DIY Clamp on amp meter

Discussion in 'The Projects Forum' started by Bear_2759, Jul 22, 2010.

  1. Bear_2759

    Thread Starter Active Member

    May 23, 2008
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    Hi Guys,

    I'm wanting to make a clamp on amp meter, as acurate as possible with a high resolution as I'll only be monitoring low amp stuff e.g. tv, pc, phone etc. max 10A
    are there any projects or circuit diagrams out there you would suggest?
     
  2. retched

    AAC Fanatic!

    Dec 5, 2009
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    Take a look at HALL effect sensors.

    They will allow you to measure the current in a wire without contacting the conductor.
     
  3. Bear_2759

    Thread Starter Active Member

    May 23, 2008
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  4. pilko

    Senior Member

    Dec 8, 2008
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  5. hgmjr

    Moderator

    Jan 28, 2005
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  6. marshallf3

    Well-Known Member

    Jul 26, 2010
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    Not many do, it takes a hall effect sensor pickup to measure a steady state magnetic field.
     
  7. hgmjr

    Moderator

    Jan 28, 2005
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    I agree. I suspect that the kit that bear_2759 is interested in is probably not likely to measure DC current with any great accuracy for that reason.

    hgmjr
     
  8. chriso

    New Member

    Aug 6, 2010
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    I've just bought a DC Clamp
    Another link here

    Got it for finding a high current leak in a cable bundle that couldn't be turned off or cut.
     
  9. marshallf3

    Well-Known Member

    Jul 26, 2010
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    It's not going to measure DC current at all, the pickup relies upon an AC signal flowing through the wire to act as the primary of the transformer you create with the ferrite core that has the secondary winding on it.

    I hand wound three toroids to monitor each leg of the 480V 3 phase feed to a 90 ton chiller the other day. It's a simple design but one thing is rarely mentioned about these things I had to learn on my own.

    Solid ferrite core with 100 turns connected to a 1 ohm 3 watt resistor. In theory 100 amps through the supply wire would give you 1 Amp out of the 100 turn secondary thus read 1V across the load resistor.

    I was only reading around 0.64V with ~100 amps flowing through the primary leg, what I forgot to consider in the design is that the 100 turn secondary winding had a DC resistance of around 0.36 ohms.

    In the end I changed to a thicker magnet wire for the secondary and reduced the number of turns to around 84.

    Reads perfectly now. If the chiller is drawing 22A through a certain leg I get 0.22V across that toroidial transformer when fed into 1 ohm, likewise 60A = 0.6V etc.

    This is the way the AC clmap on probes work to begin with.
     
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  10. hgmjr

    Moderator

    Jan 28, 2005
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    The kit that bear_2759 linked to states that it can measure AC/DC current using a clip-on probe. Hence my scepticism.

    hgmjr
     
  11. marshallf3

    Well-Known Member

    Jul 26, 2010
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    I bought one of the eBay ones this morning, $8 shipping on a $6 meter. I've got a nice Fluke but it doesn't hurt to have an extra sitting around, especially if it's anywhere near as accurate as it claims to be.

    Some time back I bought a really inexpensive LC meter off eBay, it turned out to be well built in a tough case and trying it out on several known values of inductors and capacitors showed it to be pretty darn accurate. Again, just an extra to have around.

    I've got a nice B&K portable that does caps, resistors, transistor Hfe, LED currents and is also a small battery tester with preset loading for the battery type, also have an old HP bridge but it's a bench piece. It seems my garage has a lot of older HP rackmount test equipment too, extremely accurate stuff but bulky as heck. I've even got a pair of Tektronix 545 oscilloscopes with some assorted plugins. These were vacuum tube builds and there's over 100 tubes just in the mainframe part alone.

    http://www.messmuseum.de/tek545b.htm
     
    Last edited: Aug 6, 2010
  12. JoeJester

    AAC Fanatic!

    Apr 26, 2005
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    You bought a Tek 545? Cool. I haven't used one of those since the early 70s.
     
  13. marshallf3

    Well-Known Member

    Jul 26, 2010
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    I've had them for ages, got them and several plug-ins at a state surplus auction around 1977. They haven't been powered up for almost 30 years now so the main filter caps would all need to be replaced in lieu of risking what I've seen happen before. While a big electrolytic can explode it's rare, they usually build up a really high pressure which eventually ruptures the safety vent then you've got enough acrid smoke to clear a decent sized building.

    I also have an old DuMont 304A but last time I used it some sort of time delay tube in the power supply had bit the dust.
     
  14. BillB3857

    Senior Member

    Feb 28, 2009
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    From the looks of the kit unit, the torroid core does not have a coil around it, but is, instead, being used as a magnetic field concentrator. The hall device would fit into an air gap in the core.
     
  15. Bear_2759

    Thread Starter Active Member

    May 23, 2008
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    thanks for the replies guys, bill you're 100% correct as there's no coil and hall sensor is in a gap in one end.

    for everyone else since it seems to be a common theme, I don't need it to measure DC. only 240v AC (Australian mains). the reason I got this kit was simply so I could go around all the commonly used devices throughout my home and calculate the exact ammount that each is costing me. intend on getting my money back from the kit by saving on the elec bill. last 1/4 it was close to $800...
    I'm most of the way through the kit now, just building the clamp itself. question for the pro's, does it matter if I have chipped a bit off the core? as part of the kit I'm required to cut the core in half, and as I'd almost finished the cut the last 1-2mm chipped off leaving a bevel at about 45 deg on one side.
     
  16. marshallf3

    Well-Known Member

    Jul 26, 2010
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    It's probably OK but remember, only one of the two wires used to power a device goes through the ampmeter - in other words you can't just snap it across a lamp cable or refrigerator cord.

    A long time ago I made a setup that has a plug and identical socket to fit our common outlets, the two are connected with indvidual #12 wires so I can just grab one if I need to measure something that plugs in.
     
  17. Bear_2759

    Thread Starter Active Member

    May 23, 2008
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    hmm... interesting as that's what I had intended... it's so easy to misread things sometimes:( as the site says
    " it can measure AC and DC currents simply by clipping onto the wire you wish to measure"
    I might just try it and see what happens. if not I'll take the outer insulation off a short 2m extension cord and expose the 3 inner wires. then I just need to pick the right one. should be easy enough as long as the extension cable manufacturer has stuck to aus standards. if not then it'd still be easy as to identify.
     
  18. eblc1388

    Senior Member

    Nov 28, 2008
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    This argument cannot be correct.

    A good 100:1 current transformer will not care(within practical limits) about the external load impedance and circulate 1 amp in its secondary regardless. Thus you will get the same 1 amp even if you use a 0.2Ω or 2Ω load.

    Perhaps ferrite might not be a good choice of material to use for 50/60Hz measurement afterall.
     
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