zener trouble

[Erik Durling]

We're a couple of students doing an electronics project. We are to
construct a battery indicator circuit. We had a finished circuit on
paper and we had simulated it with Multisim 7. But when we tested the
circuit in real life, it didn't work at all like we had planned (or
according to simulation). The circuit was based upon the idea of
zenerdiodes having a constant voltage drop, and not conducting if the
circuit isn't able to supply the needed voltage across it. But the
zener-diodes doesn't stop conducting at all, and the voltage drop is
far from constant.

This is the main circuit: http://ersker.com/circuit.gif

The zener diodes used in real life are: BZX55 and not BZC55, but that
ought not change anything (right?).

We did a DC Transfer Analysis in the simulator and got three nice
curves showing the voltage at the three collectors (compared to
ground) as the voltage from the source goes from 0-4.5 volts, and
everything seems right there: http://ersker.com/dctrans.gif

So, the problem we're having with our real circuit is that the LEDs
never turn off (the transistors are always saturated). The zener
diodes doesn't stop conducting when the voltage across them goes under
their specified value.

What's our mistake/misconception?

Do the zeners only operate correctly at a certain current?

 

[Roger Johansson]

egd.bydalen@swipnet.se (Erik Durling) wrote:

We're a couple of students doing an electronics project. We are to
construct a battery indicator circuit. We had a finished circuit on
paper and we had simulated it with Multisim 7. But when we tested the
circuit in real life, it didn't work at all like we had planned (or
according to simulation). The circuit was based upon the idea of
zenerdiodes having a constant voltage drop, and not conducting if the
circuit isn't able to supply the needed voltage across it. But the
zener-diodes doesn't stop conducting at all, and the voltage drop is
far from constant.

Zeners conduct at all voltages, but the current is very small when the
voltage is under the zener voltage.
(It is actually an exponential relationship between voltage and current)

With your circuit the hFE of the transistors amplify the base current
very much, so the little current through the zeners is enough to turn on
the LEDs.

This is the main circuit: http://ersker.com/circuit.gif

You should design a better circuit.
The one you have is based on faulty assumptions.

(The reason why it worked in multisim is probably that is uses simplified
simulation which does not work in real life when you create a design
based on using components in a non-regular way.)

Zeners are made to provide a fairly constant voltage, not current.
Do not use zeners to control current.


--
Roger J.

 

[Fred Bartoli]

"John Larkin" <jjlarkin@highlandSNIPtechTHISnologyPLEASE.com> a écrit dans
le message de news:nn32o0t5skfdo7ao712vhlh894i0vmklne@4ax.com...

On 28 Oct 2004 06:03:21 -0700, egd.bydalen@swipnet.se (Erik Durling)
wrote:


But I'm impressed that you're actually verifying your simulations by
experiment; some people just quit when the sim works.

This is the better case.
Some other people run into prod when the sim works.

Anyone having some nice story about direct run from sim to prod ?


On the contrary, I once did the expertise of a GTO firing hybrid that had
probably been designed with the pencil/paper method and not enough tested.
Few of them had the interesting feature of being permanently on at elevated
temperature.
As those were for railways traction, I've been nice fireworks stories.
2 hours simulation work sorted the pb out. Then 2 more weeks to convince the
customers staff.


--
Thanks,
Fred.

 

[John Larkin]

On Thu, 28 Oct 2004 21:31:06 GMT, "petrus bitbyter"
<p.kralt@reducespamforchello.nl> wrote:


This would probably be an improvement over the zener thing:

+-------+--------+
| |
.-. .-.
| | | |
| | | |
'-' '-'
| |
| |
| V LED
| -
| |
| |
| |/
+--------------| Si
| |
| |
| |
| |
.-. |
| | |
| | |
'-' |
| |
+-------+--------+

John

 

[Erik Durling]

no-email@home.se (Roger Johansson) wrote:

Zeners conduct at all voltages, but the current is very small when
the
voltage is under the zener voltage.
(It is actually an exponential relationship between voltage and
current)

With your circuit the hFE of the transistors amplify the base
current
very much, so the little current through the zeners is enough to
turn on
the LEDs.

You should design a better circuit.
The one you have is based on faulty assumptions.

Zeners are made to provide a fairly constant voltage, not current.
Do not use zeners to control current.

I think I explained the purpose of the zener improperly. We have it
there to increase the potential needed to saturate the transistor so
it needs a smaller change in the source to turn off the LED.
But the "leakage" was still the big problem eitherway.

---
jjlarkin@highlandSNIPtechTHISnologyPLEASE.com (John Larkin) wrote:

Low voltage zeners, below maybe 5 volts, are terrible. Their
conduction knee is sloppy as hell...

Would it work better by connecting ordinary diodes in series?

And your open bases are *very* sensitive to small leakages;
open bases are considered bad form by most designers.

What's meant by "open bases"?

Add b-e resistors to divert some of the leakage current

Thanks! That made a huge difference.

