voltage regulation through zener

[[email protected]]

We're designing a DC power supply through the use of a transformer, a
rectifier and a capacitor. We've finished that, and would now like to
regulate the voltage to *almost* pure DC through the use of a zener
diode in parallel with the capacitor.

The problem is that in PSPICE, the software we're using to model the
circuit, the voltage waveforms we're getting from using a zener diode
don't look like they "should." I say they don't look like they
"should," because from what we've learned, the response doesn't make
sense.

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.

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.
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.

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

Thanks,
Ben

 

[Jim Thompson]

We're designing a DC power supply through the use of a transformer, a
rectifier and a capacitor. We've finished that, and would now like to
regulate the voltage to *almost* pure DC through the use of a zener
diode in parallel with the capacitor.

The problem is that in PSPICE, the software we're using to model the
circuit, the voltage waveforms we're getting from using a zener diode
don't look like they "should." I say they don't look like they
"should," because from what we've learned, the response doesn't make
sense.

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.

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.
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.

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

Thanks,
Ben

Take a "peek at the peak" current in the zener, then draw some
conclusions ;-)

...Jim Thompson

 

[kell]

regulate the voltage to *almost* pure DC through the use of a zener
diode in parallel with the capacitor.

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

Which is it, parallel or series? Obviously it's supposed to be
parallel.

http://pg.photos.yahoo.com/ph/benfortener/detail?.dir=1835&.dnm=1651.jpg&.src=ph

Go back and look at figure 2, labeled Voutput. That is the correct
graph you are looking for. Figure 3 looks like it's just current
through the zener or something.

A zener does work as a shunt regulator (staying within the power limit
of the zener). You are just getting graphs and/or terminology mixed up.

 

[martin griffith]

On 23 Oct 2005 13:06:53 -0700, in sci.electronics.design

We're designing a DC power supply through the use of a transformer, a
rectifier and a capacitor. We've finished that, and would now like to
regulate the voltage to *almost* pure DC through the use of a zener
diode in parallel with the capacitor.

The problem is that in PSPICE, the software we're using to model the
circuit, the voltage waveforms we're getting from using a zener diode
don't look like they "should." I say they don't look like they
"should," because from what we've learned, the response doesn't make
sense.

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.

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.
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.

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

Thanks,
Ben

You need to refresh your knowledge on zeners.
When its above the Z voltage, it is very low impedance. you need to
limit the current, otherwise its will look like a short to the
transformer
http://www.opamplabs.com/zenerreg.htm

your pic#1 is almost there, but you take the load from the top of the
zener, not from the unregulated supply.
http://www.st.com/stonline/books/pdf/docs/1707.pdf


martin

 

[John Popelish]

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.

 

[John Larkin]

Take a "peek at the peak" current in the zener, then draw some
conclusions ;-)

...Jim Thompson

Zener current will be zero, at least after the first second or so. The
1N750 will disappear in a smallish puff of smoke.

Didn't somebody make a version of Spice with little animated flames
appearing over abused parts?

John

 

[Jim Thompson]

Zener current will be zero, at least after the first second or so. The
1N750 will disappear in a smallish puff of smoke.

Didn't somebody make a version of Spice with little animated flames
appearing over abused parts?

John

You can pay extra money and get PSpice "smoke" detection.

Personally I haven't had a circuit design of mine "smoke" now for
perhaps 40 years.

But I do occasionally roll my own macro-based probes that test
sensitive small-feature-size CMOS for SOAR violations.

...Jim Thompson

 

[John Larkin]

You can pay extra money and get PSpice "smoke" detection.

Personally I haven't had a circuit design of mine "smoke" now for
perhaps 40 years.

But I do occasionally roll my own macro-based probes that test
sensitive small-feature-size CMOS for SOAR violations.

...Jim Thompson

Did you see the pic I posted a while back, of my blown fet collection?
We tested a bunch of TO-247's to destruction, while developing our
17KW NMR gradient driver. Only a few of the parts we tested actually
met their own specs.

That's KILOwatts, not MILLIwatts, for you IC types.

Spice should at least warn when any circuit voltage, current, or power
exceeds some threshold, 1000 maybe. Too many newbies design silly
things.

John

 

[martin griffith]

On Sun, 23 Oct 2005 15:44:21 -0700, in sci.electronics.design Jim

You can pay extra money and get PSpice "smoke" detection.

Personally I haven't had a circuit design of mine "smoke" now for
perhaps 40 years.

But I do occasionally roll my own macro-based probes that test
sensitive small-feature-size CMOS for SOAR violations.

