# Zener diode below-threshold reverse current

Started by June 23, 2016
```On 24 Jun 2016 08:54:27 -0700, Winfield Hill
<hill@rowland.harvard.edu> wrote:

>krw wrote...
>>
>> 6.8V zeners suck.
>
> How so, IMHO, they're close to optimum in many ways.

Heat one up and see what the leakage does.
```
```On Fri, 24 Jun 2016 19:16:08 -0400, rickman <gnuarm@gmail.com> wrote:

>On 6/24/2016 6:33 PM, Tim Wescott wrote:
>> On Fri, 24 Jun 2016 17:46:05 -0400, rickman wrote:
>>
>>> On 6/24/2016 1:41 PM, Tim Wescott wrote:
>>>> On Fri, 24 Jun 2016 10:11:31 -0700, Jim Thompson wrote:
>>>>
>>>>> On Thu, 23 Jun 2016 21:07:54 -0500, Tim Wescott <tim@seemywebsite.com>
>>>>> wrote:
>>>>>
>>>>>> On Thu, 23 Jun 2016 16:20:27 -0700, Jim Thompson wrote:
>>>>>>
>>>>>>> On Thu, 23 Jun 2016 17:42:40 -0500, Tim Wescott
>>>>>>> <seemywebsite@myfooter.really> wrote:
>>>>>>>
>>>>>>>> I know the theory, more or less.
>>>>>>>>
>>>>>>>> In practice, what's the reverse current through a zener diode at
>>>>>>>> levels well below the breakdown voltage?  I've got a circuit where
>>>>>>>> I'm feeding a 12V linear regulator with a 25V line.  It would be
>>>>>>>> handy to put a zener diode in there so that when the 25V line drops
>>>>>>>> to 4V the current into the regulator drops to tens of microamps.
>>>>>>>> I'm wondering if a series zener will do it.
>>>>>>>
>>>>>>> Clear as mud ;-)  Post a schematic of what you mean... 25V -> 4V ??
>>>>>>>
>>>>>>>                                         ...Jim Thompson
>>>>>>
>>>>>> Sorry.  When the circuit is nominally on I want to make 12V from the
>>>>>> 25V rail with a linear regulator.  When the circuit is nominally off
>>>>>> the 25V rail is at 4V, and I would like to effectively shut off the
>>>>>> 12V line entirely.
>>>>>>
>>>>>> A 6.8V zener would make for around 18V at the input to the regulator,
>>>>>> so the "on" part works fine.  I'm just wondering if, with 4V on one
>>>>>> side and a regulator input on the other, if the zener will flow some
>>>>>> predictably low current.
>>>>>>
>>>>>> There seems to be ones that advertise 100nA at a 5V drop -- that
>>>>>> would be enough for me.
>>>>>
>>>>> How about a resistor from regulator input to ground?
>>>>>
>>>>>                                         ...Jim Thompson
>>>>
>>>> I'm trying to cut the current consumption from the regulator.  When the
>>>> line is at 25V it's powering a bunch of stuff.  "4V" is really just a
>>>> diode drop from a LiPo cell (yes, it's not really 4V) after a boost
>>>> converter has been shut down.  I'm looking at stuff I can easily turn
>>>> off.
>>>
>>> So what's wrong with turning off the regulator?  Regulators are often
>>> designed to be turned off for exactly this reason.  Why ignore all those
>>> devices and instead kludge something with a Zener?
>>
>> You mean all three of them that are listed in DigiKey?  Out of the
>> hundreds of 3-terminal ones available?
>
>Read my other post.  If you can't find more than 3 devices you don't
>understand how to find parts.

+1!

IME, there are more with enables than without.  Most don't have an
accurate threshold but many do, also.
```
```On Fri, 24 Jun 2016 00:52:39 -0400, rickman <gnuarm@gmail.com> wrote:

>On 6/23/2016 10:08 PM, Tim Wescott wrote:
>> On Thu, 23 Jun 2016 21:29:33 -0400, rickman wrote:
>>
>>> On 6/23/2016 6:42 PM, Tim Wescott wrote:
>>>> I know the theory, more or less.
>>>>
>>>> In practice, what's the reverse current through a zener diode at levels
>>>> well below the breakdown voltage?  I've got a circuit where I'm feeding
>>>> a 12V linear regulator with a 25V line.  It would be handy to put a
>>>> zener diode in there so that when the 25V line drops to 4V the current
>>>> into the regulator drops to tens of microamps.  I'm wondering if a
>>>> series zener will do it.
