overvoltage protection circuit

Discussion in 'General Electronics Chat' started by pjshah72, Dec 17, 2012.

  1. pjshah72

    Thread Starter New Member

    Dec 17, 2012
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    Hi,
    I have a circuit that operates from 0-5VDC. I need overvoltage protection for input of my circuit. (In other words, I do not want to apply anything above 5V as an input of my circuit. so what is the easy way?)

    I really appreciate your response.
    Thank you,
    pj
     
  2. Dodgydave

    Distinguished Member

    Jun 22, 2012
    4,963
    743
    use two 5V zener diodes back to back on the input, or a crowbar circuit.
     
  3. pjshah72

    Thread Starter New Member

    Dec 17, 2012
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    Thank you Dodgydave!!!!!!
    What is the reason for two zener diode? (One is not enough?)
    Should zener(what value?) diodes connected in series or in parallel?
     
  4. JMac3108

    Active Member

    Aug 16, 2010
    349
    66
    By back-to-back he means in parallel but with opposite polarity - to protect against positive and negative voltages greateer than 5V (using 5V zener).

    BUT be aware this only protects against a transient spike on the input. If someone connects a voltage higher than 5V to this it will blow the zener.
     
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  5. ErnieM

    AAC Fanatic!

    Apr 24, 2011
    7,386
    1,605
    And also be aware that a 5V zener comes with a tolerance...meaning it may work under 5V.

    You should be more realistic in your needs too. Hitting 5 volts exactly is first off impossible, but getting very close is expensive.

    Of course, if you want a cheap and dirty solution you just drop a 5 volt low voltage regulator there and forget about it.
     
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  6. pjshah72

    Thread Starter New Member

    Dec 17, 2012
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    Hi JMac3108 and ErnieM,
    Appreciated your inputs.
    I agreed that exact 5V is not possible.

    So basically, I am looking something that protect my circuit fully, evenif someone connects greater voltage than 5V and also it should reset itself (kind of regulator).
    Any known regulator or circuit?
    Any suggestion?

    Thanks,
    pj
     
  7. #12

    Expert

    Nov 30, 2010
    16,247
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    You are being way too vague. From what you've told us so far, "My circuit" could be anything from a Schmidt trigger to a motor for a fishing reel. There isn't a magic protection circuit that protects everything from anything. Give us some details.
     
  8. davebee

    Well-Known Member

    Oct 22, 2008
    539
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    Minor correction: back-to-back refers to diodes connected in series, not parallel.

    Also, some voltage regulators will ignore overvoltage applied to their output pin, so you can't always use one as a voltage clamp.

    If you can limit the overvoltage input current by using a series resistor before the back-to-back diodes that are connected to ground, it will improve the protection a lot. Whether you can do that depends on your circuit, of course.
     
  9. pjshah72

    Thread Starter New Member

    Dec 17, 2012
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    I apologize for using verbal language. I am talking about 4-20mA Input Circuit.
     
  10. bance

    Member

    Aug 11, 2012
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    34
    As Dodgydave sugested try a crowbar circuit, here's a link, you could use a resettable fuse.

    HTH Steve
     
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  11. pjshah72

    Thread Starter New Member

    Dec 17, 2012
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    Hi Everyone,
    I have attached picture of the circuit. (I am not sure where exactly it attached. This is my first time exp. with this site.)

    In the circuit, I have 4-20mA Input that goes into the Opamp through 200ohm resistor. This Opamp works on +5Vdc.

    Now, what should I put in between resistor and opamp so that opamp can survive from overvoltage. (for ex. if someone connects more than 5V or reverse connection.)

    Can I use Clamping circuit? If yes, can any one explain me.

    Thanks in advance.
    pj
     
  12. BillB3857

    Senior Member

    Feb 28, 2009
    2,400
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    I just jumped in to this thread and please correct me if I'm wrong....... Back To Back Zener diodes will NOT protect at the rated zener voltage. The reason being that no matter what the polarity, one or the other diode will be FORWARD biased and look similar to a standard diode limiting at somewhere near 0.7 volts. The proper way to add zener protection using two zener diodes would be to put them in SERIES, either head to head or tail to tail. You would need to subtract 0.7 from the desired protection level in order to select the zener voltage.
     
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  13. JMac3108

    Active Member

    Aug 16, 2010
    349
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    Yep. DaveBee pointed this out this a few posts back :)
     
  14. BillB3857

    Senior Member

    Feb 28, 2009
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    Sorry! I didn't read ALL of the posts, apparently. Just glad that it was cleared up.
     
  15. pjshah72

    Thread Starter New Member

    Dec 17, 2012
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    Thanks BillB3857!!!!!!
    I am not sure but why we need to use two zener diode? why not one zener diode of 5.1V or 5.6V?
    As I said, I am currently working on 4-20mA Input circuit and I am using Resistor of 200ohm. (which means it allows only 0.8V to 4.0V at max.)

    Now I want to simply protect circuit/device beyond 5.0V (keeping 1V margin) if someone apply high voltage (ex. 6, 8, 15 etc.).

    What is the advantage of using two zener diode?

    I will post my first experiment circuit tomorrow.
    Thank you,
    pj
     
  16. BillB3857

    Senior Member

    Feb 28, 2009
    2,400
    348
    The reason for two zeners is to protect the input from either a positive or negative level at the Vz+0.7. If the polarity will always be the same, only one zener would be needed and protect at Vz. It would protect at its rated voltage for one polarity and protect at about 0.7 volts for the other since it would forward bias.
     
  17. pjshah72

    Thread Starter New Member

    Dec 17, 2012
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    Can anyone help me, how to draw or attach schematic here?
    Or how can I create link for the schematic?

    thanks,
    pj
     
  18. pjshah72

    Thread Starter New Member

    Dec 17, 2012
    24
    0
    Hi
    I have attached word document. I have drawn my circuit in it. Please advice whether it is ok or not.
    I am confused for R1 value.

    Thank you,
    pj
     
  19. BillB3857

    Senior Member

    Feb 28, 2009
    2,400
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    Does the line going from the zener/resistor junction indicate going the Gate of the SCR? The way it is drawn, it goes to ground and the item labeled SCR is a just a diode. To post a drawing, it is strongly suggested that it be in the PNG format and you can upload it by using the Go Advanced tab at the bottom of the Quick Reply box.
     
  20. pjshah72

    Thread Starter New Member

    Dec 17, 2012
    24
    0

    Hi BillB3857,

    Yes, the line going from the zener/resistor junction to the Gate of SCR. (My fault that I did not mention about SCR. It is a SCR but I do not have symbol for it so I used diode and labeled SCR.)

    Is it going to work for my purpose?

    thanks for quick response.
    pj
     
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