Using zener diode to protect test circuit?

[Jim Thompson]

On Mon, 25 Apr 2005 20:30:51 -0500, "Aubrey McIntosh, Ph.D."
<newsposter@spam.vima.austin.tx.us> wrote:

Jim Thompson wrote:

On Mon, 25 Apr 2005 17:31:45 -0400, "Jonathan Westhues"
please-see-website@nospam.com> wrote:


"Aubrey McIntosh, Ph.D." <newsposter@spam.vima.austin.tx.us> wrote in
message news:116qmgpp7l3hc0a@corp.supernews.com...

Why not use a zener instead of R1, R2, DZ1? The only thing I noticed
when simulating with LTSPICE was the power dissipation at R3 at 5V.

How would you connect it? If I understand correctly then DZ1 is not being
used as a shunt regulator. As soon as the voltage at the connection of R2
and R3 goes above Vref, the TL431 turns on all the way and Q1 snaps on. The
TL431 is either all the way on or all the way off, never in between.

That means that the overvoltage trigger point is insensitive to the Vbe of
Q1. It also means that M2 will be either on all the way or off all the way,
instead of turning off slowly as Vin goes above the limit.

Jonathan
http://cq.cx/



See Aubrey, Jonathan understands.

So much for the value of a PhD ;-)

...Jim Thompson


Yeah. I remember our discussion on low voltage zeners from last year
now. I agree with you, but I'll forget again.

Jonathan explains better too. I suspect the Engineer -- Scientist gap
dominates the EE -- Chemist gap on this one.

I started with a zener because I didn't know what the TL431 even was
this morning, and didn't have a model, and I wanted to poke around in
other parts of the circuit. Not good engineering, but fine for a rainy
morning's pasttime.

I placed http://aubrey.vima.austin.tx.us/crowbar.zip online. This zip
file has an LTSPICE model for the TL431 in it. Perhaps more
interestingly are the two pdf files, one for the circuit similar to
Jim's, and one with the pesky zener. The discrepancies in part numbers
are because I used what was at hand in LTSPICE.

What I noticed is the increase in power dissipated by the control
circuit at about 5.29 - 5.34V for the zener, against lower power
dissipation overall. I don't know if the model that I grabbed off the
net gives realistic power dissipation in the TL431 or not.

Now I'm off to play with Win's hex inverter circuit. I actually may
have parts for that, left over from the Goder booster.

Track back through the posts to where I added a comment about sizing
resistors when using the TL431... you can dramatically reduce the
wasted current with some value changes.

...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona Voice480)460-2350 | |
| E-mail Address at Website Fax480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |

I love to cook with wine. Sometimes I even put it in the food.

 

[John Woodgate]

I read in sci.electronics.design that Jim Thompson
<thegreatone@example.com> wrote (in
<urpn61l4bu57o6eoe627huk4tglsmjdpv7@4ax.com&gt about 'Active bridge
rectifier using cmos hex inverters, was Using zener diode to protect
test circuit?', on Sun, 24 Apr 2005:

On Sun, 24 Apr 2005 19:09:05 +0100, John Woodgate
jmw@jmwa.demon.contraspam.yuk> wrote:

I read in sci.electronics.design that Jim Thompson
thegreatone@example.com> wrote (in
fmgn61dcomrmmbi19iuu6r216cvr12kg9h@4ax.com&gt about 'Active bridge
rectifier using cmos hex inverters, was Using zener diode to protect
test circuit?', on Sun, 24 Apr 2005:

WARNING!!!! Norton AV found W32.Beagle.AG@mm virus in this message and
has removed it. Virus scanning of outgoing messages is in force here.


[snip]

What version of NAV are you using?

2004 V7.0 Build 81 Automatic updates enabled

If it was really there it got there from something in your machine or
news server.

That's rather what I suspected. It's not on my machine. But I thought it
best to send a warning, just in case.
--
Regards, John Woodgate, OOO - Own Opinions Only.
There are two sides to every question, except
'What is a Moebius strip?'
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk

 

[John Woodgate]

I read in sci.electronics.design that Jim Thompson
<thegreatone@example.com> wrote (in
<bbqn6198ql8a64ifpkfr15nhcmk6bol8po@4ax.com&gt about 'Active bridge
rectifier using cmos hex inverters, was Using zener diode to protect
test circuit?', on Sun, 24 Apr 2005:

John, I just realized that you referenced a text message. Where's the
virus ?

I couldn't see it as an attachment or anything, but then I don't know
whether a virus can now (e.g. as of today!) hide itself in some way. I
wasn't motivated to try too many experiments before I let Norton kill
it.
--
Regards, John Woodgate, OOO - Own Opinions Only.
There are two sides to every question, except
'What is a Moebius strip?'
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk