high sensitivity current sensing

Discussion in 'General Electronics Chat' started by fridgedr, May 20, 2008.

  1. fridgedr

    Thread Starter New Member

    May 20, 2008
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    Hi,
    I’m wondering if anyone here may know of a simple electronic method, circuit, or perhaps integrated circuit that would provide an indication that a small load, around one tenth of an amp, has been turned on. Contacts closing in a relay or optically isolated switch would be perfect as only the analog state of the load needs to be sensed by a typical PC. Measuring the actual amount of current is not important.

    The sensing circuit or device can’t tamper with the load it is sensing so a current transformer such as the CR19 would have seemed to be the perfect solution. However, based on my experiments, before these transformers generate enough current to even turn on an LED, the load its sensing needs to be drawing at least one amp.

    Therefore could the small amount of current generated by the current transformer be used to trigger a transistor or perhaps be amplified? It should be mentioned the load can draw as much as 20 amps so the device or circuit may require some type of over current protection as well.
     
  2. mik3

    Senior Member

    Feb 4, 2008
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    Is the load DC or AC?
     
  3. beenthere

    Retired Moderator

    Apr 20, 2004
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    What sort of device switches the current? Can you post up a schematic?
     
  4. fridgedr

    Thread Starter New Member

    May 20, 2008
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    The load is AC. A washing machine actually. The computer needs to know if it is on or off, specifically when it is first started.

    When washers are first turned on, the water valve is the first and only thing drawing a load. This is the small load that needs to be detected. Of course later on in the cycle the 120VAC split phase motor runs which can be rated up to 3/4 HP, hence the need to make sure the device that is sensitive enough to measure the solenoid doesn't fry once the motor kicks in.
     
  5. bertus

    Administrator

    Apr 5, 2008
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  6. SgtWookie

    Expert

    Jul 17, 2007
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    Well, a solid 14 gauge conductor has a resistance of about 2.525 milliohms per foot. You might use a CMOS comparator to measure the voltage drop across a length of wire. 100mA across a 2.525 milliohm resistor = 252.5 microvolts.

    [eta]
    Oops, didn't see the AC part. Nope, comparator would have a hard time with that.

    I like the Hall-effect sensor idea. You could just put a couple of loops in L1 or L2, and the hall sensor would give you an output. There are several different types of Hall-effect sensors; some are simply on-off switches.
     
    Last edited: May 20, 2008
  7. beenthere

    Retired Moderator

    Apr 20, 2004
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    You can simply hang a device like a small relay (with the 120 VAC coil) in parallel with the solenoid valve and use the contact closure as your signal. Or a small lamp, like a neon indicator around the coil, and sense the light with a CDS cell.
     
  8. mik3

    Senior Member

    Feb 4, 2008
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    An easy solution, i can say, is to put a small voltage transformer (normal transformer) in parallel with the water valve to sense the voltage across it when it turns on. Use a 240/9 VAC transformer for example, then rectify its output (half way rectifier and capacitor will be fine) and sense this DC voltage by a transistor or whatever you want. The transformer will provide isolation from the mains supply voltage which is important because you will connect it to a PC. if you want more isolation as not to destroy the PC by a false component use an optocoupler.
    Note that you have to use a small transformer as not to draw much current from the valves control circuit because you may burn it.
     
  9. mik3

    Senior Member

    Feb 4, 2008
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    Beenthere's solution looks simpler :)
     
  10. Wendy

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    Mar 24, 2008
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  11. fridgedr

    Thread Starter New Member

    May 20, 2008
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    Going in parallel over the solenoid itself could be part of a solution, however there are two of them and on newer electronic controlled machines they are now pulsed to achieve proper temperature mixing which may complicate either of these methods. As well the computer also needs to sense when the spin cycle is over, long after the machine has finished filling.

    I wasn't previously aware of the existence of hall effect detectors or inductive pickups, I'll check into them.

    Thanks to all for your enlightenment
     
  12. thingmaker3

    Retired Moderator

    May 16, 2005
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    I have some 35Watt 0.06 Ohm resistors I bought via eBay a while back. I just now checked, and the seller still has some in stock.

    0.1A * 0.06\Omega = 6mV

    20A * 0.06\Omega = 1.2V

    Dissipation at 20A = 400*0.06 = 24W
     
  13. hgmjr

    Moderator

    Jan 28, 2005
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    I offer this suggestion which runs along the lines of beenthere's thoughts.

    Dave Johnson has an excellent website at www.discovercircuits.com and there he has this circuit that might offer you a solution or a hint toward a solution to your application.

    hgmjr
     
  14. anne

    Active Member

    Apr 20, 2008
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    I know this is an old thread, but just in case you missed this somehow...
    http://www.iaqsource.com/product.php?product=112834

    If there's a market for something, it will get made, and made cheaply. You can find these for as little as 15USD.

    Now where can I find a low currrent ac/dc version of this?
     
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