Forums

Zener diode below-threshold reverse current

Started by Tim Wescott June 23, 2016
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.

-- 

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

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.
Clear as mud ;-) Post a schematic of what you mean... 25V -> 4V ?? ...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 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.
I don't understand your numbers. Higher voltage zeners, like 8 volts and up, have very low leakage just below the rated voltage. A 1w 12 volt zener might hit 1 uA at 11.5 volts. Low voltage zeners are awful. Test some! Or consider using a mosfet. -- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
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.
Disable the regulator when the input voltage drops below some threshold? Regulator with a UVLO? A regulator that drops out instead of saturating? BTW, why a linear regulator? With any current, you're dissipating a lot of power (in both the zener and regulator).
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. -- Rick C
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
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. -- 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
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.
"Tim Wescott" <tim@seemywebsite.com> wrote in message 
news:daKdnf8UoYG2C_HKnZ2dnUU7-eudnZ2d@giganews.com...
>> 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 don't know about that. Tons of LDOs have enables. Enable, AND >25V rating, is probably on the rarer side though. I know! Drop the Vin with a zener so you can use a modest rated (20V?) LDO! ;-) As for zeners, yeah, a >5V zener will do what you want. On a related subject: there's a good reason why you don't see TVS diodes for very low voltages (under 5V). Zener devices cannot be made with a usefully steep curve. They "leak" like a sieve, and under surge conditions, the peak clamping voltage isn't much less than a regular 6.2V zener diode's (which is mainly what an SMAJ5.0A is, give or take the qualifications for surge duty instead of voltage regulation). You can only get a steep, low voltage knee, using active circuitry (think TL431). But you can't handle huge pulses with small, cheap and fast parts: the circuitry incurs delay, and you end up using a power device in its linear range -- there's no current-multiplying breakdown effect available. The traditional TL431 + thyristor crowbar is the closest, but is only good for surges (slow enough to deal with) on self-clearing circuits (since the thyristor stays latched on otherwise). So consider that, too. Your 4V requirement and 13V overhead means this is easily solved with standard zeners. :-) Tim -- Seven Transistor Labs, LLC Electrical Engineering Consultation and Contract Design Website: http://seventransistorlabs.com
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. -- Rick C