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Zener shunt regulator - was 'Is my Zener broken' (it's not) — Parallax Forums

Zener shunt regulator - was 'Is my Zener broken' (it's not)

W9GFOW9GFO Posts: 4,010
edited 2009-10-20 14:56 in General Discussion
I have exactly one 5.1V zener diode to experiment with. I dug it out of an old Radio Shack electronics kit. I have breadboarded a simple circuit so that I can see how it behaves and if it will work the way I want. I'm using a 12V power supply going into a pot to adjust voltage.

My understanding is that if there is 4 volts applied to the reversed biased 5.1V zener that it should show no voltage across the 1K resistor that connects it to ground. Instead I show 50mV. At 4.98V input it increases to 360mV.

Do I just have a crappy zener or is that typical behavior?

Rich H

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Post Edited (W9GFO) : 10/18/2009 12:34:08 AM GMT

Comments

  • PJAllenPJAllen Banned Posts: 5,065
    edited 2009-10-17 01:41
    There'll be a small amount of leakage current as reverse bias increases·(hence the voltage across your resistor) till you hit zener breakdown.
  • PJAllenPJAllen Banned Posts: 5,065
    edited 2009-10-17 01:55
    Take data, every 200mV from 0V to 7V, and report back same.
  • W9GFOW9GFO Posts: 4,010
    edited 2009-10-17 02:40
    PJ Allen said...
    Take data, every 200mV from 0V to 7V, and report back same.
    Sure. Data is good.
    INPUT       Voltage across resistor (millivolts)

0V to 2.0V       0.0
2.2V             .2
2.4V             .4
2.6V             .7
2.8V              2
3.0V              3
3.2V              6
3.4V             11
3.6V             20
3.8V             33
4.0V             53
4.2V             80
4.4V            135
4.6V            193
4.8V            281
5.0V            376
5.2V            488
5.4V            627
5.6V            768
5.8V            924
6.0V           1096
6.2V           1255
6.4V           1425
6.6V           1607
6.8V           1787
7.0V           1961



    Rich H

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  • Mike GreenMike Green Posts: 23,101
    edited 2009-10-17 03:33
    That's about what I would expect. By about 6.8V of applied voltage to the whole circuit, the Zener voltage seems to have settled down to about 5.1V. The Zener you have probably has at best a 10% tolerance. That's +- 0.51V. By 5.0V of applied voltage, the voltage across the Zener is already within this.
  • PJAllenPJAllen Banned Posts: 5,065
    edited 2009-10-17 03:57
    Did you learn anything?
  • W9GFOW9GFO Posts: 4,010
    edited 2009-10-17 05:47
    PJ Allen said...
    Did you learn anything?

    I learned that adjusting voltage by way of a 5k ohm trimpot to mV resolution seventy times may not have been the best usage of my time. I am going to change my plan slightly and use the zener as a shunt regulator since it will not work as I originally intended due to it "turning on", if only slightly, before it's breakdown voltage.

    I also learned that zener diodes are not quite what I had understood them to be. Which is to say they are not devices which only conduct once a certain voltage is reached.

    Thanks for the help,

    Rich H

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  • Mike GreenMike Green Posts: 23,101
    edited 2009-10-17 06:09
    For that, you need a comparator like the LM339 quad comparator. This compares an input voltage to a threshold voltage and produces a digital high/low signal when that threshold is passed. Since the LM339 draws essentially no current from it's voltage inputs, you can use a simple voltage divider off the regulated +5V supply for the reference voltage.
  • W9GFOW9GFO Posts: 4,010
    edited 2009-10-17 07:06
    Originally I wanted to use the zener and a transistor to either send voltage directly to power an LCD or send it through a regulator if it was over 5 volts - that didn't work out.

    The LCD needs about 4.3 volts to be readable so if I have a five volt battery pack as the power supply I am left with 4.3 volts after going through the protection diode. That would work out quite well except that sometimes the input may be up to nine volts so there needs to be some regulation. It is looking like I can just ditch the 5 volt regulator and instead use the zener to shunt any excess voltage. It is a little wasteful at higher voltages but usually the input power will be 5 to 6 volts.

    Anything wrong with that approach?

    Rich H

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  • Carl HayesCarl Hayes Posts: 841
    edited 2009-10-17 12:50
    W9GFO said...
    PJ Allen said...
    Did you learn anything?

    I learned that adjusting voltage by way of a 5k ohm trimpot to mV resolution seventy times may not have been the best usage of my time. I am going to change my plan slightly and use the zener as a shunt regulator since it will not work as I originally intended due to it "turning on", if only slightly, before it's breakdown voltage.

    I also learned that zener diodes are not quite what I had understood them to be. Which is to say they are not devices which only conduct once a certain voltage is reached.

