Forums

Zeners in Series

Started by Cursitor Doom November 9, 2019
On Sunday, November 10, 2019 at 8:49:43 PM UTC+11, upsid...@downunder.com wrote:
> On Sat, 9 Nov 2019 21:42:38 -0000 (UTC), Cursitor Doom > <curd@notformail.com> wrote: > > >Whilst mere hobbyists like myself can easily get away with making up a > >desired zener value from putting two smaller value ones in series to add > >together their rated voltages, can such a dodge be employed by > >professional designers without losing face? I mean, in the case where > >there is no commercially available zener with precisely the desired > >breakdown voltage? Would a pro designer be open to ridicule and risking > >his rep by pulling such a stunt? > > Zeners around 5 V have nearly zero temperature coefficient, those > above have a positive TC and those below a negative TC. Thus making a > 10 V zener from two 5 V zeners would have zero TC. Using less than 4 V > and over 6 V in series may also reduce the TC. Alternative put some > forward biased diode(s) (TC -2.2 mV/K) in series to compensate for the > positive TC of more than 5 V zeners.
1N821 - 1N829 put a 5.6V zener in the same package as the forward biased diode pdf.datasheetcatalog.com/datasheet/motorola/1N823.pdf You get a stable 6.2V reference voltage at precisely 7.5mA. How precisely it's worth hitting 7.5mA depends on the price of the diode - the 1N829 sells for quite a bit more than the 1N821. Alternatively you can buy one of the cheaper parts and find the exact current gives the lowest temperature coefficient, but it requires a fairly expensive test rig to cover the temperature range. There are integrated circuits around that can do better. -- Bill Sloman, Sydney
On Saturday, November 9, 2019 at 6:49:32 PM UTC-5, Phil Allison wrote:
> alan.ye...@gmail.com wrote: > ------------------------------------- > > > > > > > But, is it really good practice to use only a stack of zeners? > > Should there not be a hi impedance bleeder path in parallel to > > each of the diode(s) stack so they all start to conduct and zener > > properly. > > Phil I'm surprised you did not mention this. > > > ** Surely you jest ? > > Might as well fit bleeders in parallel with cells in a battery. > > Make the bastards share the damn current .... > > > > ... Phil
Suppose one of the zeners in the stack has very low leakage. So low that the others in the stack don't turn on. The bleeder resistors would then allow the other diodes to turn on and eventually the bad actor would turn on too. Once the series string of diodes turns on they can 'share the same damn current'. Bleeders among the cells in a battery would be counter productive wasting the energy with no beneficial purpose.
<alan.yeager.2013@gmail.com> wrote in message 
news:44553b93-c61c-4150-8396-e7262d9644db@googlegroups.com...
> > But, is it really good practice to use only a stack of zeners? Should > there not be a hi impedance bleeder path in parallel to each of the > diode(s) stack so they all start to conduct and zener properly. Phil I'm > surprised you did not mention this. >
Yes, it is. In fact, many TVSs in higher ratings (>=48V maybe?) are stacked die construction! The absence of a safe breakdown mechanism is precisely what causes leakage problems for some devices. You're literally using the best possible device to stack in series. ;-) The opposite sutation is of a bit more concern: TVSs in parallel. But there too, they are pretty good. The only downside is, at modest currents (where quiescent power dissipation is the most likely failure mode, i.e., a given part just eventually overheats), parallel diodes will tend not to share current. And that's only down to mismatch in breakdown voltage. Avalanche breakdown has a positive tempco, so it's not a current-hogging issue like it is for paralleled transistors or (forward biased) diodes. And at high peak currents, internal resistance dominates, so the sharing under surge is okay. Tim -- Seven Transistor Labs, LLC Electrical Engineering Consultation and Design Website: https://www.seventransistorlabs.com/
On Monday, November 11, 2019 at 12:39:47 AM UTC+11, alan.ye...@gmail.com wrote:
> On Saturday, November 9, 2019 at 6:49:32 PM UTC-5, Phil Allison wrote: > > alan.ye...@gmail.com wrote: > > ------------------------------------- > > > > > > > > > > > But, is it really good practice to use only a stack of zeners? > > > Should there not be a hi impedance bleeder path in parallel to > > > each of the diode(s) stack so they all start to conduct and zener > > > properly. > > > Phil I'm surprised you did not mention this. > > > > > > ** Surely you jest ? > > > > Might as well fit bleeders in parallel with cells in a battery. > > > > Make the bastards share the damn current .... > > Suppose one of the zeners in the stack has very low leakage. So low that the others in the stack don't turn on. The bleeder resistors would then allow the other diodes to turn on and eventually the bad actor would turn on too. Once the series string of diodes turns on they can 'share the same damn current'.
Zener current increases rapidly with voltage around the rated breakdown voltage. Look at the data sheets. In so far as zener diodes have a "leakage current", it has to measured at voltages well below the voltage at which is designed to operate. If one zener in a string has a lower "leakage current" than the others in a nominally identical string it will have a slightly higher operating voltage at whatever current you chose to run the string at. The only way that the other diodes in the stack "wouldn't turn on" would be if the offending diode had blown open circuit. Bleeder resistor wouldn't be a good way to solve that problem.
> Bleeders among the cells in a battery would be counter productive wasting the energy with no beneficial purpose.
Equally true of a zener diode string. -- Bill Sloman, Sydney
I made these current-limiting switches:
https://www.seventransistorlabs.com/Images/SwitchingCurrentLimitUnits.jpg
https://www.seventransistorlabs.com/Images/SwitchingCurrentLimitUnits2.jpg
The row of three SMDJ15CA in series gobbles up a surge of 600W and begs for 
more.  I can mash the "fire" button as fast as I can, and it simply thermal 
limits, no damage.  I can actually see skin oils evaporate from the top of 
the diodes, following a surge -- they get nice and toasty internally, but 
still well before failure.  (Reliability, who knows -- the thermal cycling 
is likely a factor, but this isn't a million-cycle product either.)

