Questions about Zener Diodes

Discussion in 'General Electronics Chat' started by beeson76, Oct 12, 2010.

  1. beeson76

    Thread Starter Member

    Apr 19, 2010
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    This is kind of a followup to a post that I made back in April 2010:

    http://http://forum.allaboutcircuits.com/showthread.php?t=37304

    by me (beeson76)

    I made up a simple little circuit thanks to the help from you guys. It consists of 2 LEDS (Green and Amber--changed the color from Red to Amber.

    There is a tilt switch on the "Green LED" circuit, to where when that tilt switch closes, the Amber light turns off, and the Green light turns on...and vice versa. So with the suggestion of SgtWookie, I replaced the extra LED with a Zener Diode. It worked exactly the way I wanted. But I only had a 1N4733 Zener Diode rated at 5.1 volts. When I put that in it dimmed the lights. So I put 2 in circuit, which completely shut the Amber LED off. I have finally gotten 1N4728 Zener Diodes which are rated at the 3.3v suggested. The same thing happened with putting 1 in (it only dimmed the Amber LED). Putting 2 in completely shut the LED off which is what I am wanting. So the circuit is working exactly the way I am wanting once again--which there is no problem me keeping 2 Zener Diodes in the circuit. But this brings up some questions that I cannot find the answer too about Zener Diodes.

    And instead of giving me answers straight out, I feel that reading would be to my advantage...so if you can point me in the right direction I would appreciate it.

    1. Zener Diodes have a "Zener Voltage" or "Breakdown Voltage"--3.3 for the 1N4728 and 5.1 for the 1N4733. The Voltage in my circuit is 7.5v. From what I understand, anything over the Zener Voltage is allowed to "go through". Is that correct?

    The more I think about it the more confusing it is for me. So I will start with the 1 question and go from there. The schematic to my circuit is also included.
     
  2. Audioguru

    New Member

    Dec 20, 2007
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    You connected your zener diodes backwards so instead of their rated breakdown voltage, they are just two ordinary 0.7V diodes that are conducting with forward bias.
     
  3. beeson76

    Thread Starter Member

    Apr 19, 2010
    185
    1
    You know what AudioGuru. You just answered a bunch of my questions, even without me asking them::)) This is awesome. Thank you very much.

    What was happening was that no matter what Zener Diode I put in, it always acted the same, and that was really confusing me. Now I can put in just one Diode (1N4728) and it turns off the LED completely, without me having to put 2 in. So I am starting to understand the Zener Diode a whole lot more::)) and its starting to make a lot more sense.

    A question though. Where did you get the information that a Zener Diode is .7v? (if put in backwards).

    Thanks again for the reply.
     
  4. Audioguru

    New Member

    Dec 20, 2007
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    A zener diode is supposed to have a reverse voltage across it so it can have its rated breakdown voltage. When it has a forward voltage like shown in your schematic then it behaves the same an an ordinary forward-biased diode with a voltage of 0.7V.

    Simply measure the forward voltage of any zener diode and compare it to the forward voltage of an ordinary diode. They are the same.

    I didn't read the text on your schematic because your schematic was shown side-ways (because the schematic has the switches spread out too much).
    The schematic is supposed to be with the parts with the correct polarity, not backwards regardless of which way you think of the current flow.

    With two 3.3V zener diodes in series with a 1.8V red LED then the red LED will not light (or it will be very dim) when the supply is only 7V because at least 9.4V is needed (including 1V for the 330 ohm resistor). But when the zener diodes are connected backwards then the red LED will be bright unless the 2.2V green LED is turned on.
     
  5. beeson76

    Thread Starter Member

    Apr 19, 2010
    185
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    Please excuse the basic questions. How did you figure that I need 1 volt for the resistor?--is that standard voltage?

    The forward voltage for the Amber LED is 2 volts and Forward Current is 20mA.

    The forward voltage for the Green LED is 2.1volt and the Forward Current is 20mA.

    My AC Adapter is rated at 7.5 but I checked actual voltage of the AC Adapter and is shows about 13 volts. That puts me (which I am pretty sure about my calculations) that I need a 550ohm (560 is the next largest) resistor in my circuit.

    So by what you are saying, and please correct me if Im wrong, is that I need at least 6.4 volts:

    1. 2.1 volts for at least 1 LED being lit at a time.
    2. 1 volt for the Resistor
    3. 3.3 volts for Zener Diode--sorry but schematic is wrong. I only have 1 Zener Diode in circuit.

    Here is an updated schematic with changes so far. And it is now in portrait layout.
     
    Last edited: Oct 12, 2010
  6. beeson76

    Thread Starter Member

    Apr 19, 2010
    185
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    Here is the updated schematic with all changes.

    Is this correct, especially the way the Zener Diode is "pointing". This change was made after reading Volume 1 Chapter 7 of the Manual here on this website under Conventional Flow Vs. Electronic Flow.
     
  7. Audioguru

    New Member

    Dec 20, 2007
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    I used simple Ohm's Law. 1V/330 ohms is produced by an LED current of only 3mA when the LED will be dim. If the LED has a current of 20mA so it is bright then the 330 ohm resistor will have a voltage of 6.6V across it (20mA x 330 ohms= 6.6V). You don't have enough voltage for 6.6V worth of zener diodes, 2.0V for an amber LED plus 6.6V for the resistor.