    Thanks for the help,

    Rich H

    Not the best use of your time?· Gosh, man, you may have saved 43 cents!· In these economic times, how much do you want for a couple hours work?

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    · -- Carl, nn5i@arrl.net
  • PJAllenPJAllen Banned Posts: 5,065
    edited 2009-10-17 13:22
    From 0V to 7V, every 200mV, is only 36 data points (0 incl.)·

    It was a genuine "teachable moment."· Some people pay good money for this sort of thing, mon ami.
  • Mike GreenMike Green Posts: 23,101
    edited 2009-10-17 13:35
    W9GFO,
    It sounds like you really need a buck/boost switching regulator. This will take voltages both above and below some design voltage and transform them to a well-regulated output at the design voltage. It's not as simple as a resistor and a Zener plus a transistor switch plus a linear regulator, but, when you're done, it'll work better.

    Here's one example: focus.ti.com/lit/ds/symlink/tps55065-q1.pdf
  • W9GFOW9GFO Posts: 4,010
    edited 2009-10-17 15:51
    PJ Allen said...
    From 0V to 7V, every 200mV, is only 36 data points (0 incl.)

    Well there were two measurements for every 200 mV right? I did cheat a bit and skip the measurements from .4V to 1.6V since there was no change between .2V and 2.0V (I skipped zero too). I should have said that I adjusted the voltage 28 times and took 56 voltage measurements, no, that wouldn't be accurate either because many of the measurements were repeated. I guess I shouldn't have said anything.

    Hmmm, now I am wondering... I had already learned that my zener leaked more current as the voltage increased (prior to reaching the breakdown voltage) before you requested the data points. Perhaps I have missed your point and not learned what you had intended. It wouldn't be the first time that a concept sailed over my head.

    I can add that I was also under the misconception that once a zener reached it's breakdown voltage it would allow that voltage to pass through it, unhindered. Instead, that voltage is consumed by the zener. It would have made more sense to me if a zener was described as a device that would drop a certain voltage through it (when reverse biased) equal to it's zener rating.

    Apparantley, I am still a noob. smile.gif

    Rich H

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  • sylvie369sylvie369 Posts: 1,622
    edited 2009-10-17 16:02
    Carl Hayes said...

    Not the best use of your time?· Gosh, man, you may have saved 43 cents!· In these economic times, how much do you want for a couple hours work?
    Boy, did I ever save money by just reading the thread after the fact. I learned a lot here.

    Of course now I desperately want hook up a diode and start measuring voltages.
  • W9GFOW9GFO Posts: 4,010
    edited 2009-10-17 16:28
    Mike Green said...
    W9GFO,
    It sounds like you really need a buck/boost switching regulator. This will take voltages both above and below some design voltage and transform them to a well-regulated output at the design voltage. It's not as simple as a resistor and a Zener plus a transistor switch plus a linear regulator, but, when you're done, it'll work better.

    I've looked into those before, so many pins!

    I tried this simple circuit;

    zener-diode-voltage-regulator.gif

    It seems to work ok using a 68 ohm resistor. I can see a disadvantage in that if the zener has a bad connection there will be no regulation other than the current limiting provided by the resistor. Also, my 5.1V zener allows the voltage to climb to 5.3V when the input is 8V. Maybe I need a lower value resistor, I don't have the specs for the current draw of the LCD so it is trial and error with the resistor value. A tighter tolerance zener would help too I'm sure.

    Rich H

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  • Mike GreenMike Green Posts: 23,101
    edited 2009-10-17 16:28
    W9GFO,
    The Wikipedia is your friend: en.wikipedia.org/wiki/Zener_diode

    Zeners are not great voltage regulators for some of the reasons you've noted. They're pretty good voltage references where the amount of current needed is small and relatively stable.
  • W9GFOW9GFO Posts: 4,010
    edited 2009-10-17 17:14
    Wikipedia said...
    Zener diodes are widely used as voltage references and as shunt regulators to regulate the voltage across small circuits.

    Forgive me for being dense, but I am not understanding.

    I only wish to use the zener to regulate (limit) the voltage going to the LCD, which is tolerant of a slightly variable voltage, everything else on the board gets regulated 3.3V. To use a buck/boost converter would require a high pin count chip, several resistors, capacitors, inductors and a shottky diode plus a fair bit of real estate on the PCB - all to provide a steady 5.0V to exactly one point. Is that really the most sensible approach? My first choice would be to use a 3.3V LCD but that is not an option at this point.

    The zener is not ideal but seems like it would do the job. Other than the reasons noted, which are tolerable I think, would it be bad to use this method?

