zener trouble

Low voltage zeners, below maybe 5 volts, are terrible. Their conduction knee is sloppy as hell, which your simulator probably doesn't model accurately. And your open bases are *very* sensitive to small leakages; open bases are considered bad form by most designers.

Add b-e resistors to divert some of the leakage current, and maybe go to bandgap pseudo-zeners (LM4040 types) which behave much better.

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

John

How low does the battery voltage have to go before the LEDs turn essentially off? I am suspicious that you may have the zeners backward.

That said, such low voltage zeners are not really switches that suddenly conduct when the voltage across them rises above the zener knee voltage, but are exponential devices, much like forward biased junctions. In fact, you may get a better knee out of a green LED, forward biased. To improve the switching effect, you could load (parallel) the base emitter junction with some resistance to divert zener current while keeping the base ot emitter voltage below about .5 volts up to the supply voltage that you want the LED to light.

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.

--
John Popelish

This brings up some interesting issues:

Why would anyone simulate a circuit this simple?

If something this simple doesn't simulate correctly, what hope is there for a serious circuit?

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

Why not look at a zener data sheet?

When you built it and it didn't work, why did you resort to a newsgroup for help? Why not measure and analyze the voltages and currents and figure out what's actually happening? The methodology here is: fiddle with the simulation until it works; then fiddle with the circuit until it works, or ask for help; no thinking required.

John

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.

(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.

This would probably be an improvement over the zener thing:

John

Don't be. If they didn't have to build it to demonstrate it in the lab they would have called it good after the simulation.

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You made some wrong assumptions about the zeners. They indeed need some current for the function you want them to fulfil but there is more. One thing you can do to get a better insight is making a real life I-V plot of (one of) the zeners you used. Pay special attention to the part below zener voltage. You will measure microamps but that's still current. Multiplied by beta (or Hfe) will give enough current through the LED to make it light. Keep in mind that although a general purpose LED likes a 20mA for optimal funtioning most of them start to light already at 1mA.

For a circuit like this I would use one zener or bandgap as a reference voltage. A comparator (an LM339 contains four of them) and a voltage divider. The reference voltage can be used for all three comparators. The voltage divider will set the voltage at which the comparator will switch.

Below I give an idea to make something like that using discretes. Beware it's just an idea.

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

| | | | +---->|--+ | | Si | .-. | .-. | | - | | | | \ Z | | '-' ^ '-' | | | +-------+--------+ created by Andy´s ASCII-Circuit v1.24.140803 Beta

petrus bitbyter

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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.

I resently had a tour of the EE department at Cornell University. I figure that computer screens outnumber oscilloscope screens by roughly

15:1.

John

the

current)

current

turn on

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.

--- I've read some of the replies to your original post, above, but your last post seems to show that you're still pretty much in the dark, so here goes:

If you hook up this very simple circuit:

+VIN>---+ | |K [Z4.3V] | +---->VOUT>---+ | | [R1] [VOLTMETER] | | GND>----+-------------+ You'll notice that if you start with +VIN at 0V and then make it more and more positive, the voltage across R1 will start to increase long before +VIN gets to 4.3V. The reason for that is because the Zener isn't a perfect switch and it will start allowing current to flow through itself before its Zener voltage is reached. Zeners are designed to be shunt regulators, and if you look at a data sheet for a Zener you'll find that the Zener voltage is only guaranteed to be within a certain range of voltages if the current through the Zener is the "test current", usually 20mA for 1/2 watt diodes with a Zener voltage of 12V or less.

Now, if we take a look at your circuit:

VIN>-----+-------------+ | | | [LED] [1K] | | [150R] | | | C +--[-----+-------------+

we can see that as VIN starts to go more and more positive, more and more current will start to flow through the base-to-emitter junction of the transistor, just because the Zener isn't a switch and will start to conduct well below its Zener voltage. If you have a transistor with a reasonably high beta (100 to 300) then the current which is flowing through the Zener (and also through the b-e junction)will cause a collector current to flow which will be 100 to

300 times higher than that. So, even if you're way below the Zener's knee and you have, say, 10µA of reverse current flowing, the collector current will be somewhere between 1mA and 3mA, which will be enough to light the LED through that 150 ohm resistor.

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?

-- John Fields

These work down to 2.7 volts:

--
John Popelish

Well. Here is a funny thing. I just simulated a circuit with a variable voltage source across a 1k resistor, connected to a 3.3V zener, and then to ground. The voltage source goes from 0 to 10V in 10 seconds. Oddly, the junction between the resistor and zener starts out at 2.3V, and goes up slowly from there. Thus, it appears that spice believes that the zener diode is a voltage source This happens for both circuitmaker and LTSpice. There is actually current flowing backwards through the resistor into the voltage source until the dc source gets to equal the 'voltage source' of the zener. I'd say that the spice implementation of zener diodes of low value is seriously flawed when used with a voltage lower than the actual zener voltage minimum.

Here is the spice netlist:

• node0--[V1=PWL]--node3--[R1=1k]--node4--[XD1=1N5226B]--node0--GND

* *Spice netlist for Circuit: C:\CM60S\Circuits\UNTITLED.CKT V1 3 0 DC 0 PWL( 0 0 10 10) XD1 0 4 X1N5226B R1 4 3 1k ..SAVE V(3) V(4) @v1[p] v1#branch @r1[p] @r1[i] *BKGND=RGB 0 0 0 *BINARY RAW FILE

• Selected Circuit Analyses : ..TRAN 20m 10 0 20m

• Models/Subcircuits Used:

*1N5226B 3.3V 500mW Si pkg:DIODE0.4 A,K ..SUBCKT X1N5226B 1 2 D1 1 2 DF DZ 3 1 DR VZ 2 3 2.536 ..MODEL DF D (IS=2.51N RS=84M N=1.7 CJO=182P VJ=0.75 M=0.33 TT=50.1N) ..MODEL DR D (IS=5.59M RS=8.4 N=15) ..ENDS X1N5226B ..END

--
Regards,
   Robert Monsen

"Your Highness, I have no need of this hypothesis."
     - Pierre Laplace (1749-1827), to Napoleon,
        on why his works on celestial mechanics make no mention of God.

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.

A base-emitter resistor will help a lot.

John

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

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

As I've pointed out elsewhere, Zener models appear to suck.

Here is a simple circuit that will do what you need:

Vin | | o------. | | | | e | .---b [1k] | c | | | | | | | {10k] [1k] | | | e | o-----b | | c zener | | | [10k] LED | | | '----o------' GND

When Vin is above the zener voltage + a bit, the left transistor will conduct. If its conducting, then the base of the right transistor will be higher than the emitter + 0.7, that transistor will be off, turning the LED off.

When the voltage drops below the zener voltage, the left PNP transitor will turn off, causing the right PNP transistor to turn 'sharply' on. As the voltage continues to decay, the LED will remain on, but will get dimmer.

--
Regards,
   Robert Monsen

"Your Highness, I have no need of this hypothesis."
     - Pierre Laplace (1749-1827), to Napoleon,
        on why his works on celestial mechanics make no mention of God.

That's even worse than the original circuit. More parts, too.

John