    I am glad you changed the red LED on your schematic to an amber one.

    You have a cheap unregulated adapter. Its voltage changes with its load current. It might be 13V with no load but its voltage drops when the LEDs are lighting so you won't know its voltage until you measure it with your load connected.

    I don't know why you post a circuit with two zener diodes when you might use only one.
    I don't know why you still show your zener diodes connected backwards.

    I cropped your schematic and made the zener diodes with the correct polarity. Your 1N4728 zener diodes are 3.3V only with a very high current of 76mA. Low voltage zener diodes regulate their voltage poorly so their voltage is less with less current. You should have selected zener diodes that are spec'd at 5mA.
     
  8. beeson76

    Thread Starter Member

    Apr 19, 2010
    185
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    Thanks for the replies AudioGuru. I changed the schematic to the correct way before your last post. You must not have seen it. Sorry about that.

    This gives me something to think about.

    I have some questions right off, but I want to let what you said sink in before I ask them. Usually the answer will come to me.

    Thanks again for helping me.
     
  9. beeson76

    Thread Starter Member

    Apr 19, 2010
    185
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    Here is a quick question that is bugging me.

    You say that a dim LED has 3mA going through it? So this is just current that "trickles" through, but is not enough to brighten the LED. Is this standardized knowledge that is learned? The only thing Im finding about 3mA LEDs are low current LED's that are bright at the low amperage.

    Thanks.
     
  10. Audioguru

    New Member

    Dec 20, 2007
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    I have never seen an LED that is bright at a low current except the cheap ones that are in a case that focusses the light beam into a very narrow angle.
    Most ordinary LEDs are spec'd to be bright when their current is 20mA (it is listed on their datasheet). An LED with only 3mA is not bright.

    The LED's forward voltage, the supply voltage and the resistor value use Ohm's Law for you to calculate the current. Your circuit uses a zener diode or two that also must be included in the calculation.
     
  11. beeson76

    Thread Starter Member

    Apr 19, 2010
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    So in order for the Zener Diode to work I need a current of 76mA? Is that why you recommend Zener Diodes that have low mA. But the voltage is way up there. Can I pull that off?
     
  12. beeson76

    Thread Starter Member

    Apr 19, 2010
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    If I went with a 1/2 watt Zener Diode(1N5226) that would give me 3.3 volts at 20 mA. But if my calculations are correct, my circuit current would be 10mA. Would that work better?

    Thanks.
     
  13. Audioguru

    New Member

    Dec 20, 2007
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    no.
    The zener diode that you selected has a voltage of 3.3V only when its current is 76ma. The high current will cause it to heat which reduces its voltage. Its voltage is less at less current but you don't know how much less until you measure it.
    Low voltage zener diodes are poor voltage regulators so its voltage might be only 3.0V at your low current. You should have selected a 1N5988B that is rated at 5mA. The 1N5226 that is rated at 20mA also will work.
     
  14. beeson76

    Thread Starter Member

    Apr 19, 2010
    185
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    I think I understand it now. So the 1N4728 needs at least 76mA for the Zener Diode to work at 3.3v. So if it has less mAs then the Zener Voltage is also lower, depending upon what the mAs are (In relation with Ohms Law). It looks like the 1N5988 has 3.3v at 5mA. So if current is higher, does this mean it has a high Zener Voltage? I am looking at Digikey for these Diodes and they have a

    Forward (Vf) (Max) @ If

    1.2V @ 200mA

    What does this mean?
     
  15. Audioguru

    New Member

    Dec 20, 2007
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    Yes, but they do not show how much higher.

    A zener diode is usually used in the reverse-biased mode so it zeners at its breakdown voltage. If it is connected like an ordinary forward biased diode then it conducts like an ordinary diode. Its forward voltage is about 0.65V at 10mA and is about 1.0V at 200mA. Its max forward voltage is 1.2V at 200mA.
     
    beeson76 likes this.
  16. beeson76

    Thread Starter Member

    Apr 19, 2010
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    Thanks AudioGuru for all the information. I understand it with your help. Thanks again.
     
  17. marshallf3

    Well-Known Member

    Jul 26, 2010
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    Easier put, a Zener diode operating in the Zener mode works best when it is conducting at least the minimum current reccomendation while not exceeding its wattage capability.
     
  18. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    I think a better way to think of it, to understand why you are using a Zener diode instead of an LED is this:

    An LED sort of acts like a Zener diode. It will not light up or conduct until the minimum Vf is reached, and can only withstand a low amount of current before it overheats and and becomes a Dark Emitting Diode (DED).

    A Zener acts similar, but it is hooked up "backwards". Zeners will not conduct until their rated voltage, and at that point, they ALSO need a current limiting resistor. Before voltage regulators became so inexpensive, a 5.1V Zener was used to prevent too much voltage going into a logic circuit.

    Zener Diodes work on the "Avalanche"/Reverse Breakdown voltage concept, and are designed to sustain multiple breakdowns, whereas standard diodes/LEDs cannot withstand a Reverse Breakdown.

    It would appear that for the tilt switch, you only want the light on in one orientation. The rest of the circuit is designed for lighting two LEDs, so replacing one LED with a Zener of the same voltage as the LED will result in the same current going through the remaining LED, while the Zener will "act" like an LED electrically, but not emit light. The current still needs to be limited through the Zener (usually higher, see spec sheet for device used), just as in an LED.
     
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