Forums

Zener diode below-threshold reverse current

Started by Tim Wescott June 23, 2016
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. >
Experience with our Codatron&reg; High Voltage Shunt Regulator indicates that the leakage should be in the tens of nanoamps at room temperature, even with a forward voltage slightly below the knee. We have not measured a 6.8V zener, but think that 100nA leakage near its knee is a reasonable expectation.
krw 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. 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). > >> There seems to be ones that advertise 100nA at a 5V drop -- that would be >> enough for me. > > Sounds pretty low for a 6.8V zener. At what temperature? Note that > the zener will have to dissipate some amount of power.
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..
rickman 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. >
Technically, the input IS the "enable"; 0V in for 0V out; min operating V in for rated V out (="enabled"). Just be picky....
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: http://www.onsemi.com/pub_link/Collateral/BZX84C2V4LT1-D.PDF For example, on page 3 a 6.8V zener is listed at a max reverse current of 2uA when at 4V. As John pointed out higher voltage zeners are better but you might not have much choice because of the drop-out voltage in your regulator. Of course, this also depends on what size zener you need. A big old fat one will have proportionately more leakage current. However, it's hard to find fat zeners that are stocked. So if you are dealing with lots of current I'd consider a transistor circuit instead. If this is for longer term production I would not rely on the continued availability of a larger size zener diode because that can turn into a boomerang. -- Regards, Joerg http://www.analogconsultants.com/
On Fri, 24 Jun 2016 07:28:50 -0700, Joerg <news@analogconsultants.com>
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: > >http://www.onsemi.com/pub_link/Collateral/BZX84C2V4LT1-D.PDF > >For example, on page 3 a 6.8V zener is listed at a max reverse current >of 2uA when at 4V. As John pointed out higher voltage zeners are better >but you might not have much choice because of the drop-out voltage in >your regulator. > >Of course, this also depends on what size zener you need. A big old fat >one will have proportionately more leakage current. However, it's hard >to find fat zeners that are stocked. So if you are dealing with lots of >current I'd consider a transistor circuit instead. If this is for longer >term production I would not rely on the continued availability of a >larger size zener diode because that can turn into a boomerang.
There are giant TVS zeners around. We use them with polyfuses to protect DC power inputs, from wall warts. I should measure leakage on some of them. -- John Larkin Highland Technology, Inc lunatic fringe electronics
On 6/24/2016 4:01 AM, Robert Baer wrote:
> rickman 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. >> > Technically, the input IS the "enable"; 0V in for 0V out; min > operating V in for rated V out (="enabled"). > Just be picky....
I wouldn't call it "picky"... -- Rick C
krw wrote...
> > 6.8V zeners suck.
How so, IMHO, they're close to optimum in many ways. -- Thanks, - Win
On 2016-06-24 08:18, John Larkin wrote:
> On Fri, 24 Jun 2016 07:28:50 -0700, Joerg <news@analogconsultants.com> > 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: >> >> http://www.onsemi.com/pub_link/Collateral/BZX84C2V4LT1-D.PDF >> >> For example, on page 3 a 6.8V zener is listed at a max reverse current >> of 2uA when at 4V. As John pointed out higher voltage zeners are better >> but you might not have much choice because of the drop-out voltage in >> your regulator. >> >> Of course, this also depends on what size zener you need. A big old fat >> one will have proportionately more leakage current. However, it's hard >> to find fat zeners that are stocked. So if you are dealing with lots of >> current I'd consider a transistor circuit instead. If this is for longer >> term production I would not rely on the continued availability of a >> larger size zener diode because that can turn into a boomerang. > > There are giant TVS zeners around. We use them with polyfuses to > protect DC power inputs, from wall warts. I should measure leakage on > some of them. >
They are mostly only spec'd at the rated standoff voltage or the unipolar ones at max allowed working reverse voltage. http://www.littelfuse.com/~/media/electronics/datasheets/tvs_diodes/littelfuse_tvs_diode_smaj_datasheet.pdf.pdf AFAIK engineers had to often stack them in telco apps to fulfill the leakage requirement. -- Regards, Joerg http://www.analogconsultants.com/
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 -- | James E.Thompson | mens | | Analog Innovations | et | | 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 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. -- Tim Wescott Wescott Design Services http://www.wescottdesign.com I'm looking for work -- see my website!