Analog input spike protection from 120VDC

Discussion in 'Analog & Mixed-Signal Design' started by franzschluter, Jul 25, 2011.

  1. franzschluter

    Thread Starter Active Member

    Jun 1, 2009
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    Hi.

    I need some advice. I have an EDM setup.. On the electrode there is a ~120VDC PWM open-voltage signal...

    Across it there is a voltage divider.. to throttle down the voltage to 5V max..
    Though I am worrisome about voltage spikes ruining my arduino board. How should I tackle this?

    1. Op-amp? (I dislike op-amps because of the need of dual-supply)
    2. Transil?

    In my current setup I use an LM339 to do the comparing of the signal? Would it be safe to assume that this signal is ok for a PICs analog input?

    Regards
    Franz
     
  2. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    A schematic or sketch would help, but I think I see what you are doing.

    It sounds OK. Did you do the divider? I would make it cut the 120V to 2.5 V just to get a bit more margin, but 5V is OK. Send 2.5V (or 5V) pulses to the LM339 so the LM isolated the arduino from the big volts. You can use a diode to 5V as protection after the divider.
     
  3. franzschluter

    Thread Starter Active Member

    Jun 1, 2009
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    0
    Hi,

    Do I do this in parallel? Your right I could bring it down to 2.5V. What scares me is if voltage spikes might be able to go through or something... Im not so cool about having the analog pins right out of a voltage divider from 120VDC PWM... I think the electrode voltage is somewhat dirty and may contain spikes..

    Originally it had/has an LM339 across the voltage divider and a potentiometer to adjust the reference.. Now I'd like to do this all digitally with the built-in ADC from the arduino kit..

    Id just like some advice if this looks safe enough for the analog input or perhaps I should place a transil? Op-amp? Or this setup should be fine?

    Take note that the voltage source 120V is PWM and that the example IC is not the IC I have. Its just an example. The electrode voltage of the EDM is usually more capacitive I think but i'm not sure...

    But in principle it functions the same as a Torch Height Control of a Plasma Cutter. Such that the distance of the part to be cut and the electrode is set by a given voltage level.

    Regards
    Franz

    [​IMG]
     
  4. be80be

    Senior Member

    Jul 5, 2008
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    I would add a 5 volt zener diode to clamp the max voltage to 5 on the ADC pin
     
    Last edited: Jul 25, 2011
  5. bertus

    Administrator

    Apr 5, 2008
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    2,347
    Hello,

    I would use a diode clamp circuit.
    The input voltage can be between - Vd to Vsupply + Vd.

    Here is an example used on the inputs of opamps:
    [​IMG]

    Bertus
     
  6. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    Well the first good news is RA0 on a PIC16F84 HAS protection diodes like bertus suggested built right in, so you may not need to add anything external. Microchip previously published schemes to detect the AC line zero crossing by directly connecting an I/O pin to the AC line thru several hundred kilo ohms, though lately they are backing off this suggestion as recent devices do not always have these built in protection diodes. They do still recommend it, but with reservations.

    I've done it myself on a breadboard for an AC light dimmer where the PIC sits on the AC line neutral to control a triac.

    120V thru 100K is only 1.2 mA, not a very excessive current. That's what would flow if your R4 opens.

    So you may be quite fine just as you are.

    (But the PIC16F84 has no analog functions so this PIC is no good if you are intending to measure the 120V with that PIC. I think you may need to PICk another PIC.)
     
  7. franzschluter

    Thread Starter Active Member

    Jun 1, 2009
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    Thanks for the confirmation. I was unsure. The PIC was just an example. I will be using the Atmega series that usually goes with an arduino kit.

    I will go through with the clamp diodes just to be sure and on the safe side

    Regards
    Franz
     
  8. be80be

    Senior Member

    Jul 5, 2008
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    Atmega series have better protection diodes then the pic chips
     
  9. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    An extra external diode can't hurt and may help. Even better is to make it a schlocky diode.
     
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