Need a DC pulse from AC Voltage

[al_alkan]

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?

 

[al_alkan]

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.

 

[mneary]

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.

 

[al_alkan]

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.

 

[Speakerguy]

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

 

[al_alkan]

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

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.

 

[al_alkan]

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.