Current Sensing Transformers

Discussion in 'General Electronics Chat' started by Kelly, Dec 7, 2008.

  1. Kelly

    Thread Starter New Member

    Dec 4, 2008
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    First post here so take it easy on me okay :)

    I'm trying to find the best way to cheaply monitor 120V AC current. First I was thinking Hall Effect, but then I figured a transformer might be a little cheaper and a little more isolated.

    Anyways, can anyone tell me if these current sensing transformers would fit the bill for 60Hz 0~30A with somewhat decent sensitivity?

    I am epically interested in the Triad CSTs since they seem to cost somewhat less or the CRMagnetics CR8400s with the wire leads.

    http://www.crmagnetics.com/8300.pdf

    http://www.crmagnetics.com/8400.pdf

    http://triadmagnetics.com/catalog_template.php?productCategoryId=91

    Let me know if I'm way off on the application of these as well.

    Any info or advice on this would be greatly appreciated!

    /Thanks
     
  2. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    They look OK to me. Understand that you only run one wire of an AC cable or power cord through the transformer, i.e., if you want to monitor an existing circuit, you have to open the circuit, pass one wire through the transformer, then reconnect the wire. It probably goes without saying, but don't do this with power on!
    Don't ever use the transformer without the burden resistor installed. Dangerous voltages will be present on the transformer secondary if you leave the resistor disconnected.
     
  3. Kelly

    Thread Starter New Member

    Dec 4, 2008
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    Check on the one wire and power off.

    Oh man, I forgot about possible high voltages on the secondary...That could have been a nasty surprise!

    Thanks!
     
  4. awright

    Well-Known Member

    Jul 5, 2006
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    How are you intending to monitor the current? What output format do you want? Computer? Chart recorder? Panel meter? Digital readout?

    What do you mean by "...somewhat decent sensitivity..."? Accuracy? Readings of very low current? The transformer is typically not what limits "sensitivity." The meter or readout is the major determinant of the characteristics of the information you get. Most panel meter AC ammeters are quite non-linear at low currents because of the characteristics of the magnetic vane movements they typically use.

    The common method of "monitoring" current is with a current transformer feeding a 5 amp full scale panel meter. The meter face is calibrated to display the current being measured even though the meter movement itself is only typically seeing 5 amps full scale out of the transformer. You buy the current transformer for the maximum current you want to measure and buy an AC ammeter that has 5 amps full scale deflection but has a meter face calibrated to match the transformer you will be using. If you are careful and have a steady hand, you can open up any available 5 amp meter movement, remove the meter face and white out the original scale and paint your new numbers. Hard to make it look good, though, unless you are an artist.

    Many current transformers and panel meters are available for this configuration. If a panel meter display is what you mean by "monitoring," this is the way to go because the components are all quite standardized and relatively inexpensive.

    The little circuit board mounted toroids in your links will not support this kind of ammeter readout - they can't drive a 5 amp meter movement to full scale. But if you are doing further active signal conditioning or going into a computer or benchtop instrument then one of the small circuit board-mounted toroids will be fine.

    Be particularly aware of the warning by Ron H about the danger to you and to the transformer insulation of energizing a current transformer without a load on the output terminals. That's why a current transformer is delivered with a shorting link on its terminals. Only remove the link when making the final connections to the current readout device.

    awright
     
    Last edited: Dec 7, 2008
  5. Kelly

    Thread Starter New Member

    Dec 4, 2008
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    I'm not too worried about accuracy, I was more concerned about low current (0-2 Amps) sensitivity as well as repeatability. Also, I'm not really worried about output format right now, since I'm just playing with this stuff for now. I will eventually apply some type of active signal conditioning.

    Are there better options besides these transformers for measuring (0-30 Amps) 120V AC at 60Hz? Circuit isolation and cost are also important factors.

    /Kelly
     
  6. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    You sound like you are speaking from experience. I'm not, but from looking at the specs, current transformers appear to be commonly monitored by a voltmeter across a burden resistor, although I suppose an ammeter would work as well or better.
     
  7. Kelly

    Thread Starter New Member

    Dec 4, 2008
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    I guess I was assuming I would measure the voltage drop across a "burden resistor" as well.
     
  8. hgmjr

    Moderator

    Jan 28, 2005
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    You will want to consider a split core if you plan to monitor an AC conductor that you do not care to disconnect and feed through the currenr transformer.

    hgmjr
     
  9. Kelly

    Thread Starter New Member

    Dec 4, 2008
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    Okay, after messing around with the CR8410-1000 current transformer, I've decided CTs aren't the way to go for me. I realized the only way to get a DC voltage output signal I can measure is by using a rectifier. I then have to remove the "turn-on current" issue with some more circuitry. This is a bit more involved than I would prefer.

    I think I'm going back to a hall-effect sensor like the Honeywell CSLA2CD. They are about double in price, but produce a linear DC voltage output relative to the input AC current. I wanted to avoid these since they are about twice what the CR8410s cost and require a power supply, but require a far simpler design on my part.
     
    Last edited: Dec 21, 2008
  10. eblc1388

    Senior Member

    Nov 28, 2008
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    Current transformers are usually connected to ammeters directly. Connecting to a burden resistor depends on applications rather be the norm.

    The large current flow usually can create sufficient magnetic force to move a meter pointer easily. If signaling is required, it is connected to a current transducer(yet another current transformer with burden resistor in its secondary) with low impedance input.
     
  11. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    Why would this be necessary? What do you mean by "signaling"?
    I have no experience with current transformers.
     
  12. eblc1388

    Senior Member

    Nov 28, 2008
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    The current from the current transformer secondary is usually used directly for metering or protection purposes. If it is necessary to read the current value for data logging or remote metering, then a current transducer will convert the AC current into a 4-20mA current source for displaying on a meter far away.
     
  13. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    I was questioning why the second current transformer would be needed. The output current would seem to be miniscule,
     
  14. eblc1388

    Senior Member

    Nov 28, 2008
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    It is required because some of these transducers are "self powered" which means that they do not require external AC auxiliary power for them to send out an output current. They rely on the input current to power them.

    They use this secondary current transformer(which is inbuilt inside the transducer) to provide a sufficient voltage drop across a load resistor so that a constant current source can be powered from. Also this provide electrical isolation between the input current and the 4-20mA current output.

    If this voltage (10V or more to power the circuit) is to be generated via a burden resistor directly, then it will result in a high load on the original current transformer.
     
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