Simple Current Detection Device

Discussion in 'General Electronics Chat' started by bd525, Mar 16, 2010.

  1. bd525

    Thread Starter New Member

    Nov 11, 2009
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    I am working on a project for a device that will monitor a LP power motor and send a notification when the motor has started to run. I need a way to detect that the motor is running but I cannot due this intrusively(ie they wont let me splice into any existing wires). My idea was to create device that could detect current being generated by the alternator through the wire that is recharging the battery using maybe a hall effect sensor (kind of like a mini amp clamp). I have never had any experience with this and was wondering if anyone my be able to point in the right direction...

    My device can provide a 5 volt supply to a hall effect sensor, but which one would be best to use? Should I use a transducer?

    Thanks for all your help!
     
  2. hgmjr

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  3. bd525

    Thread Starter New Member

    Nov 11, 2009
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    That would work but the current coming from the alternator to the battery has already passed through a rectifer and is DC current... I dont think this will work on AC will it???
     
  4. hgmjr

    Moderator

    Jan 28, 2005
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    Here is the operation description of the circuit included with the schematic. It clearly indicates that the circuit is designed to monitor the current flowing in the AC voltage line powering the AC powered pump motor. It should work just fine as an "AC current" detector when the motor is running...

    hgmjr
     
  5. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    Did you mean to say
    If not, you are not making any sense.
     
  6. bd525

    Thread Starter New Member

    Nov 11, 2009
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    Yes that is what I meant to say... it wont work with DC not AC... sorry for the confusion... it was one of those days where I was on my 36th hour without sleep...
     
  7. Bychon

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    Mar 12, 2010
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    The CURRENT from the alternator to the battery is PULSED. Detecting the pulses is the same thing as detecting AC. It will work.
     
  8. bd525

    Thread Starter New Member

    Nov 11, 2009
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    I completely forgot about the pulsating DC... what a moron... thanks man!
     
  9. Jojo_B

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    Nov 17, 2009
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    Just out of curiosity/clarification, for this circuit, for initial sensing, you do a wirewrap around the insulated wire...then wire wrap an un-insulated inductor? Does it matter which direction you wrap it?
     
  10. Bychon

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    Mar 12, 2010
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    I don't know why you'd wire wrap anything. Probably because I don't understand what "wire wrap" means in this case. Do not install any shorted loops of wire near your pickup.

    The principle is that a coil next to a changing current carrying wire will pick up the differences in magnetic field around the conductor. These are used in car alarms where the only thing used to detect the current changes is an insulated wire, wrapped once around the battery cable. Nothing fancy. Just a common, insulated wire and a zip tie to make it look nice. Don't think this to death. And, no, it doesn't matter which way you wrap the turns, as long as they all go the same direction. You bring the coil wires into the amp board, grounding one lead, and the induced voltage shows up on the other lead and dances around the zero volt line. Then the opamp does its job. It doesn't matter if you hand wind 3 or 4 turns of coil or zip-tie a prefab coil next to the battery cable.
     
  11. bd525

    Thread Starter New Member

    Nov 11, 2009
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    With the example of the loop in car alarms... so they just loop a piece of wire once around the positive or ground wire? and then that loop of wire has voltage when current in running though the battery..? what kind of voltage would you see on the loop wire? I mean how many volts? How might you go about doing this as an experiment? Sorry for the nube questions... I am trying to understand.
     
  12. Bychon

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    Mar 12, 2010
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    I don't know how many volts a single turn would produce, and I don't know how many amps are inducing the voltage. I believe there is a math way to do this, but it is faster and easier for me to wrap 10 turns of scrap wire around a battery cable and hook it to a volt meter.

    I am a practical person. I fix things. I don't math them to death unless that is the only way, like, what kind of voltage will spike when I turn this triac off? I can math that easier than I can buy a dozen triacs and watch them melt until I get the snubber right. My advice is, do it the practical way. Even if you want to do the math, you have to get out a meter and measure the current first. (Darn, back to practical again.)
     
  13. bd525

    Thread Starter New Member

    Nov 11, 2009
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    I hear what your saying and I am all about practicle... I was asking what sort of voltage to expect because I tried this on my truck and saw no voltage reading on my meter... just not sure what I am doing wrong. I took a wire with alligator clips on and wrapped it around the battery cable, both positive and ground (one at a time) but I got no voltage... (yeah the truck was running)...
     
  14. Bychon

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    Mar 12, 2010
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    Being a practical person, I made 10 turns around the battery cable of my car and set the meter on millivolts AC, max hold. I started the car and when I got out and looked, I saw the meter registering 3.2 millivolts RMS AC. This being a 6 banger Aerostar, it probably uses 200 amps to start. 3.2 millivolts per 200 amps with 10 turns driving a meter with 10 megohms of input impedance.

    I can't hand it to you any better than that.
     
  15. retched

    AAC Fanatic!

    Dec 5, 2009
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    You may want to try 20 turns and trying again. The MAX-HOLD option on digital meters is a nice way to see what happened while you were starting the truck. Once the vehicle is started, the amperage through the battery cable is going to drop pretty low. The alternator would be the area to track current after starting. However, the greatest draw, as Bychon said, will be from the battery to the starter during starting.

    Try increasing you amount of turns, using insulated wire, and try again. If you want to stick with 10 turns, fine. You will then want to use an amplifier to up the gain to a measurable level.

    If your meter is not sensitive enough to pick up under 10mV or 5mV better yet, You will read nothing on your meter. Check the meters specs to be sure your not spitting into the wind.
     
  16. bd525

    Thread Starter New Member

    Nov 11, 2009
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    Thanks guys thats what I needed to know... I think I need to get a different meter and try again with more turns... I'll let you know how it goes
     
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