Forums

Zeners in Series

Started by Cursitor Doom November 9, 2019
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?
It's perfectly fine. Adding a new part to the stockroom is a big deal, only worth it if you expect to need a lot of them. Two zeners in series may be good economics. It spreads the power dissipation, too. I sometimes put resistors or caps in series or parallel, to avoid adding another line to the BOM of a board, even if all the right values are in stock. Saves loading up a reel on the pick-and-place to slam down one cheap part. I even -gasp- sometimes use only half of a dual diode! But ridicule? Reputation? Who would know? -- John Larkin Highland Technology, Inc lunatic fringe electronics
On Sunday, November 10, 2019 at 1:23:56 PM UTC+11, Phil Allison wrote:
> Bill Slowman 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. > > > > About the only problem with using a stack of zeners is running them at high enough current to prevent the avalanche process from turning itself off. > > ** My one problem was making sure 3W zeners had enough heat sinking so they did not de-solder themselves and fall right off the tag strip.
That does involve working out the power being dissipated in each zener. The data sheets mostly have thermal derating curves, which indirectly tells you how warm the zener gets at a given power dissipation. Covering problems like this is the boring but unavoidable part of circuit design.
> Simple fix was to use more, lover voltage types. > > Damn those heavy metal guitarists !!
You need a theologian for that. I could put you in contact with at least one, but Methodists don't go in for applied theology. -- Bill Sloman, Sydney
Bill Sloman wrote:

--------------------
 > 
> > ** My one problem was making sure 3W zeners had enough heat sinking > > so they did not de-solder themselves and fall right off the tag strip. > > That does involve working out the power being dissipated in each zener. >
** This was back in 1981. I was then working for a Scientologist - Allen Wright. Long story. It first of all involved knowing the max possible average current flow into the screens of four EL34s inside Marshall. The answer is about 80 mA with the tubes driven into hard distortion from a sine generator. I had used 5 x 18V, 5W zeners in series at fist - worked fine on the bench for 15 minutes. Case temp was hot but not sizzling and I knew power zeners were tough buggers. Only 1.44 watts each dissipation, mounted well spaced on a large sized tag board that soaked up much of the heat. Must be OK. But with the chassis back in the wood case, no through ventilation whatever, lots of heat dripping from the power tubes, a hot night and with a loony guitarist who did not take a break for hours - one of the solder joints got a few degrees too hot. Sound stopped, musicians wept. A change to 7 x 12 volt zeners was the fix. All came about because tube factories in Europe and the UK closed down, leaving one US plant going - Philips /Sylvania. They made an odd version of the EL34 /6CA7 tube that hated high screen voltages and would quickly burn out. My "zener mod" made it possible to use them safely, I know of no other tech who came up with the same or any solution. The very first customer got to do some Beta testing for me. ..... Phil
On Sunday, November 10, 2019 at 1:38:41 PM UTC+11, jla...@highlandsniptechnology.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? > > It's perfectly fine. Adding a new part to the stockroom is a big deal, > only worth it if you expect to need a lot of them. Two zeners in > series may be good economics. It spreads the power dissipation, too. > > I sometimes put resistors or caps in series or parallel, to avoid > adding another line to the BOM of a board, even if all the right > values are in stock. Saves loading up a reel on the pick-and-place to > slam down one cheap part. > > I even -gasp- sometimes use only half of a dual diode! > > But ridicule? Reputation? Who would know?
John Larkin does expect rather more praise than he deserves, and does seem to be correspondingly sensitive about his reputation. Ridicule doesn't work on him. He writes it off as an expression of personal animosity and ignores it, which does mean that he posts the same nonsense time after time, which does make him ridiculous, but not in a way that he can detect. -- Bill Sloman, Sydney
On Saturday, 9 November 2019 23:49:32 UTC, 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
maybe he's thinking of neons. Zeners certainly have no use for such things NT
>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?
There are programmable zeners, like the TL431 for example. That said, I have some zener with a red and green LED in series in my PIC programmer.... In the old days temperature compensation and stuff was done by putting things in series. But I'd use the TL431 for small currents. I got 50 of those on ebay for 1$84 free shipping in 2015...
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.
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! -- 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.
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 -- 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.
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