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    ULN2803 as Input Protection

    Hello everyone!

    The ULN2803 is commonly used in MCU projects to drive high-voltage or high-current loads. But I have been thinking about using the ULN2803 as a high voltage input protection device to drive MCU digital input pins. The schematic is attached.

    Click image for larger version. 

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    Is this possible? Is this feasible? Is there a better way (with less components, with less costly components) to do the same thing?

    Thanks in advance to all who reply.

    Cheers.

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    Re: ULN2803 as Input Protection

    A series resistor with clamping diodes has the same or better protection effect.



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    Re: ULN2803 as Input Protection

    Hi,

    For sure it's possible..

    But I don't see the benefit.
    The protection grade is doubtful. Is it able to protect itself against negative ESD?
    High leakage current, slow switching speed, unnecessary high drive current, not extremely cheap, no precise switching thresholds.

    Why not use a 74hc14?
    Or a resistor capacitor network and true ESD protection devices...with specified protection grade?

    Klaus
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    Re: ULN2803 as Input Protection

    I agree with FvM and Klaus, it will work but why waste board space and money when a cheaper solution is better.
    Just add a series resistor the a pair of small signal Schottky diodes (BAT54 for example) from the MCU pins to GND and VSS. If space is very short, you can use a simple Zener diode rated at VDD or a little less at the MCU pin to clamp excess voltage, it will protect against reverse voltages and excess forward voltage although not as well as the two diode solution.

    Brian.
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    Re: ULN2803 as Input Protection

    So, putting all your comments (thank you) together, would you agree that the following would be a better circuit design?

    Click image for larger version. 

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    - - - Updated - - -

    Quote Originally Posted by KlausST View Post
    Hi,
    Why not use a 74hc14?
    Or a resistor capacitor network and true ESD protection devices...with specified protection grade?
    What do you mean exactly by true ESD devices?



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    Re: ULN2803 as Input Protection

    Yes, but all you really need is one series resistor and the two diodes, the other components won't stop it working but they probably are not needed. Also note that if your MCU needs to respond quickly to a change in input, the capacitor has to charge/discharge first through the resistors so it will delay the signal a little.

    If you really need a capacitor, for example in a very noisy environment, drop its value and increase the resistor value so you get the same filtering effect and also get better protection against input over-voltage. For example, change 1K to 10K and 10nF to 1nF. If you need the pull-up resistor R2, make it much higher in value, maybe 47K or 56K so you can still pull the MCU input low enough when the input is grounded.

    Brian.
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    Re: ULN2803 as Input Protection

    Hi,

    What do you mean exactly by true ESD devices?
    Not "ESD device" but "ESD protection device".
    --> http://lmgtfy.com/?q=esd+protection+device

    ESD = electrostatic discharge (one possible cause of destroying inputs)

    Example: IP4220 (one of countless others)

    Klaus
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    Re: ULN2803 as Input Protection

    Quote Originally Posted by KlausST View Post
    Hi,
    Not "ESD device" but "ESD protection device".
    My bad. I obviously meant ESD protection devices. I am familiar with TVS diodes, but they tend to be expensive, even in bulk. The only ones I know to be remotely affordable are the SMAJ5.0A series. Are there any others you'd recommend (bearing cost in mind)?



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    Re: ULN2803 as Input Protection

    Hi,

    Are there any others you'd recommend (bearing cost in mind)?
    Many thousands.
    And why? Because there are many applications with different requirements.
    There is no "one and only" best, cheapest, most robust, high speed.....

    Nowadays we have internet. So we are able to simply find everything we need on our own.
    Semiconductor manufacturers as well as distributors build online selection tools exactly for you.
    So use it.

    Klaus
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