Re: Simple power supply overheating components, shorting zener

Not to beat a dead horse, but to summarize:

(1) The circuit has multiple problems. You have to fix all of them.
400V film caps can't take the voltage or current. The diodes can't
take the 1uF 700V switching on spikes. Putting a capacitor directly
across a zener is mostly pointless (the diode will keep the capacitor
from storing the charge you need for use during the diode off time--
you need a resistor before the zener). The resistor can't be
guaranteed to take the 700V voltage or current spikes.

(2) Just because you can't see spikes doesnt mean they're not there,
or won't be at the customer's site. Especially in an industrial
environment, there can be 1KV spikes every time some inductive load
gets switched on or off.

I would patch the design by using a tiny 240V transformer, available at
digi key for $12. About a cubic inch.

Or I'd start over with a quad comparator, about 7 resistors, five
1N4148 diodes, one zener and three capacitors should do the trick.
Rough description:

A microwatt power supply-- say a 100K 2Watt resistor, a 1N4007 diode,
10uf 50V capacitor, a 100K 1/2 Watt resistor, then a 25uf 15V capacitor
with a 12V zener across it.

A mickey-mouse logic voltage selector:

A five resistor voltage divider across the 50V capacitor, totaling
about 1 meg. The four taps go to four of the inputs of a 74C14
directly to the odd inputs, thru 1meg resistors for the even inputs.
Resistor values chosen so we got 4 VDC at each tap at the magic AC
voltages.

The outputs of the even numbered gates go to the cathode end of a
diode, anode ends go to the odd numbered inputs.

The odd numbered outputs go to cathode ends of two more diodes, the
anodes go together, up to +12 volts thru a 1 Meg resistor and signal
"voltage BAD". That point goes to the input of a fifth gate, the
output of that gate is a nice strong "voltage OK".

How it works: the voltage divider trips the gates in sequence at 90,
130, 200, and 270 VAC, at the lower schmitt trip voltage (about 4
volts).

The even numbered gates (that trip at the overvoltage for each range)
turn off the respective "voltage ok" gate for that range.

The output diodes negative logic OR together the results of low v ok
and high v ok.

You can probably lower the power dissipation even more by upping the
value of the first 100K resistor until the 12V barely regulates at
80VAC.

.

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