>>>
>>> There are a number of regulators which have a quiescent current in the
>>> 10's of microamps and an enable input.  Connect the enable input to the
>>> Vin with a voltage divider and the regulator will shut down with low
>>> input voltages dropping the input current to very low levels without a
>>> Zener diode.
>>
>> That number is hugely smaller than the number of 3-terminal regulators
>> out there, though.
>
>I think that is pretty irrelevant.  Digikey lists over 58 thousand
>linear voltage regulators.  How many do you think will meet the
>requirements?  I don't have any idea how it matters that there are many
>that don't fit the requirements.  The design only needs one.
>
>BTW, by definition there are NO 3 terminal regulators with an enable.

Switch ground? ;-)
```
```On Thursday, June 23, 2016 at 10:08:03 PM UTC-4, Tim Wescott wrote:
> On Thu, 23 Jun 2016 16:20:27 -0700, Jim Thompson wrote:
>
> > On Thu, 23 Jun 2016 17:42:40 -0500, Tim Wescott
> > <seemywebsite@myfooter.really> wrote:
> >
> >>I know the theory, more or less.
> >>
> >>In practice, what's the reverse current through a zener diode at levels
> >>well below the breakdown voltage?  I've got a circuit where I'm feeding
> >>a 12V linear regulator with a 25V line.  It would be handy to put a
> >>zener diode in there so that when the 25V line drops to 4V the current
> >>into the regulator drops to tens of microamps.  I'm wondering if a
> >>series zener will do it.
> >
> > Clear as mud ;-)  Post a schematic of what you mean... 25V -> 4V ??
> >
> >                                         ...Jim Thompson
>
> Sorry.  When the circuit is nominally on I want to make 12V from the 25V
> rail with a linear regulator.  When the circuit is nominally off the 25V
> rail is at 4V, and I would like to effectively shut off the 12V line
> entirely.
>
> A 6.8V zener would make for around 18V at the input to the regulator, so
> the "on" part works fine.  I'm just wondering if, with 4V on one side and
> a regulator input on the other, if the zener will flow some predictably
> low current.
>
> There seems to be ones that advertise 100nA at a 5V drop -- that would be
> enough for me.
>
> --
> Tim Wescott
> Control systems, embedded software and circuit design
> I'm looking for work!  See my website if you're interested
> http://www.wescottdesign.com

You could interpose a PFET between the Li BATT and the boost to kill the battery drain entirely. The low voltage hefty current types are numerous and cheap in quanitity. Fairchild FDS9934C or Vishay Si9939 or Si2329DS or ... gazillions in stock all over the place.
```
```In article <MYSdnS7qps99-PHKnZ2dnUU7-R2dnZ2d@giganews.com>,
Tim Wescott <seemywebsite@myfooter.really> wrote:

> I know the theory, more or less.
>
> In practice, what's the reverse current through a zener diode at levels
> well below the breakdown voltage?  I've got a circuit where I'm feeding a
> 12V linear regulator with a 25V line.  It would be handy to put a zener
> diode in there so that when the 25V line drops to 4V the current into the
> regulator drops to tens of microamps.  I'm wondering if a series zener
> will do it.

Zener diodes start looking a bit like resistors at the lower voltages.
High efficiency illumination LEDs are much better if the exact voltage
doesn't matter.

Recent regulators with UVLO and battery monitors will do this better
than anything else.  They have a low duty cycle polling interval for
UVLO so that the resistors setting the threshold don't waste power.  The
feature is primarily for keeping LiPo batteries safe during storage so
the drain is extremely low.

--
I will not see posts from astraweb, theremailer, dizum, or google
because they host Usenet flooders.
```
```On Thursday, June 23, 2016 at 3:42:49 PM UTC-7, Tim Wescott wrote:

> In practice, what's the reverse current through a zener diode at levels
> well below the breakdown voltage?  I've got a circuit where I'm feeding a
> 12V linear regulator with a 25V line...

If you might be taking 100 mA through the zener, it'll have to be
a 1W rated unit; power semiconductors are usually intended for
up to 150C temperatures, and won't have low leakage limits.

So, could you use a transistor follower and a low current zener, maybe
with a collector resistor to share the heat load?
7V Zener to base of 1A NPN pass transistor, The pulldown
on the base (to ground) will sink maybe 5 mA when powered (18V), and
at 0.4V (base turned off) would still handle 100 uA,    Small zeners
are often spec'ed at 100nA leakage.
```
```On Thu, 23 Jun 2016 22:49:35 -0400, krw <krw@nowhere.com> wrote:

>On Thu, 23 Jun 2016 21:07:54 -0500, Tim Wescott <tim@seemywebsite.com>
>wrote:
>
>>On Thu, 23 Jun 2016 16:20:27 -0700, Jim Thompson wrote:
>>
>>> On Thu, 23 Jun 2016 17:42:40 -0500, Tim Wescott
>>> <seemywebsite@myfooter.really> wrote:
>>>
>>>>I know the theory, more or less.
