Inductive Current Measurement

Discussion in 'General Electronics Chat' started by 95se5m, Jul 24, 2006.

  1. 95se5m

    Thread Starter New Member

    Jul 24, 2006
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    0
    Hello all. I am trying to measure the current of the output of a 15KV NST (Neon sign transformer) It's 60Hz, I would love to drop a coil on one of the output legs and using a VM measure a reasonably accurate current. Here's my issue. I need a calculation to find the number of turns on a ferrite coil to give me a 1v per ma output with a 30ma max from the NST as it is current limited.

    The current circuit measures voltage through a 1000:1 resistive divider in oil, I tried using a 1k resistor in series with the low side of the DC output to measure 1v per ma but ended up with a really toasty VM panel meter.

    Any thoughts? Ideas? Anything?

    Thanks in advance,
    Jim
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    Hi,

    Just to make things a bit less scary - the current transformer you want to build may be purchased as an off-the-shelf item. If you want to pursue making your own, remember that current is current, and you can set up to experiment with the coil using a low voltage transformer output and a variable resistance to track linearity.

    Ferrite will tend to "soak up" the magnetic field, and so winding your project on a ferrite bobbin may not give good results. I'd use a plastic form that was a close fit over the monitored wire.
     
  3. 95se5m

    Thread Starter New Member

    Jul 24, 2006
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    0
    All the "Off the shelf" clamp on current meters read in Amps, not miliamps. I need to create or find a coil/clamp on for miliamps. Remember the NST I am using is current limited to 30ma @ 15Kv full load. I just need to find out the current running from the NST at any one time through a ballast resistor and negitive resistance gas discharge tube. I would be more than happy to use an off the shelf item if I could. Any direction into a clamp style for this small an amount of current?
     
  4. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    Hi,

    Sorry, Ithought you wanted something left on the lead for periodic monitoring. Clamp-on meters are way too coarse for 30 mills. Let me hit the books on Hall effect sensors....
     
  5. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    Hi,

    It doesn't look good for a clamp-on device. Anything I can find that is rated in milliamps requires a wound bobbin around the wire being measured. You can get on www.gmw.com/electric_current to see their stuff as an example.
     
  6. 95se5m

    Thread Starter New Member

    Jul 24, 2006
    8
    0
    Okay all, I have done a little more research into this. A bobbin I believe would work perfectly, however I can't find any information into how many turns on the bobbin should I use? I was thinking of using a bobbin, to a amplifier circuit of some kind, to a V to I converter, to a digital readout for 0 to 99V I am looking to get a 1ma to 1v resolution.

    For those of you who are ready to start preaching I am building a PSU for a CO2 laser of my own construction. I have a Variac controlling a 15Kv @ 30ma NST, I have 2 0 to 99V meters, one measuring the voltage using a 1000:1 400Meg voltage divider. I need to measure the current to allow for control of the power output.

    Now, again to stem off the neigh sayers, I have been working with HV for sometime, and working with lasers for as long, so I have a working knowledge of what’s going on and will not kill myself.
     
  7. atlas

    Member

    Jul 18, 2006
    28
    0
    Hi there,

    you can use a hall effect sensor if you want, as it has the ability to do that and it's safe..But remember some hall effect sensor like the one i'm using multiply your current with some factor that depends on the numbre of turns used.. For example if you want to measure 5 ampere, the hall effect sensor will multiply your current with factor of (4/1000), where 4 indicates the number of turns.
     
  8. Thooper

    New Member

    Jul 16, 2006
    1
    0
    I'm by no means an authority, but it seems to me (and I believe beenthere was suggesting it) that you could set up an experimental unit with a KNOWN measurable fixed current. Something, for instance, that's not 15kV.
    You could leave a meter right inline, confirming that your current is stable, and adjust your coil on the fly. Then you can have a steady reference as to how many turns gives you what sort of ratio.
    Once you have your ratio the way you want it, you are good to go and apply it to your permanent situation.
     
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