zener diode voltage regulator

[barry]

Hi Barry,

Thanks, I'd realised that point. I just thought it might be helpful to fm101 to understand that - as FvM briefly said - every part, including a Transistor Universal NPN (TUN), has its maximum ratings, and 12V is double the 6V maximum Vbe for BJTs.

Again, there is 0.6V across B-E, not 12V

 

[crutschow]

12V is double the 6V maximum Vbe for BJTs

Then you need to more carefully read the data sheet as you suggested we all do.
That maximum is for a reverse bias Vbe voltage.
As Barry has repeatedly said, the Vbe voltage here is about +0.7V forward bias.

 

[Easy peasy]

TUN is a TLA I have never seen used before, npn xtor would be a far better use of text.

 

[dick_freebird]

Hi,

1) Yes, for your purpose here.

2) Unregulated DC would be e.g. a steady-ish 5V that goes from e.g. 4.9V to 5.1V, what you see and use as the input voltage to the voltage regulator after the bridge rectifier and smoothing capacitors you called diode ring, etc.

Read about how to calculate smoothing, or bulk, or filtering, capacitance in power supplies, it will help, and that in voltage regulator datasheets there is a useful to know parameter called line rejection - how much input ripple a regulator can handle and with regard to frequency before it can't keep up any more and regulate the output.

I'm sure you know that Zener + BJT regulators are fossilized circuits and (very) bad due to various weaknesses Zener and BJT each have, plus lack of real Error Amplifier feedback, but useful as introductory learning concepts and tools. Have fun!

Having designed products of both zener and bandgap type, I can say there are application care-abouts which might make you prefer the zener reference (and it's not like the bandgap is a spring chicken either).

Fast boot applications is one. You can be up in under a microsecond with a zener while a buffered bandgap has op amp slew rate / BW limitations that give a much longer time-to-tolerance.

"Back in the day" (and still used, a little) you could buy temperature compensated zener "diodes" which have junctions on both faces of the die, to get a zener and forward diode in series, cancelling zener PTC and diode NTC.

--- Updated Nov 11, 2022 ---

... 12V is double the 6V maximum Vbe for BJTs.

A couple of points here.

BVebo will generally be higher, like 7-8V. Vbe(r) limits of 6V are derived from hot carrier reliability testing (or just following the pack). The BV will follow base doping inversely, old HV wide base transistors will have higher BV (but not necessarily better hot carrier resistance, as that involves lighter surface conc and oxide chemistry as well).

And your lateral PNP, substrate PNP on "standard linear" bipolar technology has BVebo=BVcbo (>40V) owing to its very low base (= N 3-5 epi) doping.

Totally device technology specific, the DS must be consulted.

 

[FvM]

"TUN" is an acronyme used by Elektor magazin, stands for "transistor universal npn", in other words small signal general purpose npn, e.g. BC107.

As stated by Barry, the transistor isn't reverse biased in normal operation, there's however a situation where the transistor may be destroyed by reverse base current. It can happen when the output voltage has a larger filter capacitor and the input voltage is suddenly shorted.

 

[crutschow]

It can happen when the output voltage has a larger filter capacitor and the input voltage is suddenly shorted.

But that is not something that would not normally happen.
Just removing the input voltage would not do that.

If an input short could really happen, then a reverse-bias diode across from base to emitter junction would protect against that.

 

[d123]

Hi,

Yes, okay. OP seems to have fled, anyway.

I know, I have a DMM and so on and know Vbe is a diode drop, but BVebo is in datasheets for a reason, I guess. Wish I hadn't mentioned it, but thanks for deeper description.

I meant fm101 perhaps wasn't aware of the importance of datasheet before doing, not anyone else in mind when I said that. I can relate to what I think is their level of knowledge from personal experience, and it seems people new to electronics, self included, take a while to click that 'blindly' doing stuff with components and breaking/burning some by accident is less productive than acquiring relevant information first.

TUN and TUP are terms I've seen in several places, and I think they are useful abbreviations.

1us, interesting point, amongst others mentioned, as evet, even if aspects of die-level design go over my head a bit.

Thanks for inputs, everyone, really, some very interesting points brought up in the discussion.