Reducing an Input Voltage

[Peter]

Newbie question. I'm running a small timer circuit (555, CD4017 and
some output Mosfets) from a 35v AC supply to sequence Christmas
lights. At present I reduce the input to 15V DC as follows:

35V AC (the Christmas lights input supply - typical Downunder outdoor
lights).
Bridge rectifier
330uF smoothing cap
2.2k 2W dropping resistor
15V 5W zener
100uF smoothing cap
555 and 4017 (small cap, high resistors on 555 to reduce current
drain).

All of the above values were chosen by trial and error, the main
criteria being
- it would work (it does)
- it would be as small as possible (eg. 330uF rather than 1000uF).

I saw another posting referring to the use of a voltage divider
resistor pair to reduce the voltage, with the zener in parallel with
the grounded resistor.

Would that allow a lower wattage zener to be used?
Would it allow a lower wattage dropping resistor(s) to be used?
Would it be a "better" design?
Any other suggestions on minor improvements? (Still keeping it
simple!)

 

[Baphomet]

"Peter" <pmolsen@one.net.au> wrote in message
news:6ba442b.0307271923.98bd84@posting.google.com...

I saw another posting referring to the use of a voltage divider
resistor pair to reduce the voltage, with the zener in parallel with
the grounded resistor.

Would that allow a lower wattage zener to be used?

YES (no yelling intended. Fred Abase got on my case for using HTML tags)

Would it allow a lower wattage dropping resistor(s) to be used?

NO

Would it be a "better" design?

DEPENDS upon what you mean by better. It would allow a lower wattage zener
to be used at the expense of the voltage range of regulation. You would
probably save a couple of cents on a lower wattage zener, or perhaps, you
have a 15v. zener lying around but it's stand alone wattage is insufficient
to the task, then using resistive shunting would be the way to go. If this
isn't a consideration, I would go with an unshunted higher wattage zener and
use one less resistor.

Any other suggestions on minor improvements? (Still keeping it
simple!)

 

[Bill Bowden]

pmolsen@one.net.au (Peter) wrote in message news:<6ba442b.0307271923.98bd84@posting.google.com>...

Newbie question. I'm running a small timer circuit (555, CD4017 and
some output Mosfets) from a 35v AC supply to sequence Christmas
lights. At present I reduce the input to 15V DC as follows:

35V AC (the Christmas lights input supply - typical Downunder outdoor
lights).
Bridge rectifier
330uF smoothing cap
2.2k 2W dropping resistor
15V 5W zener
100uF smoothing cap
555 and 4017 (small cap, high resistors on 555 to reduce current
drain).

All of the above values were chosen by trial and error, the main
criteria being
- it would work (it does)
- it would be as small as possible (eg. 330uF rather than 1000uF).

I saw another posting referring to the use of a voltage divider
resistor pair to reduce the voltage, with the zener in parallel with
the grounded resistor.

Would that allow a lower wattage zener to be used?
Would it allow a lower wattage dropping resistor(s) to be used?
Would it be a "better" design?
Any other suggestions on minor improvements? (Still keeping it
simple!)

The 2.2K resistor is only dissipating 0.56 watts, so you could
use a 1 watt size, and the zener worst case current is 16mA,
so you could use a 1/2 or 1 watt size.

Also, the 330uF cap can be much smaller, at 20uF for 6 volts
ripple, or 50uF for about 2 volts ripple. And the cap across
the zener could be smaller at maybe 1uF since all it does
is bypass the ICs.

But since you have it working, might as well leave it as is.


-Bill

 

[Rich Grise]

pmolsen@one.net.au (Peter) wrote in message
....

I saw another posting referring to the use of a voltage divider
resistor pair to reduce the voltage, with the zener in parallel with
the grounded resistor.

Would that allow a lower wattage zener to be used?
Would it allow a lower wattage dropping resistor(s) to be used?
Would it be a "better" design?
Any other suggestions on minor improvements? (Still keeping it
simple!)

It depends on what your intent is. If the goal is to light/
sequence some lights, then if it ain't broke, don't fix it. If
the point is to futz around with stuff to learn stuff, then
not only go for it and see what happens, but try an LM317 or
7815 or something as well.

Have Fun!
Rich