissipate 1/4 W.
The current through R8 at the trip
point is:
2.5 V/R8 = 531 µA
Since Q1 is on as the trip point is
approached,
(5.1 V - 2.5 V) × R7 = 56 µA
Therefore, R4 and R6 must supply:
531 µA 56 µA = 475 µA
Using the values of R4 and R6,
(560 kΩ) × 475 µA = 266 V
Because the power supply uses a voltage doubler, the trip point is 94 V ac.
That is:
(2 × √2) × 94 = 266).
Since Q1's base current is only about
900 A and U1's current transfer ratio is
100% minimum, the optocoupler's output should not exceed 900 A. Because
the current transfer ratio of the optocoupler can deteriorate with time, this
should be derated.
Note that the ripple on the dc bus is
affected by load and capacitance. As a result, the final selection of resistor values is based on testing. Also, not all
manufacturers' parts will work for U2 in
this application, due to the fact that bias
current drops below the device's recommended minimum. This design used
National Semiconductor's LM431.
The power supply housing this circuit
has been produced for the past five
years and continues to be used in new
products. To date, the power supply hasn't had any problems in manufacturing.