Zener diode

Discussion in 'General Electronics Chat' started by Webby, Aug 31, 2008.

  1. Webby

    Thread Starter Active Member

    Jun 15, 2008
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    Reading an article about semi conductors, what is meant by Zener diode breakdown voltage (avalanche point)?

    How is the determined breakdown point controlled?
     
  2. hgmjr

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    Jan 28, 2005
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    This link to the topic of zener-diodes located in the AAC ebook covers gives a good explanation of the zener breakdown voltage along with several other characteristics of the device.

    hgmjr
     
  3. Webby

    Thread Starter Active Member

    Jun 15, 2008
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    Hi hgmjr, read through the manual and cant get my head around it. Understand the principle of a convential diode struggling with the Zener type.
     
  4. Wendy

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    Mar 24, 2008
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    I'm a firm believer in keeping it simple. Basically instead of burning out as a conventional diode does when its Peak Inverse Voltage rating is exceeded, the zener allows anything over its PIV rating to flow through, until it can no longer handle the heat (basically it's wattage rating).

    The graph of this property isn't perfect, but it can come close. The low voltage versions (around 3.3V) are pretty mushy (the knee of the curve is sloppy), but it still works.
     
    Last edited: Aug 31, 2008
  5. Webby

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    Jun 15, 2008
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    Hi Bill, getting without now slowly... what type of circuits would use such type of diode? When in a circuit does reverse current start to flow in a causing the diodes to burn out?
     
  6. Wendy

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    Mar 24, 2008
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    The burn out is a wattage issue, similar to what a resistor has. Matter of fact, most zeners have a wattage rating.

    The primary use for zeners is that of voltage reference for a voltage regulator. In it's simplest form it is a voltage regulator, but you get better results keeping it's current stable and then powering drive circuitry from it.

    In a conventional diode when the diode begins to conduct backwards the damage is done, the PN junction has been compromised.
     
  7. Webby

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    Jun 15, 2008
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    Hi Bill, thats great thanks for the information with a conventional diode when would it conduct backwards for instance?
     
  8. Wendy

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    Mar 24, 2008
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    When it died because too much voltage was applied backwards. The PN junction has distructively broken down and the little molten bits are shorting the inside of the case. :D This is why PIV is fundimental rating, they don't recover from these failures.

    Another use for zeners is since they don't distructively die the same way they can (in theory) recover. They also make good low voltage surge supressors for the same reasons.
     
  9. Webby

    Thread Starter Active Member

    Jun 15, 2008
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    Hi Bill, is this the same when a diode is fitted in reverse bias in a circuit causing current to flow the wrong way?
     
  10. Wendy

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    Mar 24, 2008
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    Conventional diodes will not conduct if biased backwards, until their PIV rating is exceeded. They are then ruined. Basically it is all or nothing.

    This property is why they are considered switching devices.

    Zeners act like conventional diodes until their zener rating is reached, at which point they begin to conduct nondistructively. At this point wattage is important, voltage (zener rating) X current (amps) = Wattage.

    In many cases you can use a zener as a conventional diode and never know the difference.
     
  11. Webby

    Thread Starter Active Member

    Jun 15, 2008
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    Hi Bill, been studying up on these semiconductors can you please explain what is meant by the forward voltage drop on a zener diode. If it is between 0.5v-0.7v and lets say connected to a 12v circuit does this mean the voltage is 11.5v?
     
  12. Wendy

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    Mar 24, 2008
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    Nope, like I said, they look like a regular diode most of the time. When you back bias them and exceed their zener voltage they begin to conduct. Like regular diodes, they drop .7 volts FORWARD biased. The mode most zeners are used in is back biased, this is where their unique properties assert themselves.
     
  13. SgtWookie

    Expert

    Jul 17, 2007
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    Zener diodes are seldom used forward biased. An exception to that would be when they are used back-to-back in order to limit peak-to-peak voltages between two signal lines.

    If you connected two 4.7v Zener diodes back-to-back across a pair of signals, once the voltage reached near the sum of Vf + Vr, they would conduct and "clamp" or limit the voltage peaks.

    [eta]
    Have a look at the attached schematics. Basically, the circuit on the left is equivalent to the circuit on the right.

    Note that the outputs are so near to identical that the traces even overlap each other on the O-scope simulation.
     
    Last edited: Aug 31, 2008
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