Floating Variable "Zener" - Page 1
Forum section: electronics / projects

author: mawyatt
author_rank: Super Contributor
timestamp: October 28, 2022, 07:09:40 pm
content: In circuit design one may encounter a need for a floating 2 terminal Zener with a specific voltage drop. As Murphy would have it often this voltage is not something you have on hand and need to order/wait for the appropriate componentMany designer folks likely have the versatile TL431 or similar on hand which is a very useful chip, but what if you don't??Recalling a very old Vbe Multiplier concept from the early op-amps days (used to bias the complementary outputs with something a little less than two Vbes) may come to the rescue, which it did in our case!!We needed a 14 volt zener but lacked this component so employed a 12 volt zener we had and a 2N3904, adding a 10K and 100K resistor and we had a 2 terminal 14 volt zener equivalent. Of course the below breakdown current will be much higher than a normal zener (set by R1 and R2), and the dynamic impedance may be higher (could be actually lower due to the negative C to B feedback), and the temperature effects now include the Vbe ( ~ -2mv/C) which may be beneficial since zeners above ~ 5 volts tend to have a +TC which the -TC of the Vbe can help reduce.Anyway, here's a couple images, good use for our newly refurbed Tek 577Hopefully, this technique may help in a pinch!Best,

author: jbb
author_rank: Super Contributor
timestamp: October 29, 2022, 06:55:30 pm
content: That’s a good one to keep in the back of the toolbox.I wonder if some temperature compensation could be arranged by using a Zener with a tempco around +2mV / C (I guess that would be somewhere around 7V2?) and squishing the Zeber diode and BJT together with a bit of heatshrink?

author: magic
author_rank: Super Contributor
timestamp: October 29, 2022, 07:14:35 pm
content: That's about what you get in theSZA263The optimal zener choice is apparently more like 6V2.Per my datasheets, 12V has approx +8mV/°C on average. So it seems that TC could be reduced if the transistor is set for 3x gain without including the zener (that's about 2V) and then connected in series with the zener.On the other hand, if you are replacing a 14V zener, maybe you want something with even more positive TC.Or most likely, you just don't give a damnQuote from: mawyatt on October 28, 2022, 07:09:40 pmThere is probably one somewhere within your reach right now, just takes a hammer and a soldering iron to get it out

author: DavidAlfa
author_rank: Super Contributor
timestamp: October 29, 2022, 07:47:53 pm
content: Searching "adjustable zener transitor" drops infinite results.Anyways, in what universe wouldn't you find a TL431?

author: fourfathom
author_rank: Super Contributor
timestamp: October 29, 2022, 09:25:30 pm
content: Quote from: DavidAlfa on October 29, 2022, 07:47:53 pmDarn! At first glance I was hoping that this was a way to cancel out the VBEtempco, but no such luck. The "Zener" voltage still has the tempco gain set by the voltage divider, and the top transistor actually adds very slightly to it.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 29, 2022, 09:38:05 pm
content: Note that putting more transistors in the loop invites the same problems a TL431 does: higher dynamic impedance, and stability.I don't like the un-degenerated emitter in cascade: the gain varies too much with operating point. A little degeneration puts a ceiling on that, and then it can be designed to guarantee stability.Speaking of dynamics, I suppose a cap across the top divider (P1 + R1 in the last one above) would be the most effective, and maybe an R+C there would be necessary to stabilize a 2 or 3 stage one, in some conditions.The tempco isn't easy to cancel (short of building a whole-ass bandgap circuit, or the matching zener), but it can be tweaked with an NTC thermistor partially in parallel with R2. Namely, an NTC plus some normal (non-sensitive) resistance, and that pair in parallel with another (non-sensitive) resistance, makes up the whole equivalent of R2, and this can be tuned to give pretty reasonable behavior (like <10% error from 0-85°C). Or at relatively high ratios, I think a PTC (Ni or Pt RTD?) is about right, but it has to be the entirety of P1+R1? So your adjustable range has to be done at R2.Adjusting R2 is generally safer anyway, as long as it's by small increment still -- in general, only use what adjustment range, or V/I operating range, or etc. -- you need. Design not just the desired operating point but its extremes as well. For the general case, that means making, like, amplifier stage input and output ranges match each other, so that they tend to saturate about the same point. Anyway for this, adjusting R2 means, if the wiper goes loose (as they inevitably do), it defaults to a lower voltage -- in an amplifier Vbe mult, this increases distortion instead of causing runaway bias.Tim

author: mawyatt
author_rank: Super Contributor
timestamp: October 30, 2022, 01:37:27 am
content: If you ignore base bias and other non-ideal transistor effects (Early voltage, finite Beta and so on), then the effective zener equivalent:Vzx ~ (Vbe +Vz)(1 + R2/R1)Then dVzx/dT is (dVbe/dT + dVz/dT)(1 + R2/R1) assuming d(R2/R1)/dT ~ 0 which should be relatively easy to achieve with modest like type resistors.So if a 6~6.8 volt zener is selected with ~ +2mv/C , this will cancel the ~ -2mv/C of the transistor and the overall Vzx should be approximately temperature independent to a 1st order. Of course we've made lots of assumptions and such, so don't expect absolute zero TC!!!Here's quick LTSpice simulation of such with a 6.2V Zener and 2N3904 at 1ma, seems to work for a 14 volt effective "Zener". Edit, added another with R1 changed to ~14K and R2 to 100K and HV transistor for a ~ 54V Zener equivalent.Best

author: David Hess
author_rank: Super Contributor
timestamp: October 31, 2022, 01:49:01 am
content: Another method I have seen when a low impedance source is driving a high impedance source is to use a resistor and then drive a constant current through the resistance into the low impedance source. The resistor may be bypassed to lower the AC impedance.

author: Terry Bites
author_rank: Super Contributor
timestamp: October 31, 2022, 06:27:34 pm
content: Any zener can be floated. The series resistance can be on one side or the other or split between both. It make no difference to the zener voltage.The 431 is the component of choice if you want a precise predetermined voltage is a LM4040/1. Both of these components are ubiquitou and cheap as chips- why would you not use them? You can even add a pass transistor to get arbitrarily high currents.2*6.8=? If its a 14V zener, then its not likely to be very accurate or precise. Add in a series 1N914 and your tempco will improve.

author: jonpaul
author_rank: Super Contributor
timestamp: October 31, 2022, 06:31:01 pm
content: Use TL431, ass low tempco trimpot if precise adj needed.https://www.ti.com/product/TL431Jon