>>>>
>>>>In practice, what's the reverse current through a zener diode at levels
>>>>well below the breakdown voltage?  I've got a circuit where I'm feeding
>>>>a 12V linear regulator with a 25V line.  It would be handy to put a
>>>>zener diode in there so that when the 25V line drops to 4V the current
>>>>into the regulator drops to tens of microamps.  I'm wondering if a
>>>>series zener will do it.
>>>
>>> Clear as mud ;-)  Post a schematic of what you mean... 25V -> 4V ??
>>>
>>>                                         ...Jim Thompson
>>
>>Sorry.  When the circuit is nominally on I want to make 12V from the 25V
>>rail with a linear regulator.  When the circuit is nominally off the 25V
>>rail is at 4V, and I would like to effectively shut off the 12V line
>>entirely.
>>
>>A 6.8V zener would make for around 18V at the input to the regulator, so
>>the "on" part works fine.  I'm just wondering if, with 4V on one side and
>>a regulator input on the other, if the zener will flow some predictably
>>low current.
>
>6.8V zeners suck.  I'd just use an enable pin on the regulator.

And if the regulator of choice has no ENABLE pin, then what?

>Some have an accurate reference so you can program the dropout voltage with
>just a voltage divider.  When the input drops below that value, the
>regulator shuts off (see: UVLO - Under Voltage Lock Out).

---
Wescott was obviously referring to a three terminal series regulator,

As to your damnation of 6.8 volt Zeners, 6.8 volts is pretty close to
where, with the specified reverse current through the diode,  the
Zener's tempco goes away, so why would you think 6.8 volt Zeners suck?

John Fields

```
```On 7/10/2016 5:01 PM, John Fields wrote:
> On Thu, 23 Jun 2016 22:49:35 -0400, krw <krw@nowhere.com> wrote:
>
>> On Thu, 23 Jun 2016 21:07:54 -0500, Tim Wescott <tim@seemywebsite.com>
>> wrote:
>>
>>> On Thu, 23 Jun 2016 16:20:27 -0700, Jim Thompson wrote:
>>>
>>>> On Thu, 23 Jun 2016 17:42:40 -0500, Tim Wescott
>>>> <seemywebsite@myfooter.really> wrote:
>>>>
>>>>> I know the theory, more or less.
>>>>>
>>>>> In practice, what's the reverse current through a zener diode at levels
>>>>> well below the breakdown voltage?  I've got a circuit where I'm feeding
>>>>> a 12V linear regulator with a 25V line.  It would be handy to put a
>>>>> zener diode in there so that when the 25V line drops to 4V the current
>>>>> into the regulator drops to tens of microamps.  I'm wondering if a
>>>>> series zener will do it.
>>>>
>>>> Clear as mud ;-)  Post a schematic of what you mean... 25V -> 4V ??
>>>>
>>>>                                         ...Jim Thompson
>>>
>>> Sorry.  When the circuit is nominally on I want to make 12V from the 25V
>>> rail with a linear regulator.  When the circuit is nominally off the 25V
>>> rail is at 4V, and I would like to effectively shut off the 12V line
>>> entirely.
>>>
>>> A 6.8V zener would make for around 18V at the input to the regulator, so
>>> the "on" part works fine.  I'm just wondering if, with 4V on one side and
>>> a regulator input on the other, if the zener will flow some predictably
>>> low current.
>>
>> 6.8V zeners suck.  I'd just use an enable pin on the regulator.
>
> And if the regulator of choice has no ENABLE pin, then what?

Then "choise" another regulator...  There is no shortage of suitable
regulators in spite of the inability of some people to find them.

>> Some have an accurate reference so you can program the dropout voltage with
>> just a voltage divider.  When the input drops below that value, the
>> regulator shuts off (see: UVLO - Under Voltage Lock Out).
>
> ---
> Wescott was obviously referring to a three terminal series regulator,
> so your rsponse is irrelevant.
>
> As to your damnation of 6.8 volt Zeners, 6.8 volts is pretty close to
> where, with the specified reverse current through the diode,  the
> Zener's tempco goes away, so why would you think 6.8 volt Zeners suck?

If someone wants to limit themselves to unsuitable regulators, then it
can be hard to help them with electronics design.  Tim is willing to
consider regulators with an enable, but he can't see to get past the
limitations of the Digikey selection tool.  I recommend starting with
the TI tool on their web site.  It's easier to find a part on their site
that meets the requirements.

