How well does a zener diode regulate - Page 1
Forum section: electronics / beginners

author: brownt
author_rank: Regular Contributor
timestamp: July 06, 2021, 03:48:46 am
content: I thought the output of the attached circuit would be stable at 2.7 volts with an input of 5 - 9 volts.However, the output ranges from 2.7 - 3.2vIs that what is expected?

author: bdunham7
author_rank: Super Contributor
timestamp: July 06, 2021, 04:13:47 am
content: Zener diodes typically have very high temperature and current coefficients, more so if their value is significantly more or less than 6.9V or so. A 2.7V zener will not be a very stable regulator, as you have observed.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 06, 2021, 04:16:54 am
content: Low voltage zeners have a soft knee; read the datasheet. This is normal behavior, and a good reason to avoid them for signal clamping or voltage regulation applications.Note that the zener is only its rated voltage and tolerance, at its rated current (Vz at Iz). All bets are off outside that range, though 5-8V zeners are quite stable (6.2V being nearly ideal, i.e., within tolerance over a fairly wide range of Iz), and the slope is always one-to-one, i.e. voltage drop is always higher at higher currents.If you need a sharper knee, consider a TL431 adjustable shunt regulator. (Note that, despite it usually being symbolized as an adjustable zener, it is anything but; a better description is a three-terminal op-amp with a suspiciously large, yet stable, 2.50V (or 1.24V for TLV431) input voltage offset. As an op-amp, it does not respond instantaneously, but takes some microseconds; it can be unstable into certain loads (capacitance); or it may require compensation between input and output pins, while controlling something greater than itself, like an isolated switching power supply. So there may be a little engineering to use it for any particular application.)Tim

author: bob91343
author_rank: Super Contributor
timestamp: July 06, 2021, 04:52:58 am
content: Even a series group of silicon diodes might be better at such low voltages. Alternatively, drive the zener with a constant current and use an opamp to deliver to the load.

author: EEVblog
author_rank: Unknown
timestamp: July 06, 2021, 05:14:57 am
content: Quote from: T3sl4co1l on July 06, 2021, 04:16:54 amI mention this in my video, it's the difference between zener breakdown and avalanche breakdown. Roughly around the 5V mark.There is a reason my the zener voltage it marked as "nominal" in the datasheet.

author: brownt
author_rank: Regular Contributor
timestamp: July 06, 2021, 05:50:14 am
content: I see. Good to know.I wonder what I can do then. TL431, maybe. I don't know enough about it.A device outputs a voltage. The voltage is divided with a potentiometer, and a proportion is sent back to the device.I want to connect two devices to the same potentiometer.It is unknown whether a device will output 5v or 3.3vHence, I was trying to lock it down to 2.7 volts to prevent the possibility of a lower voltage device being damaged if a high voltage device was connected.The fact that the full voltage will never be delivered back to the device, is a smaller issue than damaging a device.There is very little current available to do much with. Only 2 - 3mAWhat might I be able to do in this instance?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 06, 2021, 07:38:17 am
content: Two devices at the same time, or two different devices, one at a time (you just don't know which)?Why does it matter what the voltage is, at the input or output of this network? What is it doing?What are the device(s)? I don't understand the secrecy if it's just like some dumb Arduinos or something. The more concrete you can make the problem, the better answered your question can be.Tim

author: brownt
author_rank: Regular Contributor
timestamp: July 06, 2021, 09:12:33 am
content: Hi, no secret, just keeping it simple through being generic. The devices are guitar effect units, or keyboards, or anything else that has an expression port. There are so many different brands of the devices, that I can't cater for the specifics of their internals, other than saying they all have either 5v or 3.3v outputs.There are two devices, or more, that I want to connect to a single potentiometer. The output of the devices may be 5v or 3.3 volt. The potentiometer will feedback a proportion of the output anywhere from zero to maximum. I assume that if you hold a 5-volt pressure to a 3.3-volt circuit there is potential damage. I want to prevent that potential damage bearing in mind that the devices may present a combination of 5v and 3.3v, or all 5v, or all 3.3vTwo, or more, devices at the same time. thanks

