Forums

Zener diode below-threshold reverse current

Started by Tim Wescott June 23, 2016
On Wed, 27 Jul 2016 14:05:39 -0500, John Fields
<jfields@austininstruments.com> wrote:

>On Sun, 10 Jul 2016 17:15:51 -0400, rickman <gnuarm@gmail.com> wrote: > >>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. > >--- >Actually, raw 6.8 volt Zeners pretty much rule in terms of voltage >tempco, so they don't suck as much as you claim.
Actually, tempco isn't of any importance for the application. Leakage is, though the solution is a really poor one.
>--- > >>> 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. > >--- >"Choise"??? >--- > >>>> 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. > >Rather than damning one tool and blessing another, wouldn't it be >better to teach a man how to fish?
First you have to point them to the lake.
On Wed, 27 Jul 2016 14:49:40 -0500, John Fields
<jfields@austininstruments.com> wrote:

>On Sun, 10 Jul 2016 17:58:29 -0400, krw@attt.bizz wrote: > >>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. > >--- >That's no help, since hostility leads, reductio ad absurdum, to war. > >Do you want to declare war with your limited resources? >--- > >>When you want to discuss electronics, let us all know. > >--- >I'm always up for that, but your invitation is preumptuous, as if you >were running the game. >
It's always been clear that you aren't here to discuss electronics either.
On Wed, 27 Jul 2016 12:35:29 -0700 (PDT), Lasse Langwadt Christensen
<langwadt@fonz.dk> wrote:

>Den onsdag den 27. juli 2016 kl. 21.29.30 UTC+2 skrev John Fields: >> On Sun, 10 Jul 2016 14:40:27 -0700, John Larkin >> <jjlarkin@highlandtechnology.com> wrote: >> >> >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, >> >>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? >> >> >> >>John Fields >> > >> >Good grief, you only post here to be deliberately obnoxious. What a >> >sucky way to live. >> >> --- >> Au contraire. >> >> Instead of just flapping your gums, as usual, why don't you state what >> technical quarrels you have with a 6.8 volt Zener diode, properly >> biased, having close to zero TC? >> > >looking at a few datasheets, zero TC is ~5.6V, lowest impedance ~6.8V > >-Lasse
--- Indeed. Thanks, my memory isn't what it used to be. John Fields
On Wed, 27 Jul 2016 14:57:59 -0700, John Larkin
<jjlarkin@highlandtechnology.com> wrote:

>On Wed, 27 Jul 2016 14:49:40 -0500, John Fields ><jfields@austininstruments.com> wrote: > >>On Sun, 10 Jul 2016 17:58:29 -0400, krw@attt.bizz wrote: >> >>>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. >> >>--- >>That's no help, since hostility leads, reductio ad absurdum, to war. >> >>Do you want to declare war with your limited resources? >>--- >> >>>When you want to discuss electronics, let us all know. >> >>--- >>I'm always up for that, but your invitation is preumptuous, as if you >>were running the game. >> >>JF > >That's silly. I'm running the game.
--- Then you create unhappiness because you wallow in it? JF
On Wed, 27 Jul 2016 16:00:57 -0400, rickman <gnuarm@gmail.com> wrote:

