Page 30 of 36
Figure 13-DC Good Detector snippet
DC Voltage Good Detection
The following discussion refers to Figure 13. The DC Voltage good detector allows C1 to
charge, and the relay to be energized, whenever VCC is greater than about 53 volts.
Here’s how it works.
R5 and R6 form a voltage divider that places 1/5 of VCC on the cathode of D3, a 10 Volt
zener. When the cathode of D3 is more than 10.6 volts positive (with respect to PGND)
Q2 turns on. This turns Q1 off, allowing C1 to charge via R1. That says that the threshold
of DC voltage good is approximately VCC>5*10.6=53 volts. This calculation ignores the
attenuation of R4, but that provides less than a 10% change. R4 is there to assure that
Q2’s leakage currents cannot turn it on.
AC Voltage Present Detection
The following discussion refers to Figure 14Figure 13. The AC Voltage Present
Detection Circuit allows C1 to charge, and the relay to be energized, whenever the peaks
of the X1-X2 transformer voltage are much greater than 40 volts. Here’s how it works.
R11 and R10 form a voltage divider that places 2/7 of the voltage from X1 to X2 at the
input of D4, a 10 Volt zener. Thus any time that the peaks of the transformer voltage
exceed 39.2 volts, opto-isolator U1’s LED conducts, which turns on the opto-isolator’s
transistor, discharging C2. (7/2*11.2=39.2, where 11.2 is the sum of the 10V zener
voltage and the 1.2 volt drop across the input LED of the opto isolator.).
Discharging C2 keeps Q3 turned off, which allows Q3 to charge.