author: theHWcave
author_rank: Regular Contributor
timestamp: July 06, 2021, 10:44:12 am
content: Not sure if I understand how this is supposed to work. If you feed 0..3.3V into an expression port that expects 0..5V, you only get up to 66% of the expression?As far as I know, expression ports actually deliver the required voltage. For example on one of these standard TRS plugs they may have 5V on ring, the variable expression voltage on tip and ground on sleeve.For 2 units, why don't you use a simple stereo potentiometer where each of the two expression ports is wired independently to one of the pots. That way it is completely irrelevant what voltage they use.If simultaneous control of more expression ports are needed, your choice is either mechanically to gang more potentiometers together but a neater electronic solution would be digital potentiometer chips. For example you could use a X9C103PZ chip that acts like a 10K pot which is controlled in 100 steps by an up/down signal and a count pulse. You could control any number of these together, either with a simple 2-button interface or maybe a little Arduino that reads a normal pot on one of its A/D inputs and then sends out the necessary signals to control an array of digital potentiometers. I think there are also other digital pot chips that may have I2C or similar interfaces which may be easier to control from an Arduino.

author: gcewing
author_rank: Regular Contributor
timestamp: July 06, 2021, 10:49:52 am
content: Perhaps not use the 5V or 3.3V provided by the devices at all, and use an external 3.3V power source.

author: brownt
author_rank: Regular Contributor
timestamp: July 06, 2021, 12:02:57 pm
content: Thanks, but that is not the solution I need.Yes, I realise that full modulation will not be achieved, but its often not important anyway. Effects such as tremolo and phaser don't sound good fully modulated. And most multi-effect units have a calibrate function that will maximise whatever is available anyway.Two potentiometers, or digital pots are not suitable for this specific project. They will work of course, but the requirement is one potentiometer, and a potential mix of 3.3v and 5v.

author: brownt
author_rank: Regular Contributor
timestamp: July 06, 2021, 12:06:47 pm
content: hmmm good thought, though not for this project. Any power has to come from the devices outputting the voltage. 5 or 3.3v at 2 - 3 mAOne of the helpful parts of this project, is it doesn't have to be perfect. But it can't blow up a device either. Less than full modulation is ok.Maybe a passive resistor arrangement would do, but you don't know which output voltage devices might be chosen at any given time

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 06, 2021, 12:20:42 pm
content: Simpler solution -- why use one pot? Use a dual ganged pot. They'll track alright, and more importantly they'll track over their own respective range. Feed each unit's supply/reference to the respective pot, no problem. One component, however many connectors, and just wiring. Simple as could be.The next best solution is probably a multiplier,which can be done in a couple of ways without too much trouble, if certain assumptions are met (like, if the ref level is DC, not an audio signal -- which it sounds like this may be the case). And yes, you can use some kind of digital control (DAC), the downside is programming an MCU to run it, which isn't terribly complicated, but it is more stuff needed.Specifically, I'm thinking of a PWM multiplier. You have a local 0-100% pot signal (just one, no gangs needed), fed to a pulse width modulator (which can be as simple as a LM393 or TLC556 plus support components). Using a pair of analog switches per output channel, chop the ref by the PWM. Then filter it with an op-amp (a 3rd order lowpass is very reasonable) to get a smooth output again. This effectively multiplies the reference by the PWM signal, which after filtering, gives a nice smooth 0-100% of the reference (this is ultimately why a multiplying function is required: you're scaling one signal by another).The digital pot does exactly the same thing -- another name for this is an mDAC (multiplying DAC), it just does it in digital steps, rather than high/low pulses. You certainly wouldn't want to build one yourself, but ICs are cheap and plentiful. I suppose it's not even that horrible a solution, to use a pot into a classic (parallel output) ADC, into an mDAC. No MCU and programming required, just.. not the most satisfying solution in terms of how much stuff goes into the thing. But eh, when you've got building blocks as powerful as these, readily available, it can be the right approach.Alternately, if you're sure that a 0-3.3V range will work for either device, you can use a common pot, and a 3.3V supply -- use a low dropout regulator (LDO, a literal one, not like an ancient LM1117 or 317 or 78L33 or anything) of the same voltage, and it will pass 3.3V with, well, low dropout; or regulate anything higher (namely 5V) down to 3.3 as desired. Two such LDOs can have their outputs wired in parallel, and whichever one is powered or higher, will do the job. No problem. Then it doesn't matter which box is which ref, the inputs and outputs are both interchangable.Tim