---
jpopelish@rica.net (John Popelish) wrote:

How low does the battery voltage have to go before the LEDs turn
essentially off?

The fresh-voltage is about 4.5, then one LED turn off every 0.5 volt.
Though we have potentiometers connected between the two resistors in
the picture (and the values differ a little), but I left that out on
purpose, those values aren't that important for the "problem".

I am suspicious that you may have the zeners backward.

Isn't that the point with zeners (that they're backwards)?

But using a comparator (comparing your divider voltage to a fixed
reference instead of a transistor sensing zener current) would give
a
lot cleaner threshold.

That was what we planned to use at first, but they themselves needed a
higher voltage than the battery was able to provide (less than 3 volts
at minimum).

---
jjlarkin@highlandSNIPtechTHISnologyPLEASE.com (John Larkin) wrote:

Why would anyone simulate a circuit this simple?

Cause we wanted to know what components we needed before we spent our
money.

Did your instructor deliberately give you a circuit that would
simulate correctly but not work in real life? If you're very lucky,
yes.

Actually, our instructor doesn't know shit about electronics (he
thinks he does though).

Why not look at a zener data sheet?

We trusted in the simulator (won't do that again =) )

When you built it and it didn't work, why did you resort to a
newsgroup for help?

Cause we didn't have access to anyone competent to help us.

---

Thank you for making things clearer to a bunch of confused students!

 

[Erik Durling]

John Fields <jfields@austininstruments.com> wrote in message news:<oog5o015ou8nog98m14fh8d1bj7qmta9db@4ax.com>...

If what you're trying to do is build something with LEDs which light
sequentially as the supply voltage increases, then you need to use
three comparators, a voltage reference, three LEDs and a handful of
resistors.

Want a schematic?

We understand this would be easier to build with comparators and
voltage reference components, but we really want to build it using
only discreete components.

 

[john jardine]

"Erik Durling" <egd.bydalen@swipnet.se> wrote in message
news:32840f19.0410300123.67940f6b@posting.google.com...

John Fields <jfields@austininstruments.com> wrote in message
news:<oog5o015ou8nog98m14fh8d1bj7qmta9db@4ax.com>...

If what you're trying to do is build something with LEDs which light
sequentially as the supply voltage increases, then you need to use
three comparators, a voltage reference, three LEDs and a handful of
resistors.

Want a schematic?

We understand this would be easier to build with comparators and
voltage reference components, but we really want to build it using
only discreete components.

Something like this has quite a clean switch-on action when the battery
drops below 3.0V.
Tr1 compares the fixed 2.5V supply with a divided down version of the
battery voltage and starts turning on when its base voltage goes below 1.9V.
The "stable 2.5V" can come from a zener or a reference.
(the 1k and 10k at Tr2 are there just to kill any leakage currents)

Battery Volts
IN
| Stable 2.5V
| ,---------,
.-. | |
| | | .-.
5k6| | | | |
'-' | | |47
| |< Tr1 '-'
+---------| BC556 |
| |\ |
.-. | V Led
| | .-. -
10k| | | | |
'-' | | |
| '-'1k |/ Tr2
| +-------| BC546
| .-. |>
| | | |
| | | |
| '-'10k |
| | |
=== === ===
GND GND GND

created by Andy´s ASCII-Circuit v1.24.140803 Beta www.tech-chat.de

 

[Bill Bowden]

egd.bydalen@swipnet.se (Erik Durling) wrote in message news:<32840f19.0410280503.4de75f7b@posting.google.com>...

We're a couple of students doing an electronics project. We are to
construct a battery indicator circuit. We had a finished circuit on
paper and we had simulated it with Multisim 7. But when we tested the
circuit in real life, it didn't work at all like we had planned (or
according to simulation). The circuit was based upon the idea of
zenerdiodes having a constant voltage drop, and not conducting if the
circuit isn't able to supply the needed voltage across it. But the
zener-diodes doesn't stop conducting at all, and the voltage drop is
far from constant.

This is the main circuit: http://ersker.com/circuit.gif

The zener diodes used in real life are: BZX55 and not BZC55, but that
ought not change anything (right?).

We did a DC Transfer Analysis in the simulator and got three nice
curves showing the voltage at the three collectors (compared to
ground) as the voltage from the source goes from 0-4.5 volts, and
everything seems right there: http://ersker.com/dctrans.gif

So, the problem we're having with our real circuit is that the LEDs
never turn off (the transistors are always saturated). The zener
diodes doesn't stop conducting when the voltage across them goes under
their specified value.

What's our mistake/misconception?

Do the zeners only operate correctly at a certain current?

Maybe you can just put the LEDs in the emitter side of the
transistor, so they light at some base voltage set by a
couple resistors? No zener needed since the LEDs light
at around 2 volts. Something like this:

+ Battery +
| |
| \
| / R3
\ \
R1 / |
\ C
| |/
+-------B| NPN
| |\
\ E
R2 / |
\ LED
| |
GND GND


-Bill