# Zener kinda turns on

Started by April 17, 2013
```I was making a circuit that when you power it up with 5V a 5V LED indicator=
turns on and if you power it up with 12V the 5V LED goes off and the 12V i=
ndicator goes on.

But... it's not coming on with 5V...=20

I spiced it, and it doesn't work in simulation either...

if you look at the image on the link

I put a 5.6V zener D1 in the circuit thinking that this Zener would be OFF =
until I provide it a voltage drop of 5.6V, and since it's in series with so=
me other stuff (resistors and an LED) I thought it would definitely be off =
at 5V, but it's on just enough to turn on my FET M1 and not let my D4 LED l=
ight up... I'm basically measuring a little over 1V at the gate.

I think I can remedy this problem by changing R2 to a lower value... like 1=
k maybe, but why is D1 turning on at all?  Is it leakage current that's cau=
sing this circuit not to work at 5V?  D3 the 12V LED is definitely off at a=
5V supply (as it should be) and the circuit works at 12V... but not at 5V.=
..

http://vampiresquidlabs.com/cms/temp/

much thanks!
```
```On Apr 17, 11:42=A0am, panfilero <panfil...@gmail.com> wrote:
> I was making a circuit that when you power it up with 5V a 5V LED indicat=
or turns on and if you power it up with 12V the 5V LED goes off and the 12V=
indicator goes on.
>
> But... it's not coming on with 5V...
>
> I spiced it, and it doesn't work in simulation either...
>
> if you look at the image on the link
>
> I put a 5.6V zener D1 in the circuit thinking that this Zener would be OF=
F until I provide it a voltage drop of 5.6V, and since it's in series with =
some other stuff (resistors and an LED) I thought it would definitely be of=
f at 5V, but it's on just enough to turn on my FET M1 and not let my D4 LED=
light up... I'm basically measuring a little over 1V at the gate.
>
> I think I can remedy this problem by changing R2 to a lower value... like=
1k maybe, but why is D1 turning on at all? =A0Is it leakage current that's=
causing this circuit not to work at 5V? =A0D3 the 12V LED is definitely of=
f at a 5V supply (as it should be) and the circuit works at 12V... but not =
at 5V...
>
>
> http://vampiresquidlabs.com/cms/temp/
>
> much thanks!

That 12V zener is not doing very much...

But look at an I-V curve for the 5.6V zener, I'd guess there's still
some current flow at 5.0V.  If the FET gate is at 1 V then you have
~1uA of current flowing through the zener.

George H.
```
```On a sunny day (Wed, 17 Apr 2013 08:42:50 -0700 (PDT)) it happened panfilero
<panfilero@gmail.com> wrote in

>I was making a circuit that when you power it up with 5V a 5V LED indicator=
> turns on and if you power it up with 12V the 5V LED goes off and the 12V i=
>ndicator goes on.

0 to +12 V
+----------------------------------------
|         |                  |          |
|        ---                ---         |
|        \ ///              \ ///      [ ] 10 K
|        --- LED1           --- LED2    | R3
[ ] 1k     |                  |          |
| R1      c                  c          |
|------b  NPN              NPN  b-------|
|         e------------------e          |
---        Q1       |        Q2          |
/                  |                    |
/ \ 5.6V           [ ] 470              [ ] 10K
---  zener          |  R2                | R4
|                 ///                  ///
///

Differential?

At <= 5V Q1 will be on, and LED1 is powered via Q1 and R2,
wiot hR1 providing base current to Q1.
The base of Q2 will always be 1/2 the supply.
As soon as the zener conducts the voltage at the base of Q1 can no longer rise,
and Q2 will take over and light LED2 via Q2 R2.
The ratio R3 R4 sets the cross over point?
R1 also limits the zener current.,
Just estimated values...
Transistors can be super cheap small NPN.
Not tested, but looks cleaner.

>
>http://vampiresquidlabs.com/cms/temp/
>
>
>much thanks!

```
```On Wed, 17 Apr 2013 08:42:50 -0700 (PDT), panfilero
<panfilero@gmail.com> wrote:

>I was making a circuit that when you power it up with 5V a 5V LED indicator turns on and if you power it up with 12V the 5V LED goes off and the 12V indicator goes on.
>
>But... it's not coming on with 5V...
>
>I spiced it, and it doesn't work in simulation either...
>
>if you look at the image on the link
>
>I put a 5.6V zener D1 in the circuit thinking that this Zener would be OFF until I provide it a voltage drop of 5.6V, and since it's in series with some other stuff (resistors and an LED) I thought it would definitely be off at 5V, but it's on just enough to turn on my FET M1 and not let my D4 LED light up... I'm basically measuring a little over 1V at the gate.
