Re: voltage regulation through zener

BenFortener@xxxxxxxxx wrote:
(snip)

As far as I've learned, a zener will break down when it's in reverse
bias at some specified voltage (the one in pspice was determined to be
about 4.6 volts through the use of a simple DC power supply in series
with a zener and a load resistance).  Since the breakdown is so
dramatic, the current can increase a lot more with very little change
in voltage.

But there is certainly some voltage change with current increase. this is often modeled as a resistor in series with an ideal zener.

So, just for an example, this circuit:
http://pg.photos.yahoo.com/ph/benfortener/detail?.dir=1835&.dnm=16bd.jpg&.src=ph
should just limit the voltage at 4.6 volts (as long as the current
flowing through the zener is within specified power regulations), and
once the input voltage goes to more than 4.6 volts, it shouldn't
matter, and the voltage across the diode should still be about 4.6
volts.

This statement does not take the internal resistance of the zener into account.

It produces this output:
http://pg.photos.yahoo.com/ph/benfortener/detail?.dir=1835&.dnm=1651.jpg&.src=ph
The red is what I thought it should do and the black is what it does.

It looks about right to me. Check how high the current is going to drive those voltage peaks up to where they are. I think you will discover you are *way* overloading the zener at those peaks. And the voltage sags after the zener turns off each half cycle, because the capacitor is supplying current with no assistance from the transformer during those periods, so it must be discharging.

Any information anyone could offer to me on zener diodes or why this
isn't working would be greatly appreciated.


Zeners do a fair job regulating voltage only when their current stays fairly constant. Fairly constant means that the current never goes all the way to zero, and stays below the rated current for the device. If you don't know the rated current, divide the rated power by the zener voltage for a rough idea of what that is.

Now, to design a regulator with a zener, you need to have a supply that produces a voltage that is *always* higher than the zener voltage. This implys that you need to let the capacitor charge up as high as it can, without trying to clip the peaks with a zener. Then you connect a series resistor to the parallel combination of zener and load.

In this circuit, the series resistor and parallel combination of zener and load form a voltage divider with one leg (the zener) being a variable resistor whose value changes dramatically as voltage across it changes slightly.

The series resistor must be chosen such that at the peak capacitor voltage, the zener current (series resistor current minus load current) must not exceed its ratings, and at minimum capacitor voltage, the zener current must not go to zero. For best regulation, you may need to keep the zener minimum current well above zero, say, no less than 1/4 or 1/2 the peak current.

You should be able to design this by trial and error, by monitoring the zener current during a half cycle, and seeing if it is possible to pick the correct series resistor while keeping the zener current in bounds. Then you can go back and try variations in line voltage and see how it handles those cases. You may find that your zener needs to be a higher power rated device to do a reasonable job under all conditions. And you will find out that this type or regulator is very wasteful with power.

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