Need a DC pulse from AC Voltage

I have a DC device that needs a pulse of 12 Volt DC voltage
(anywhere between 0.5 seconds to 10 seconds long).

The pulse requires a 2 milliampere current. The pulse needs
to be created by turning on a 120 V AC supply.

I want the cheapest possible method of getting such a low
current 12 V DC pulse.

So far the cheapest method I built is a Rectifier Diode in series
with three electrolytics like shown below:

(120 V AC hot)----->|--||----||--/--||--\-----(AC neutral);

Where the DC voltage across the last electrolytic at /--||--\ is
about 20 volts DC and the Capacitor is paralleled at the / \ points
across the last Capacitor by a resistor in series with a 12 Volt Zener Diode.

This creates the required length of 12 Volt DC pulse across the Zener Diode

The first two capacitors have very high resistance parallel bleeder resistors -- and the
device requiring the dc pulse has an impedance that drains the 12 Volts across the
Zener in the needed time for the pulse -- each time I turn on the AC Voltage.

This device costs about a dollar or more for parts. I want the DC supply to last
for years and I worry about the electrolytics failing over time and heat.

The question is:
As there is no concern about AC Voltage shock in the application and I want a small
device, "Is this the best and cheapest method and would a tiny 120 Volt AC step down
transformer -- or other method work better and last longer?

Found a 250 volt electrolytic capacitor to replace the left two caps -- so now it just uses
two caps, two diodes, one bleeder resistor, and one resistor in series with the zener diode.

This also works well -- and exactly the same with the 12 volts dc pulse across the zenar
diode supplying the short dc pulse when the 120 V ac is turned on. The cost is still low.

I still worry how many hours these 2 electrolytics will last at 85 to 90 degrees C?

It has 6 parts -- however, the two diodes and two resistors should last for years.

Why are you operating electronics in a 85 degrees C environment? Do you have an alternative location?

The diodes and resistors won't last very long either unless you've derated them accordingly.

Thanks, yes, it is quite warm where the circuitry is located and can not be moved. I suppose at 85 degrees C I will just have to use the special electrolytics and other components rated for high temp... unless there is a simpler method.

I once read that for every 10 degrees C the life of an electrolytic is cut in half.

Tantalum are great at high temperature. But not at those voltages unfortunately.

speakerguy79 said:

Tantalum are great at high temperature. But not at those voltages unfortunately.

Click to expand...

Thanks, I wish aluminum electrolytics held up better. I thought about putting a fuse in series with the ac just in case an electrolytic failed and the current went up. I wouldn't want one exploding. A low resistance low wattage resistor in series with the ac might act as a fuse but wonder about using that.

Maybe someone will think of a better way to make a 12 V dc low current pulse when 120 V ac is turned on.

Update:

Using two caps in series (one low mfd with a large dc drop and one high mfd with a low dc drop)...
I tried using the high voltage drop cap with too low a voltage rating. This caused the cap to fail.

When the high voltage drop cap failed the other cap failed and blew out fluid.

I am now using a 250 volt cap for high voltage drop and have a low ohm resistor in series to act like a fuse with no problem.

Exploding caps are no fun -- even with goggles -- and make a bad substitute for fireworks.