>
>I think I can remedy this problem by changing R2 to a lower value... like 1k maybe, but why is D1 turning on at all?  Is it leakage current that's causing this circuit not to work at 5V?  D3 the 12V LED is definitely off at a 5V supply (as it should be) and the circuit works at 12V... but not at 5V...
>
>
>http://vampiresquidlabs.com/cms/temp/
>
>
>much thanks!

You need a lower R2 or a different circuit. Low voltage zeners don't
have super sharp knees.

--

John Larkin         Highland Technology, Inc

jlarkin at highlandtechnology dot com
http://www.highlandtechnology.com

Precision electronic instrumentation
Picosecond-resolution Digital Delay and Pulse generators
Custom laser drivers and controllers
Photonics and fiberoptic TTL data links
VME thermocouple, LVDT, synchro   acquisition and simulation
```
```panfilero wrote:
> I was making a circuit that when you power it up with 5V a 5V LED indicator turns on and if you power it up with 12V the 5V LED goes off and the 12V indicator goes on.
>
> But... it's not coming on with 5V...
>
> I spiced it, and it doesn't work in simulation either...
>
> if you look at the image on the link
>
> I put a 5.6V zener D1 in the circuit thinking that this Zener would be OFF until I provide it a voltage drop of 5.6V, and since it's in series with some other stuff (resistors and an LED) I thought it would definitely be off at 5V, but it's on just enough to turn on my FET M1 and not let my D4 LED light up... I'm basically measuring a little over 1V at the gate.
>
> I think I can remedy this problem by changing R2 to a lower value... like 1k maybe, but why is D1 turning on at all?  Is it leakage current that's causing this circuit not to work at 5V?  D3 the 12V LED is definitely off at a 5V supply (as it should be) and the circuit works at 12V... but not at 5V...
>
>
> http://vampiresquidlabs.com/cms/temp/
>
>
> much thanks!
Err...it takes about 4V to turn on a "typical" LED, so use of a 5V
(or even 3V) zener will guarantee no light from the LED.

```
```"Robert Baer"

>  Err...it takes about 4V to turn on a "typical" LED, so use of a 5V (or
> even 3V) zener will guarantee no light from the LED.

**  D4 is the 5V LED,  D3 is the 12V LED.

The dopey OP has failed to allow for leakage current in the 5.6 V zener and
is trolling us as usual.

...  Phil

```
```On Wed, 17 Apr 2013 08:42:50 -0700, panfilero wrote:

> I was making a circuit that when you power it up with 5V a 5V LED
> indicator turns on and if you power it up with 12V the 5V LED goes off
> and the 12V indicator goes on.
>
> But... it's not coming on with 5V...
>
> I spiced it, and it doesn't work in simulation either...
>
> if you look at the image on the link
>
> I put a 5.6V zener D1 in the circuit thinking that this Zener would be
> OFF until I provide it a voltage drop of 5.6V, and since it's in series
> with some other stuff (resistors and an LED) I thought it would
> definitely be off at 5V, but it's on just enough to turn on my FET M1
> and not let my D4 LED light up... I'm basically measuring a little over
> 1V at the gate.
>
> I think I can remedy this problem by changing R2 to a lower value...
> like 1k maybe, but why is D1 turning on at all?  Is it leakage current
> that's causing this circuit not to work at 5V?  D3 the 12V LED is
> definitely off at a 5V supply (as it should be) and the circuit works at
> 12V... but not at 5V...
>
>
> http://vampiresquidlabs.com/cms/temp/

Call it leakage or what you will -- zeners do not have sharp corners in
their I-V characteristic.

_Always_ study the data sheet of the part you're going to use.  In the
case of two-terminal parts, _always_ know what the I-V characteristic
is.  It's _never_ as simple as the basic books would have you believe.

--
My liberal friends think I'm a conservative kook.
My conservative friends think I'm a liberal kook.
Why am I not happy that they have found common ground?

Tim Wescott, Communications, Control, Circuits & Software
http://www.wescottdesign.com
```
```On Wed, 17 Apr 2013 08:42:50 -0700 (PDT), panfilero
<panfilero@gmail.com> wrote:

>I was making a circuit that when you power it up with 5V a 5V LED indicator turns on and if you power it up with 12V the 5V LED goes off and the 12V indicator goes on.
>
>But... it's not coming on with 5V...
>
>I spiced it, and it doesn't work in simulation either...
>
>if you look at the image on the link
>
>I put a 5.6V zener D1 in the circuit thinking that this Zener would be OFF until I provide it a voltage drop of 5.6V, and since it's in series with some other stuff (resistors and an LED) I thought it would definitely be off at 5V, but it's on just enough to turn on my FET M1 and not let my D4 LED light up... I'm basically measuring a little over 1V at the gate.
>
>I think I can remedy this problem by changing R2 to a lower value... like 1k maybe, but why is D1 turning on at all?  Is it leakage current that's causing this circuit not to work at 5V?  D3 the 12V LED is definitely off at a 5V supply (as it should be) and the circuit works at 12V... but not at 5V...