author: Zero999
author_rank: Super Contributor
timestamp: July 06, 2021, 12:48:09 pm
content: Quote from: brownt on July 06, 2021, 05:50:14 amYes, a proper shunt regulator, such as the TL431 should, rather than a zener diode.The TL431 is just an op-amp, connected to an internal 2.495V reference, with a shunt transistor output. When connected, the op-amp ensures its output equals 2.495V.The website linked below can help you calculated the values of R1 and R2, which form a potential divider. VOUTon the TL431 schematic, becomes VINin the calculator and enter 2.495 into VOUTon the calculator.https://www.random-science-tools.com/electronics/divider.htm

author: magic
author_rank: Super Contributor
timestamp: July 06, 2021, 12:55:08 pm
content: TL431 is a power hog which needs to be fed 0.4mA (typically) and up to 1mA (guaranteed) before it reaches nominal output.If you don't mind 2.5V instead of 2.7V, LM385-2.5 is an "almost perfect 2.5V zener", regulating over a range of currents from 10~20µA to 30mA.

author: Veketti
author_rank: Regular Contributor
timestamp: July 06, 2021, 01:43:22 pm
content: Not meant to hijack the topic, but as this is also about zener diode regulate, I was wondering how does Zeners fail? If they fail getting short it would cause the regulated device probably fry due to getting the input voltage?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 06, 2021, 03:33:43 pm
content: Semiconductors generally die by short circuit. Sometimes the short is extremely small (a blown gate on a MOSFET can manifest as some kohms between pins), sometimes gross (milliohms). If fault current is available (large capacitors, battery or mains power), the short may subsequently blow open (this takes a few milliseconds), usually by vaporizing the bond wires or component leads. The first kind of failure can be silent, the second is likely to damage the package (release magic smoke, crack the package, even propel the package as shrapnel). A shorted zener in a shunt regulator may dissipate additional power in the source (resistor or CCS); one should design with enough margin that this does not cause a cascading failure.Tim

author: theHWcave
author_rank: Regular Contributor
timestamp: July 06, 2021, 03:47:19 pm
content: I fail to understand why a stereo pot doesn't do the job? Its one pot (turning knob) for two outputs.Also you should always use the voltage that comes from the units, be it 3.3V or 5V and neither mix or combine them. Neither should you send voltage generated elsewhere (and that includes voltage "mixed/combined" from other units) back into a unit. Just imagine that a unit is turned off and the other(s) still on. Now the powered off unit receives voltage through its effects input. That may very well break something because they are not designed for that. It is expected that the effects voltage goes to zero when the power is off because the effects pedal (with the pot inside) is powered by the unit.

author: Zero999
author_rank: Super Contributor
timestamp: July 06, 2021, 03:49:17 pm
content: Quote from: magic on July 06, 2021, 12:55:08 pmThat's still a lot less than a zener diode.https://assets.nexperia.com/documents/data-sheet/BZX79.pdfQuoteThere are lower current alternatives to the TL431, with a variable output voltage, such as the TLV431 and TLVH431.

author: David Hess
author_rank: Super Contributor
timestamp: July 07, 2021, 12:37:07 am
content: Temperature compensated zener diodes at about 6.8 volts get down into the 5 to 10 ppm/C range, but still have an associated impedance which limits line and load regulation. Precision circuits rely on closely regulating the zener current and buffering the output if necessary which makes a considerable improvement; under such conditions the change in input voltage can be practically ignored.Fixed 1.2 and 2.5 volt integrated bandgap references like the LM336 and LM385 are suitable replacements for low voltage zener diodes in many applications.

author: brownt
author_rank: Regular Contributor
timestamp: July 07, 2021, 12:40:08 am
content: Good point with the power off point.Yes a dual pot will work, and other solutions too. However, the project is such that only a single potentiometer can be used, and devices will be mixed. It's just the way it is, and I need to find a way to deal with that.

author: gcewing
author_rank: Regular Contributor
timestamp: July 07, 2021, 10:42:04 am
content: Connect your single pot to a device that drives a dual pot with a servo.

author: Vovk_Z
author_rank: Super Contributor
timestamp: July 10, 2021, 03:01:21 pm
content: Quote from: brownt on July 06, 2021, 03:48:46 amSorry for being a bit late with my answer: regulating effect (and filtering effect) of a simple Zener regulator is normally somewhere between 5 and 50.