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Surge / overvoltage input protection?

Discussion in 'Electronic Basics' started by MRW, Jul 25, 2007.

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  1. MRW

    MRW Guest

    Hi. I've read some articles about surge input protection; I think it
    was regarding switches. Basically, they had a diode's anode connected
    to the input terminal and the cathode connected to the voltage source.
    It looks like this:

    http://bayimg.com/kaebDaaBj

    My question is that most diode datasheets that I've seen shows the
    minimum forward voltage drop as somewhere around 700mV. Another article
    that I read (I think this is about opamps this time), mentioned that
    the input is capable of handling a maximum signal amplitude that is VCC
    + 0.3V.

    In this situation, does that mean that I would need a diode with a
    lower forward voltage drop?

    Also, I seem to be confused as to how this type of diode protection
    scheme works. Using the picture link, if the signal is higher than the
    VCC voltage by an amount greater than the diode forward voltage, then
    the diode conducts. So that would mean that the input is now at VCC
    voltage levels? If so, then wouldn't the diode "open" up again since
    the anode is now at the same voltage level as VCC?

    Thanks!



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  2. Often a second diode is connected between the input and the
    ground rail to do the same thing for negative spikes.
    Usually, the 300 mV limit is an operational one (possible
    malfunctions if exceeded), and there is a higher absolute
    maximum (possible damage if exceeded) spec.
    A lower drop diode, such as a Schottky or germanium, is
    sometimes used, but these also have higher leakage current,
    so may not by useful for very low current signals.

    But in many cases, the 300 mV limit can be exceeded a bit,
    without damage, if the input current is limited. This might
    be accomplished by using an ordinary silicon diode (that may
    drop a volt or so while it is passing a surge current) and
    adding a current limiting resistor between the diode and the
    chip input. Sometimes it is possible to choose such a
    resistor that will safely limit the surge current from a
    source 1 volt higher then Vcc but below enough to not
    interfere with normal signals.
    Right. And if the surge source impedance is considerably
    higher than the impedance of the Vcc source, most of the
    rest of the surge voltage is dropped across the surge source
    impedance. However, if the Vcc source impedance is high,
    the surge may just jack up Vcc to damaging levels. For this
    reason, some designs use a zener diode to ground, that
    breaks down and conducts the surge current to ground when
    the voltage gets close to Vcc. I have seem 4.8 volt zeners
    used with 5 volt Vcc rails, this way, especially for logic
    signals that don't have to go all the way to Vcc to work.
    For negative spikes, the zener acts like a normal diode in
    forward conduction. The weakness of the zener approach is
    that it lets the input go all the way to zener break down
    voltage, even if the chip being protected is unpowered. In
    that case, the chip ends up seeing an input that is a lot
    more than a volt higher than its Vcc.
    No, Vcc plus the diode forward drop voltage.
     
  3. Eeyore

    Eeyore Guest

    And why do you think your equipment's inputs needs protecting from these
    'surges' ?

    Graham
     
  4. MRW

    MRW Guest

    Thanks, John!


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  5. MRW

    MRW Guest

    Hi Graham. Oh, I was just curious. I figured it'd come handy sometime
    in the future when I encounter something like this.


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  6. Eeyore

    Eeyore Guest

    Powerline 'surges' (let's call them transients shall we?) if that's what you
    meant are very unlikely to be an issue for equipment inputs (or outputs).

    The biggest problems for your ins and outs are likely to be electrostatic
    discharges and RF pickup.

    Graham
     
  7. MRW

    MRW Guest

    Gotcha. I was actually thinking more of the CMOS switch application
    notes that I've seen. They had were depicting a sine wave input that
    was much greater than VCC. One solution they mention was using diodes.
    I've probably seen that configuration before in some textbook, but I
    guess I paid attention to them the second time around. Thanks!


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  8. Eeyore

    Eeyore Guest

    There's a couple of things worth noting here.

    Firstly it's not a good idea to use CMOS switches with voltages larger than
    their power supplies !

    Secondly, mamy ICs have an internal 'parasitic' diode structure to the supply
    rails on input nodes. Combined with a modest value current limiting resistor
    these may be able to handle at least transient excursions beyond the power
    supply voltage.

    If you regularly expect excessive voltage inputs then it's a good idea to fit
    additioanl diodes to deal with it. Do bear in mind that the input has to be
    current limited for these to be effective.

    It's good to see you're aware of this issue.

    Graham
     
  9. Manoj

    Manoj Guest

    Dear friends

    Surges may come due to many reasons. they can be as high as hundred of
    volts. Suppose the connection is made to wires going outside the
    equipment. these wires might be long. the longer the worst is the
    case. when the switch opens (the worst condition is opening and not
    closing), due to inductance of the wire, there will be a spike good
    enough to damage the cmos ics. surges can also be due to noisy
    envoirnment.

    please correct me if i am wrong.

    the internal protective diodes of the cmos are not very great. if you
    expect surges / spikes you should always put external diodes. but a
    series resistor before the diode is also a must. without this series
    resistance a spike of >50 Volts would mean the whole current passes
    through the diode with negliglible source resistance. this can damage
    the diode itself.

    with regards

    manoj
     
  10. Eeyore

    Eeyore Guest

    You're quite correct about that possibility.

    Exactly.

    Graham
     
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