>
>
>http://vampiresquidlabs.com/cms/temp/
>
>
>much thanks!

A wee bit overkill...

http://www.analog-innovations.com/SED/Panfilero_LED_5V_12V_Question_SED.pdf

but Perfectionist 'R' Us  >:-}

...Jim Thompson
--
| James E.Thompson                                 |    mens     |
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    |
| Phoenix, Arizona  85048    Skype: Contacts Only  |             |
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  |
| E-mail Icon at http://www.analog-innovations.com |    1962     |

I love to cook with wine.     Sometimes I even put it in the food.
```
```On Thu, 18 Apr 2013 11:55:54 -0700, Jim Thompson
<To-Email-Use-The-Envelope-Icon@On-My-Web-Site.com> wrote:

>On Wed, 17 Apr 2013 08:42:50 -0700 (PDT), panfilero
><panfilero@gmail.com> wrote:
>
>>I was making a circuit that when you power it up with 5V a 5V LED indicator turns on and if you power it up with 12V the 5V LED goes off and the 12V indicator goes on.
>>
>>But... it's not coming on with 5V...
>>
>>I spiced it, and it doesn't work in simulation either...
>>
>>if you look at the image on the link
>>
>>I put a 5.6V zener D1 in the circuit thinking that this Zener would be OFF until I provide it a voltage drop of 5.6V, and since it's in series with some other stuff (resistors and an LED) I thought it would definitely be off at 5V, but it's on just enough to turn on my FET M1 and not let my D4 LED light up... I'm basically measuring a little over 1V at the gate.
>>
>>I think I can remedy this problem by changing R2 to a lower value... like 1k maybe, but why is D1 turning on at all?  Is it leakage current that's causing this circuit not to work at 5V?  D3 the 12V LED is definitely off at a 5V supply (as it should be) and the circuit works at 12V... but not at 5V...
>>
>>
>>http://vampiresquidlabs.com/cms/temp/
>>
>>
>>much thanks!
>
>A wee bit overkill...
>
>http://www.analog-innovations.com/SED/Panfilero_LED_5V_12V_Question_SED.pdf
>
>but Perfectionist 'R' Us  >:-}
>
>                                        ...Jim Thompson

That is, as they say, "component rich."

This should work:

https://dl.dropboxusercontent.com/u/53724080/SED/LEDs_5_12.JPG

It could have a bandgap in the "13K" leg, but it really doesn't need
it.

--

John Larkin         Highland Technology, Inc

jlarkin at highlandtechnology dot com
http://www.highlandtechnology.com

Precision electronic instrumentation
Picosecond-resolution Digital Delay and Pulse generators
Custom laser drivers and controllers
Photonics and fiberoptic TTL data links
VME thermocouple, LVDT, synchro   acquisition and simulation
```
```On 4/18/2013 11:55 AM, Jim Thompson wrote:
> On Wed, 17 Apr 2013 08:42:50 -0700 (PDT), panfilero
> <panfilero@gmail.com>  wrote:
>
>> I was making a circuit that when you power it up with 5V a 5V LED indicator turns on and if you power it up with 12V the 5V LED goes off and the 12V indicator goes on.
>>
>> But... it's not coming on with 5V...
>>
>> I spiced it, and it doesn't work in simulation either...
>>
>> if you look at the image on the link
>>
>> I put a 5.6V zener D1 in the circuit thinking that this Zener would be OFF until I provide it a voltage drop of 5.6V, and since it's in series with some other stuff (resistors and an LED) I thought it would definitely be off at 5V, but it's on just enough to turn on my FET M1 and not let my D4 LED light up... I'm basically measuring a little over 1V at the gate.
>>
>> I think I can remedy this problem by changing R2 to a lower value... like 1k maybe, but why is D1 turning on at all?  Is it leakage current that's causing this circuit not to work at 5V?  D3 the 12V LED is definitely off at a 5V supply (as it should be) and the circuit works at 12V... but not at 5V...
>>
>>
>> http://vampiresquidlabs.com/cms/temp/
>>
>>
>> much thanks!
>
> A wee bit overkill...
>
> http://www.analog-innovations.com/SED/Panfilero_LED_5V_12V_Question_SED.pdf
>
> but Perfectionist 'R' Us>:-}
>
>                                          ...Jim Thompson

Here' in the trailer park, we'd have just painted the 5V power plug red
and the 12V one green.

Over in town, they'd have used a dual-color LED, put a zener in series
with the red one and called it green and orange.

Or use rectangular LEDS and call it a bar graph.  Probably works better
for the color-blind.

I've hosted a bunch of design reviews.
Seen a lot of "solutions" that were WAY out of proportion to the root