It's also got a ground-return connection, so if you want to use it as a 
current-limiting buck converter instead, you can save a lot of power 
dissipation and operate at higher duty cycle.  Otherwise, it works as a 
two-terminal, bidirectional current-regulating diode, gobbling the excess 
power internally.

Tim

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

"Tom Gardner" <spamjunk@blueyonder.co.uk> wrote in message 
news:SaHxF.69412$om6.5287@fx27.am4...
> On 09/11/19 21:42, Cursitor Doom wrote: >> Whilst mere hobbyists like myself can easily get away with making up a >> desired zener value from putting two smaller value ones in series to add >> together their rated voltages, can such a dodge be employed by >> professional designers without losing face? I mean, in the case where >> there is no commercially available zener with precisely the desired >> breakdown voltage? Would a pro designer be open to ridicule and risking >> his rep by pulling such a stunt? > > My question would be "what is the problem to which you think > that is a good solution?". > > Only then I would consider whether that is an adequate > thing to do. > > If nothing else, you should assure yourself that the > voltage will be sufficiently accurate and stable as > temperature and current vary.
On Sun, 10 Nov 2019 10:18:59 -0000 (UTC), Cursitor Doom
<curd@notformail.com> wrote:

>On Sat, 09 Nov 2019 18:38:31 -0800, jlarkin wrote: > >> I even -gasp- sometimes use only half of a dual diode! >> >> But ridicule? Reputation? Who would know? > >Everyone here, now you've admitted to still using dual diodes! >:-D
Oh, the shame! BAV23S, dual series, 250V, 4 cents BAV99, dual series, 70V, 1.4 cents The BAV23S is great in C-W high voltage boosted flyback supplies. I'd post an example but people would whine. -- John Larkin Highland Technology, Inc lunatic fringe electronics
Cursitor Doom wrote:
> On Sat, 09 Nov 2019 19:43:40 -0500, Tom Del Rosso wrote: > >> Cursitor Doom wrote: >>> Whilst mere hobbyists like myself can easily get away with making >>> up a desired zener value from putting two smaller value ones in >>> series to add together their rated voltages, can such a dodge be >>> employed by professional designers without losing face? I mean, in >>> the case where there is no commercially available zener with >>> precisely the desired breakdown voltage? Would a pro designer be >>> open to ridicule and risking his rep by pulling such a stunt? >> >> Win and others here have noted that zeners above 7V produce much more >> noise. >> >> You were in this thread too. > > Indeed I was. ISTR I started it. But this is a completely different > matter!
But isn't the anser the same? Obviously it is done routinely.
On Sun, 10 Nov 2019 07:59:28 -0600, "Tim Williams"
<tiwill@seventransistorlabs.com> wrote:

><alan.yeager.2013@gmail.com> wrote in message >news:44553b93-c61c-4150-8396-e7262d9644db@googlegroups.com... >> >> But, is it really good practice to use only a stack of zeners? Should >> there not be a hi impedance bleeder path in parallel to each of the >> diode(s) stack so they all start to conduct and zener properly. Phil I'm >> surprised you did not mention this. >> > >Yes, it is. In fact, many TVSs in higher ratings (>=48V maybe?) are stacked >die construction! > >The absence of a safe breakdown mechanism is precisely what causes leakage >problems for some devices. You're literally using the best possible device >to stack in series. ;-) > >The opposite sutation is of a bit more concern: TVSs in parallel. But there >too, they are pretty good. The only downside is, at modest currents (where >quiescent power dissipation is the most likely failure mode, i.e., a given >part just eventually overheats), parallel diodes will tend not to share >current. And that's only down to mismatch in breakdown voltage. > >Avalanche breakdown has a positive tempco, so it's not a current-hogging >issue like it is for paralleled transistors or (forward biased) diodes. And >at high peak currents, internal resistance dominates, so the sharing under >surge is okay. > >Tim
Some big diode packages are multiple paralleled diodes. Diodes get ohmic, and have positive TCs, at high current, so it works out. -- John Larkin Highland Technology, Inc lunatic fringe electronics
On Sun, 10 Nov 2019 07:28:49 -0800, jlarkin wrote:

> Oh, the shame! > > BAV23S, dual series, 250V, 4 cents > > BAV99, dual series, 70V, 1.4 cents > > The BAV23S is great in C-W high voltage boosted flyback supplies. I'd > post an example but people would whine.
ISTR you already did a few months ago; one of your 'Dremmelboard' [TM] prototypes. I hope you take proper precautions with the dust! -- This message may be freely reproduced without limit or charge only via the Usenet protocol. Reproduction in whole or part through other protocols, whether for profit or not, is conditional upon a charge of GBP10.00 per reproduction. Publication in this manner via non-Usenet protocols constitutes acceptance of this condition.
alan.ye...@gmail.com wrote:

--------------------------
> > > > > ** Surely you jest ? > > > > Suppose one of the zeners in the stack has very low leakage. > So low that the others in the stack don't turn on. >
** Crikey - he wasn't jesting. Maybe he struggles with the concept of current. ..... Phil