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I am trying to clarify the difference between some current sensors, would i be correct in stating that Current Transformers induct the flux of collapsing magnetic fields caused by AC, but cannot sense DC, while hall effect sensors direct the magnetic field onto a semiconductor so they detect both AC and DC?

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  • \$\begingroup\$ A CT needs a changing magnetic flux to produce a signal whereas Hall sensors don't. \$\endgroup\$
    – Andy aka
    Feb 17, 2014 at 17:43

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Yes, Hall effect sensors sense the magnetic field caused by current, and can therefore measure absolute current. A current transformer can only sense current down to some minimum frequency below which gain falls off rapidly. A current transformer can't sense a fixed current.

If you are measuring current of something that is inherently AC, like the power line, then a current transformer can be appropriate. If you really need to sense DC current, then you can't use a current transformer and a Hall effect sensor may be appropriate. Keep in mind that a Hall sensor requires separate power to operate.

The other major way to sense current is by using a sense resistor. The resulting voltage is not isolated, so that may need to be dealt with. In the case of a sense resistor, it is often convenient to put some detection and processing circuitry together with the sensor. That can the convert the result to digital and send the digital information over some isolation barrier, like a opto-coupler.

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  • \$\begingroup\$ Would you say a Sense Resistors or shunts (?) are pretty much a voltage divider, you are measuring the proportional current? Am I on the right track with that? (While I am here, I recognise your avatar and I wanted to thank you for your help in previous questions where I was being difficult because of "aesthetic considerations" :D you are very patient with half-arsed barely educated 'electronic artists' haha! I really appreciate it, and I am slowly learning.) \$\endgroup\$ Feb 17, 2014 at 20:03
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    \$\begingroup\$ @Oliver: No, I wouldn't call a current sense resistor a voltage divider. A voltage divider is two resistors working together, for one thing. To see what a resistor does, look at Ohm's law. One way to write that is Volts = Amps x Ohms, which describes a resistor as a current to voltage converter. THat is how a resistor is used to sense current. For example a 100 mOhm resistor will produce 100 mV per amp passing thru it. \$\endgroup\$ Feb 17, 2014 at 20:17
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    \$\begingroup\$ It looks just like a voltage divider but because the sense resistor is so low in value compared to the load resistor, it has almost no effect on the current, and you can almost ignore it as far as the load current goes. \$\endgroup\$ Feb 17, 2014 at 21:51
  • \$\begingroup\$ @Spehro: I thought we were talking about current sense resistors, so there is no load resistor. It seems like you are referring to the current transformer case? \$\endgroup\$ Feb 17, 2014 at 22:17
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    \$\begingroup\$ @OlinLathrop I was trying to think how he'd see a voltage divider- and if you consider the shunt in series with a load impedance or resistance (both across the mains) it does look like a voltage divider. Both a shunt resistor and a CT affect the current a teeny bit in just about any real situation. \$\endgroup\$ Feb 17, 2014 at 22:43
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There is a way to detect DC current with a transformer - It is called a flux gate or zero flux DC current transformer. The technique was written about in 1901 in an IEEE paper, was realized in an HP 428B milliammeter from the 1950s or 1960s and used in other applications today. Essentially, toroidal cores with equal and opposite windings are excited by a sinusoidal waveform. This produces zero net flux in a winding common to both cores - the cores should be matched orthogonal cores. If a DC current flows in a wire through the center of the cores, it produces a net flux as it subtracts from one of the cores flux (the other core is saturated further). The common winding then indicates a net flux which can be bucked with a second common winding current which is proportional to the net flux/current in the sense winding. A control loop is used to close the loop and drive the net flux to zero. Pretty cool stuff. I am surprised it is not used more often and suspect it is easier/cheaper to use hall effect sensors.

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