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Figuring voltage from an current sense coil? — Parallax Forums

Figuring voltage from an current sense coil?

James LongJames Long Posts: 1,181
edited 2009-05-24 21:09 in General Discussion
I have to admit, I'm not sure where to start, so I figured the guru's here could point me in a direction.

First let me explain what I'm trying to do. There is a system which produces pulses on a wire at 2500-6000 Hz, with a voltage of about 290 volts. I would like to read the frequency of this wire.

I'm pretty sure I do not want to interface with this wire directly, but thought about using a sensor coil. I'm lost as to figuring the limiting resistor for the opto isolator from the coil. I'm being very careful about this voltage (or noise) getting into my circuitry, so I'm using a sense coil to an opto-isolator which I'll be reading the output of.

How does one figure the voltage from a sense coil. Is it a simple matter of actually reading the voltage from a meter, or can it done (relatively close), with just math?

James L

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James L
Partner/Designer

Lil Brother SMT Assembly Services

Please note: Due to economic conditions the light at the end of the tunnel will be turned off until further notice. Thanks for your understanding.

Comments

  • PrettybirdPrettybird Posts: 269
    edited 2009-05-23 01:54
    My background is industrial electronics. I know and have used doughnots (current sensing coils) in older 3 phase motor panels. They are just a basic transformer. I use to use a small doughnut from a swithching power supply to read relative current changes on AC servo motors. I just wanted to see change not really in amps. The math would be too confusing to me. I was just a machine repair electrician / technician. I hope you find this imformation usefull. Google current transformers. It may be usefull.

    The more you learn, the more you find you don't know. it is neverending.
  • PrettybirdPrettybird Posts: 269
    edited 2009-05-23 01:59
    Forgot to mention frequency. Sorry. Just get a frequincy counter. Ham radio people been using them for years. low frequencys are vere cheap now. You not in the microwave (gig range) I think.
  • James LongJames Long Posts: 1,181
    edited 2009-05-23 02:04
    I was starting to think, I could probably just use a hall effect sensor heat shrinked to the wire. That would give me frequency without the need for the opto-isolator, although probably still a good idea.

    Any other thoughts??

    James L

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    James L
    Partner/Designer

    Lil Brother SMT Assembly Services

    Please note: Due to economic conditions the light at the end of the tunnel will be turned off until further notice. Thanks for your understanding.
  • kwinnkwinn Posts: 8,697
    edited 2009-05-23 02:54
    The current transformer idea should be able to give you frequency and voltage, although you would have to find a way to produce a calibration curve. Four or five points over the voltage range should be enough unless the load current is very non linear with respect to voltage.
  • Beau SchwabeBeau Schwabe Posts: 6,568
    edited 2009-05-23 04:32
    James Long,

    Just a crude test... can you place a nearby AM radio and "hear" the frequency ... If so then there may be a simple Op-Amp configuration that might work in your application by just placing a piece of wire next to your signal wire.

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    Beau Schwabe

    IC Layout Engineer
    Parallax, Inc.
  • PhilldapillPhilldapill Posts: 1,283
    edited 2009-05-23 06:17
    That's what I was thinking, Beau. Putting a piece of wire fairly parallel to the wire to be tested should give you what you want. The current in the wire will induce current in the parallel "test" wire. This test wire would then be fed to some opamps to boost the signal, compare it to certain measurement values, output pulses in sync with the frequency, etc. etc. I've thought about making a crude device to do this very thing.

    Or were you talking about something totally different, Beau?
  • Beau SchwabeBeau Schwabe Posts: 6,568
    edited 2009-05-23 06:37
    Philldapill,

    No, that's along the lines of what I was thinking. I'm doing a full system upgrade to my laptop over the long weekend right now or else I would provide a schematic. There is an Op-Amp configuration I am thinking of that doesn't get used much, that would be perfect in a situation like this.

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    Beau Schwabe

    IC Layout Engineer
    Parallax, Inc.
  • Beau SchwabeBeau Schwabe Posts: 6,568
    edited 2009-05-24 21:09
    James Long,

    Here is a test circuit... you should be able to go down to just a few milli-Volts of fluctuation on the input that will provide a full 5V swing on the output. A small piece of wire (6 inches or so) in parallel with the signal wire should be enough to detect a signal. Keep the circuit as straightline as possible and keep the output as far away from the input as possible. Leads from the input should be as close to the first stage Op-Amp as possible, this is after all the "signal tracer probe".

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    Beau Schwabe

    IC Layout Engineer
    Parallax, Inc.
    Signal Tracer.JPG
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