...Jim Thompson

Hehe, hate to see what these guys would do with a soldering iron, and
real components....maybe a scope, even

These guys are our succesors.....( just like our parents used to say)


martin

 

[[email protected]]

Very good reply. Thank you for the in depth explanation about using a
zener in a regulated voltage supply.
Thanks to martin for those links, especially the pdf.

I think I definitely have a better understanding of it now, and I'll
probably finish the design tonight. I'll post again if I run into
problems, and I'll post the final circuit to with output.

Thanks to everyone for their replies; this is really going to help.

 

[[email protected]]

John Popelish: Your reply was the one I was citing in the previous
post.

The point of a class is to learn before you just go out and blow
something up. I don't have the 20+ years of experience yet. We just
learned what a diode was three weeks ago.

 

[Jim Thompson]

On Sun, 23 Oct 2005 15:44:21 -0700, in sci.electronics.design Jim

Hehe, hate to see what these guys would do with a soldering iron, and
real components....maybe a scope, even

These guys are our succesors.....( just like our parents used to say)


martin

SOAG(M,VDS,VGS)=(SGN(VD(M)-VS(M)-VDS)+1)*(SGN(VG(M)-VS(M)-VGS)+1)/4

...Jim Thompson

 

[Pooh Bear]

We're designing a DC power supply through the use of a transformer, a
rectifier and a capacitor. We've finished that, and would now like to
regulate the voltage to *almost* pure DC through the use of a zener
diode in parallel with the capacitor.

That's lousy practice, won't work very well and is hopelessly inefficient.

Since the breakdown is so
dramatic, the current can increase a lot more with very little change
in voltage.

Not really. The curve around breakdown is typically very sloppy IME.

Check out the 'dynmaic impedance' of zeners.

Graham

 

[John Popelish]

John Popelish: Your reply was the one I was citing in the previous
post.

(snip)

If you set your newsreader to post with quoted text, it will show who
said what (as mine did, above), who you are talking to and what you
are commenting on.

But delete everything but reminders of where the conversation is, to
keep things brief. Adding markers for your edits keeps you from being
accused of altering the meanings of someone else's words with your
clipping.

Picking the best newsgroup also increases the chance of a helpful
reply. This thread, for instance, was a better fit to
sci.electronics.basics (for beginners). You are not quite a designer,
yet. ;-)

 

[Terry Given]

Did you see the pic I posted a while back, of my blown fet collection?
We tested a bunch of TO-247's to destruction, while developing our
17KW NMR gradient driver. Only a few of the parts we tested actually
met their own specs.

That's KILOwatts, not MILLIwatts, for you IC types.

Spice should at least warn when any circuit voltage, current, or power
exceeds some threshold, 1000 maybe. Too many newbies design silly
things.

John

Hi John,

did you see the paper on TO-247 transient-related thermal failures in
one of the IEEE trans. recently? same deal as big die, the attachment
develops solder voids due to CTE mismatch and rapid dT/dt...

Cheers
Terry

 

[John Larkin]

Hi John,

did you see the paper on TO-247 transient-related thermal failures in
one of the IEEE trans. recently? same deal as big die, the attachment
develops solder voids due to CTE mismatch and rapid dT/dt...

Cheers
Terry

Interesting. We were pulsing 300 watt rated fets, bolted directly to a
machined-flat copper block, and most of them blew up in under 100
milliseconds at 300 watts.

John

 

[John Larkin]

SOAG(M,VDS,VGS)=(SGN(VD(M)-VS(M)-VDS)+1)*(SGN(VG(M)-VS(M)-VGS)+1)/4

...Jim Thompson

Where's t?

John

 

[Jim Thompson]

On Sun, 23 Oct 2005 16:21:08 -0700, Jim Thompson
[snip]

SOAG(M,VDS,VGS)=(SGN(VD(M)-VS(M)-VDS)+1)*(SGN(VG(M)-VS(M)-VGS)+1)/4

...Jim Thompson

Where's t?

John

This is the hot electron issue with very small feature size CMOS.
Foundry physicists tend to be bird brains ;-) They don't want the
device to enter the region AT ALL. Thus no "t" in the equation. This
macro spits out a "1" at such occurrences, making it easy to spot in
Probe.

A real-life BJT SOAR macro would have an integrator in there.

...Jim Thompson

 

[Jasen Betts]

http://pg.photos.yahoo.com/ph/benfortener/detail?.dir=1835&.dnm=16bd.jpg&.src=ph

that URL doesn't point to an image, links can't find an obvious image on
that page, mozilla unavailable due to a hardware fault during an upgrade,
so I gave up.

I'm guessing you should put the zener after the resevoir capacitor (with a
resistor between them) and don't draw so much current that the resistor drop
is greater than the difference betwen the capacitor voltage and what you
want.

Bye.
Jasen