Zener diode below-threshold reverse current

Started by June 23, 2016
```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.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

I'm looking for work -- see my website!
```
```On Fri, 24 Jun 2016 07:28:50 -0700, Joerg wrote:

> On 2016-06-23 15:42, 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.
>>
>>
> The datasheet is your friend:
>

<snip>

I guess my real question is how much can I trust data sheets on this one,
and what pitfalls (like temperature!) should I watch out for.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

I'm looking for work -- see my website!
```
```On 2016-06-24 10:42, Tim Wescott wrote:
> On Fri, 24 Jun 2016 07:28:50 -0700, Joerg wrote:
>
>> On 2016-06-23 15:42, 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.
>>>
>>>
>> The datasheet is your friend:
>>
>
> <snip>
>
> I guess my real question is how much can I trust data sheets on this one,
> and what pitfalls (like temperature!) should I watch out for.
>

Max is the worst case but that is for 25C in this example. For the whole
temperature range you usually have to write to the manufacturer. I had
to do this a lot over the decades. If this is for a product and a
critical parameter get it in writing, such as via email.

If you just have to maintain a low enough voltage below a certain input
voltage you could consider a circuit-side bleeder resistor to ground.

--
Regards, Joerg

http://www.analogconsultants.com/
```
```On Fri, 24 Jun 2016 12:41:49 -0500, Tim Wescott
<seemywebsite@myfooter.really> 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.

Regulator type?  Load under normal conditions?

...Jim Thompson
--
| James E.Thompson                                 |    mens     |
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    |
| San Tan Valley, AZ 85142   Skype: Contacts Only  |             |
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  |
| E-mail Icon at http://www.analog-innovations.com |    1962     |

I'm looking for work... see my website.
```
```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.

<http://www.analog-innovations.com/SED/RegulatorTurnOff4TimWescott.png>

...Jim Thompson
--
| James E.Thompson                                 |    mens     |
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    |
| San Tan Valley, AZ 85142   Skype: Contacts Only  |             |
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  |
| E-mail Icon at http://www.analog-innovations.com |    1962     |

I'm looking for work... see my website.
```
```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?

--

Rick C
```
```On 6/24/2016 1:13 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.
>
> After sorting on output (12V and adjustable), input over 25V, and cases
> with enough pins, there's 9 -- which in DigiKey parlance means 3.
>
> 3 is "a number", but it's not a large enough number for me.

You clearly didn't do the search right.  Digikey doesn't make it easy to
search for output voltage.  By selecting *only* 12 volts for the output
you eliminated some huge percentage of the potential units.
"Adjustable" is also much more exclusive than you need.  Try going to
the TI site which has (or at least had) much better search capabilities
allowing the input max and min as well as the output max and min to be
specified separately.  Or contact some FAEs and tell them what you are
looking for.  They often know their product line much better than the
search engine.

If you think there are only 3 linear regulators that suit your needs
then you clearly are not familiar with the market.

--

Rick C
```
```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?

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

I'm looking for work -- see my website!
```
```"Robert Baer" <robertbaer@localnet.com> wrote in message
>   Well, if a 6.8V zener does not meet your fancy, try a reverse-biased E-B
> transistor junction; somewhere around 8V with tens of nA leakage..

In other application (semi-precision voltage clamp), I've found that the
reverse leakage is not nearly as low as would be desirable.

I fixed that by inverting the transistors, which reduced leakage
considerably, while increasing dynamic range (limited by Vceo instead of
Vebo).

The extra bias current required to maintain the clamp 'reference' (due to
low hFE) wasn't a problem in that case.

Tim

--
Seven Transistor Labs, LLC
Electrical Engineering Consultation and Contract Design
Website: http://seventransistorlabs.com

```
```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.

--

Rick C
```