>On 7/27/2016 3:05 PM, John Fields wrote: >> On Sun, 10 Jul 2016 17:15:51 -0400, rickman <gnuarm@gmail.com> wrote: >> >>> 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. >> >> --- >> Actually, raw 6.8 volt Zeners pretty much rule in terms of voltage >> tempco, so they don't suck as much as you claim. >> --- >> >>>> 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. >> >> --- >> "Choise"??? >> --- >> >>>>> 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. >> >> Rather than damning one tool and blessing another, wouldn't it be >> better to teach a man how to fish? > >I had fish last night...
--- Which, no doubt, was stepped on three or four times before it got to your table, so you don't know how to fish, you know how to buy. JF
On Tuesday, July 12, 2016 at 3:25:34 PM UTC-7, Lasse Langwadt Christensen wrote:
> Den tirsdag den 12. juli 2016 kl. 23.56.53 UTC+2 skrev John Larkin: > > On Tue, 12 Jul 2016 13:48:32 -0700 (PDT), dagmargoodboat@yahoo.com > > wrote: > > > > >On Tuesday, July 12, 2016 at 3:14:43 PM UTC-4, rickman wrote: > > >> On 7/12/2016 3:08 PM, John Larkin wrote: > > >> > > > >> > Mo and I were sitting at a table near the window of our neighborhood > > >> > French restaurant snacking on beer and oysters and cassoulet (I'm > > >> > eating the leftover cassoulet this instant... it holds up nicely) and > > >> > we noted that the entire population of the planet could live like > > >> > this, if people weren't such jerks. > > >> > > >> How would that work exactly? How are you going to pay the person who > > >> collects your garbage or serves you those oysters the same as what you > > >> make? Sounds very socialistic to me. > > >> > > >> -- > > >> > > >> Rick C > > > > > >Why are we wealthier than the cavemen? We both started with the same > > >raw materials (and in fact they had them arguably greater abundance, > > >since none of them had been used). So why? > > > > > >The answer lies in our accumulated knowledge, and our codes of conduct, > > >with the latter a /sine qua non/ for the former. > > > > > >But socialistic codes of conduct have a poor history for creating wealth. > > >Rather, they prey on wealth once it exists, dragging it down. > > > > > >"There is in fact a manly and legitimate passion for equality that spurs > > > all men to wish to be strong and esteemed. This passion tends to elevate > > > the lesser to the rank of the greater. But one also finds in the human > > > heart a depraved taste for equality, which impels the weak to want to > > > bring the strong down to their level, and which reduces men to preferring > > > equality in servitude to inequality in freedom." > > > > > > --Alexis de Tocqueville, Ancien Regime and the Revolution > > > > > >Cheers, > > >James Arthur > > > > Even a modern socialist train wreck like Cuba or Venezuela is better > > than a primitive society whose favorite pastime is tribal warfare. > > > > https://youtu.be/tA3L7RG8bnA
Amazing- not that an Arab feels that way, but that he said that in public. Have they beheaded him yet? Mark L. Fergerson
On 7/27/2016 10:38 PM, John Fields wrote:
> On Wed, 27 Jul 2016 16:00:57 -0400, rickman <gnuarm@gmail.com> wrote: > >> On 7/27/2016 3:05 PM, John Fields wrote: >>> On Sun, 10 Jul 2016 17:15:51 -0400, rickman <gnuarm@gmail.com> wrote: >>> >>>> 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. >>> >>> --- >>> Actually, raw 6.8 volt Zeners pretty much rule in terms of voltage >>> tempco, so they don't suck as much as you claim. >>> --- >>> >>>>> 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. >>> >>> --- >>> "Choise"??? >>> --- >>> >>>>>> 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. >>> >>> Rather than damning one tool and blessing another, wouldn't it be >>> better to teach a man how to fish? >> >> I had fish last night... > > --- > Which, no doubt, was stepped on three or four times before it got to > your table, so you don't know how to fish, you know how to buy.
I know how to fish. I don't know how to keep turtles from swallowing my hook, so I don't fish. Hell, I live on a lake. But like most places it is not recommended that you eat the fish too often. -- Rick C
On Wed, 27 Jul 2016 19:55:37 -0400, krw@attt.bizz wrote:

>On Wed, 27 Jul 2016 14:05:39 -0500, John Fields ><jfields@austininstruments.com> wrote: > >>On Sun, 10 Jul 2016 17:15:51 -0400, rickman <gnuarm@gmail.com> wrote: >> >>>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. >> >>--- >>Actually, raw 6.8 volt Zeners pretty much rule in terms of voltage >>tempco, so they don't suck as much as you claim. > >Actually, tempco isn't of any importance for the application. Leakage >is, though the solution is a really poor one. > >>--- >> >>>> 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. >> >>--- >>"Choise"??? >>--- >> >>>>> 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. >> >>Rather than damning one tool and blessing another, wouldn't it be >>better to teach a man how to fish? > >First you have to point them to the lake.
So, why not do that, then identify the fish in the lake from which the querent wants bounty, and what it takes to catch the particular fish he wants? JF
On Wed, 27 Jul 2016 19:56:59 -0400, krw@attt.bizz wrote:

>On Wed, 27 Jul 2016 14:49:40 -0500, John Fields ><jfields@austininstruments.com> wrote: > >>On Sun, 10 Jul 2016 17:58:29 -0400, krw@attt.bizz wrote: >> >>>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. >> >>--- >>That's no help, since hostility leads, reductio ad absurdum, to war. >> >>Do you want to declare war with your limited resources? >>--- >> >>>When you want to discuss electronics, let us all know. >> >>--- >>I'm always up for that, but your invitation is preumptuous, as if you >>were running the game. >> >It's always been clear that you aren't here to discuss electronics >either.
--- Not at all true - check my posting record - from someone who thinks he's running the game, but I don't mind (actually, I rather relish) an occasional sidestep into the dark side just to bring you phonies to your knees. JF
On Wed, 27 Jul 2016 22:56:01 -0400, rickman <gnuarm@gmail.com> wrote:

>On 7/27/2016 10:38 PM, John Fields wrote: >> On Wed, 27 Jul 2016 16:00:57 -0400, rickman <gnuarm@gmail.com> wrote: >> >>> On 7/27/2016 3:05 PM, John Fields wrote: >>>> On Sun, 10 Jul 2016 17:15:51 -0400, rickman <gnuarm@gmail.com> wrote: >>>> >>>>> 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. >>>> >>>> --- >>>> Actually, raw 6.8 volt Zeners pretty much rule in terms of voltage >>>> tempco, so they don't suck as much as you claim. >>>> --- >>>> >>>>>> 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. >>>> >>>> --- >>>> "Choise"??? >>>> --- >>>> >>>>>>> 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. >>>> >>>> Rather than damning one tool and blessing another, wouldn't it be >>>> better to teach a man how to fish? >>> >>> I had fish last night... >> >> --- >> Which, no doubt, was stepped on three or four times before it got to >> your table, so you don't know how to fish, you know how to buy. > >I know how to fish. I don't know how to keep turtles from swallowing my >hook, so I don't fish. Hell, I live on a lake. But like most places it >is not recommended that you eat the fish too often.
--- If turtles bite on your bait, and you don't want to catch turtles, then you don't know how to fish for what you want to catch. The rest is just bullshit. JF