--

Rick C
```
```On Sun, 10 Jul 2016 16:01:50 -0500, John Fields
<jfields@austininstruments.com> wrote:

>On Thu, 23 Jun 2016 22:49:35 -0400, krw <krw@nowhere.com> wrote:
>
>>On Thu, 23 Jun 2016 21:07:54 -0500, Tim Wescott <tim@seemywebsite.com>
>>wrote:
>>
>>>On Thu, 23 Jun 2016 16:20:27 -0700, Jim Thompson wrote:
>>>
>>>> On Thu, 23 Jun 2016 17:42:40 -0500, Tim Wescott
>>>> <seemywebsite@myfooter.really> wrote:
>>>>
>>>>>I know the theory, more or less.
>>>>>
>>>>>In practice, what's the reverse current through a zener diode at levels
>>>>>well below the breakdown voltage?  I've got a circuit where I'm feeding
>>>>>a 12V linear regulator with a 25V line.  It would be handy to put a
>>>>>zener diode in there so that when the 25V line drops to 4V the current
>>>>>into the regulator drops to tens of microamps.  I'm wondering if a
>>>>>series zener will do it.
>>>>
>>>> Clear as mud ;-)  Post a schematic of what you mean... 25V -> 4V ??
>>>>
>>>>                                         ...Jim Thompson
>>>
>>>Sorry.  When the circuit is nominally on I want to make 12V from the 25V
>>>rail with a linear regulator.  When the circuit is nominally off the 25V
>>>rail is at 4V, and I would like to effectively shut off the 12V line
>>>entirely.
>>>
>>>A 6.8V zener would make for around 18V at the input to the regulator, so
>>>the "on" part works fine.  I'm just wondering if, with 4V on one side and
>>>a regulator input on the other, if the zener will flow some predictably
>>>low current.
>>
>>6.8V zeners suck.  I'd just use an enable pin on the regulator.
>
>And if the regulator of choice has no ENABLE pin, then what?
>
>>Some have an accurate reference so you can program the dropout voltage with
>>just a voltage divider.  When the input drops below that value, the
>>regulator shuts off (see: UVLO - Under Voltage Lock Out).
>
>---
>Wescott was obviously referring to a three terminal series regulator,
>
>As to your damnation of 6.8 volt Zeners, 6.8 volts is pretty close to
>where, with the specified reverse current through the diode,  the
>Zener's tempco goes away, so why would you think 6.8 volt Zeners suck?
>
>John Fields

Good grief, you only post here to be deliberately obnoxious. What a
sucky way to live.

--

John Larkin         Highland Technology, Inc

lunatic fringe electronics

```
```On Sun, 10 Jul 2016 16:01:50 -0500, John Fields
<jfields@austininstruments.com> wrote:

>On Thu, 23 Jun 2016 22:49:35 -0400, krw <krw@nowhere.com> wrote:
>
>>On Thu, 23 Jun 2016 21:07:54 -0500, Tim Wescott <tim@seemywebsite.com>
>>wrote:
>>
>>>On Thu, 23 Jun 2016 16:20:27 -0700, Jim Thompson wrote:
>>>
>>>> On Thu, 23 Jun 2016 17:42:40 -0500, Tim Wescott
>>>> <seemywebsite@myfooter.really> wrote:
>>>>
>>>>>I know the theory, more or less.
>>>>>
>>>>>In practice, what's the reverse current through a zener diode at levels
>>>>>well below the breakdown voltage?  I've got a circuit where I'm feeding
>>>>>a 12V linear regulator with a 25V line.  It would be handy to put a
>>>>>zener diode in there so that when the 25V line drops to 4V the current
>>>>>into the regulator drops to tens of microamps.  I'm wondering if a
>>>>>series zener will do it.
>>>>
>>>> Clear as mud ;-)  Post a schematic of what you mean... 25V -> 4V ??
>>>>
>>>>                                         ...Jim Thompson
>>>
>>>Sorry.  When the circuit is nominally on I want to make 12V from the 25V
>>>rail with a linear regulator.  When the circuit is nominally off the 25V
>>>rail is at 4V, and I would like to effectively shut off the 12V line
>>>entirely.
>>>
>>>A 6.8V zener would make for around 18V at the input to the regulator, so
>>>the "on" part works fine.  I'm just wondering if, with 4V on one side and
>>>a regulator input on the other, if the zener will flow some predictably
>>>low current.
>>
>>6.8V zeners suck.  I'd just use an enable pin on the regulator.
>
>And if the regulator of choice has no ENABLE pin, then what?

Then choose another, asshole.

When you want to discuss electronics, let us all know.

```