Zener diode not Zener-ing

Discussion in 'General Electronics Chat' started by Johnnymysto, Feb 1, 2007.

  1. Johnnymysto

    Thread Starter New Member

    Feb 1, 2007
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    I'm trying to use a Vishay 3.3V Zener for a reference voltage in an amp circuit. The problem is that it's not maintaining 3.3VDC. I'm working with a 20VDC supply, and using a 10K resistor in series with the Zener. As I raise the power supply to 20V, the Zener's voltage goes up and up and up - never stopping at 3.3V. I've got it connected properly, so what am I doing wrong?

    Thanks!
     
  2. hgmjr

    Moderator

    Jan 28, 2005
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    Can you provide a diagram of your circuit hookup and a part number for the zener diode you are using?

    At first blush, it sounds as though the zener has been damaged and is no longer acting as a zener.

    hgmjr
     
  3. Johnnymysto

    Thread Starter New Member

    Feb 1, 2007
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    Sure! Here's a diagram, and the part number is 1N4728A.

    Thanks,
    Eric
     
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  4. hgmjr

    Moderator

    Jan 28, 2005
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    I see nothing in the circuit that sets off any alarm bells. Your maximum zener voltage is not in excess of the max current for the zener. You may need to provide some hysteresis (positive feedback) to prevent the output of the LM358 from oscillating that is likely to occur as your signal crosses the threshold setpoint.

    Your circuit appears to be detecting when the total supply voltage falls below the 15.52 volt level.

    Suggest you try a second zener to see if it behaves in the way.

    How are you measuring the voltage across the zener? Are you using a Digital Voltmeter or some other means?

    hgmjr
     
  5. Johnnymysto

    Thread Starter New Member

    Feb 1, 2007
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    Yes, I'm using a Fluke digital multimeter. And you're right about detecting the 15.52 volt level.

    I'll try another Zener. I did have a 12V Zener laying around that I put in there - just to make sure I was wiring and measuring properly - and it was doing the same thing, i.e. not maintaining 12V. But maybe it was bad, too.
     
  6. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    What is the voltage you measure? I would try using higher current through the zener, maybe it will help.
     
  7. Gadget

    Distinguished Member

    Jan 10, 2006
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    Dunno about why the Zener is misbehaving, but there is a good chance that the LM358 is misbehaving if you have 40 volts (+/- 20 volts) supplying it !!.
     
  8. Johnnymysto

    Thread Starter New Member

    Feb 1, 2007
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    Sorry, I mislabeled the schematic. That "20V-" should actually be labeled as ground.
     
  9. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    So what is the voltage you measure at the zener?
     
  10. Johnnymysto

    Thread Starter New Member

    Feb 1, 2007
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    When the supply is at 20VDC, I measure 2.05 V across the Zener.
     
  11. Gadget

    Distinguished Member

    Jan 10, 2006
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    You've got 1.7 mA current thru the zener.
    Try reducing the value of the 10k resistor.

    Try a 1k 1/2 watt resistor..(it will dissapate a little over 1/4 watt). This will up the current by a factor of 10, the zener will still be opperating at only a 20th of its max wattage. If its still not zenering, then there may be a problem with the zener or the input on the 358.
     
  12. hgmjr

    Moderator

    Jan 28, 2005
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    So your problem is not that the zener is exceeding the 3.3 volt but rather that it never reaches 3.3 volts. Your initial posting seemed to indicate that the voltage did not stop at 3.3 volts so I interpreted your statement incorrectly.

    I agree with Gaget. If you take a look at the amount of current that it takes to cause the zener to reach the zener voltage you will see the problem. The 1n4728A takes 76 milliamps to reach the 3.3 zener voltage. Your circuit only delivers around 1.7 milliamps. You need to drop your 10K ohm resistor to 1K and see what that does for your circuit.

    hgmjr
     
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