    Rich H

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  • Mike GreenMike Green Posts: 23,101
    edited 2009-10-17 17:51
    Yes, you can use a Zener diode as a shunt regulator. The term "shunt" refers to the fact that the Zener has to be in parallel with the load and acts as a current "shunt", drawing excess current when needed. The series resistor is what drops the input voltage to the desired voltage and, for best stability, has to be chosen so that the current through the Zener is significant throughout the range of input voltages. In your case, you probably want your input voltage to be at least 6V. I don't know what the LCD draws, but you should measure it under several different conditions. I'd assume that it's at least 20-30mA when the backlight is on and maybe a couple of mA when off. You have to choose the resistor to handle this and the Zener would shunt at least 20-30mA when the backlight is off. That's 100mW to 150mW of heat to dissipate at a minimum, so make sure your Zener can handle that.
  • sam_sam_samsam_sam_sam Posts: 2,286
    edited 2009-10-17 20:53
    Here Is a small power board that I use to power My 4 Line LCD Display with back light ON ·and a Basic Stamp

    They only cost $8.00 per board


    http://www.futurlec.com/Mini_Power.shtml

    ·

    [url=:popitup('../Pictures/ET-MINI_POWER_SUPPLY_600.jpg')]ET-MINI_POWER_SUPPLY.jpg[/url]

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    ··Thanks for any·idea.gif·that you may have and all of your time finding them smile.gif

    ·
    ·
    ·
    ·
    Sam

    Post Edited (sam_sam_sam) : 10/17/2009 9:00:21 PM GMT
  • ercoerco Posts: 20,260
    edited 2009-10-17 23:29
    I must be the last guy to still use 7805 linear regulators, but they are simple, nearly bullet proof, and give you a rock-solid 5.0 volts out for about 5.5 volts in (and everything above). A 7805 functions essentially as a self-regulating resistor to deliver steady output voltage, so they become less efficient and simply get hot as

    1) current increases, and
    2) the input voltage increases

    They're cheap (maybe 25 cents each in a multi pack), ultra-simple to use (3 pins), deliver excellent regulation, and perfect for a low-current LCD display, even for a battery-powered device.

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
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  • W9GFOW9GFO Posts: 4,010
    edited 2009-10-18 00:27
    Just a reminder, what I needed was a way to provide as much voltage as possible to an LCD when the source may be as low as 4.3 volts. The voltage drop through a 7805 is too much, the LCD fades away at around 4.3 volts.

    I picked up some zeners from RS. They can handle 1 watt but require 49mA, much more than the typical 5mA ones I found on Mouser. Using it along with a 47 ohm resistor is working nicely. I'll order some 5mA ones so as not to waste so many electrons. This LCD has no backlight but if it did I think I would be able to power it via the regulated 3.3V.

    Rich H

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  • Beau SchwabeBeau Schwabe Posts: 6,568
    edited 2009-10-18 16:10
    As indicated, Zener diodes aren't really designed to handle a great deal of current. Instead they are used as a current or voltage reference to drive a device which can handle more current.

    Here is a basic regulator using a Zener.

    www.elecfree.com/electronic/misc-regulator-circuits-by-2n4401/

    reg4.thumbnail.gif

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    Beau Schwabe

    IC Layout Engineer
    Parallax, Inc.
  • Lab RatLab Rat Posts: 289
    edited 2009-10-19 03:42
    i am curious as to why the voltage is fluctuating so much. if it is battery operated just replace the batteries when you can no longer read the lcd. me personaly none of my projects are battery operated unless you count car batteriesthat i use as a way of supressing or filtering my electric along with saving some to start the motor for the generator. my curent project uses a chainsaw motor powered with propane to run the genorator to power the machine

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    ·
  • W9GFOW9GFO Posts: 4,010
    edited 2009-10-19 04:18
    It is because it will usually be powered by a four cell battery pack, but other times a five cell - or possibly even a two cell LiPo.

    Rich H

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  • Lab RatLab Rat Posts: 289
    edited 2009-10-20 02:32
    what ever happened to a good old nine volt

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    Parallax posesses power beyond belief.

    Believe in it.
    Visit me at
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    ·
  • W9GFOW9GFO Posts: 4,010
    edited 2009-10-20 05:21
    You won't find many nine volts in this application (RC receivers).

    Rich H

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  • mctriviamctrivia Posts: 3,772
    edited 2009-10-20 14:54
    nine volts suck. they don't provide much power.

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  • mctriviamctrivia Posts: 3,772
    edited 2009-10-20 14:56
    W9GFO I would use a buck boost circuit. It will let you squeak out every last drop out of your battery and give you a steady output voltage. The only down side is they do waist a bit of power but I suspect you will gain it back by being able to drain the battery down more.

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    Want to make projects and have Gadget Gangster sell them for you? propmod-us_ps_sd and propmod-1x1 are now available for use in your Gadget Gangster Projects.

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