Current Sensing Schematic

[bigkim100]

Would anyone have a schematic that would "sense" a 120V.60Hz electrical unit being turned on.
Is it possible to sense the current being used when the unit is turned on, by winding a coil of wire over the A.C. line cord?.
Thanks for the help.

[greg123]

You bet! A torroidal coil will do this for you as it senses when current is in the line. What do you want to happen then? A light to come on or ...da...da..da

[Chris Smith]

One of the easiest ways to sense current in the AC circuit is to place a solid [about a foot long] wire in line to feed the circuit in question, large enough in gauge/diameter to provide the amperage for that circuit, and wind it into a small coil configuration wrapped around a 1/4 inch dowel. <p>Place/silicone glue/ a glass encapsulated reed switch across the out side of this coil and when current flows in the coil through on the way to the circuit it will produce a magnetic field large enough to close the reed relay switch.<p>You need about eight or ten turns in the coil and perhaps as low as 1/4 of one amp going through the coils to trigger this circuit switch. <p>More, neater, and closer windings equals a larger field strength.

[Ron H]

Chris, it seems to me that the alternating magnetic field produced by your coil will cause the reed switch to open and close at a 120 Hz (100 Hz in UK) rate.<p>Ron

[Edd]

Kim-Kim_Kim:
You neglected to include one important snippet of info…..the lowest power consumption of the device(s)coming on that line to be detected. That would be paramount in knowing the potential induced into/detected by/ the sensing device.If using a ferrite toroid with ONE leg of the AC line passing thru it an amp of line current might be required for an adequate induced level into a sense winding with enough turns for an adequate output level. Or I have looped up to 6 turns of AC POSJ cord around ½ of a 1 in toroid and bound it with a twist tie, leaving the other half of the winding window open for the MULTI –turn sense winding, thus greatly multiplying the detection sensitivity for smaller AC line currents. This could even potentially provide enough output for rectification, filtering and direct passive drive of a LED and/or Sonalert type of notification device. All is dependent and scaled to the lowest AC power being detected.
As you see, the reed switch technique suggested would require a hefty load to be passing through it. And its closure continuity will be chopped up into 8 ms gaps (10 ms..UK) at 60/50 Hz~, as its magnetized contact/reed member only will be responding to one polarity of the two magnetic field polarities produced by its bipolar AC source.<p>73’s de Edd
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[Chris Smith]

Actually the reed switch cant operate at 60 hz and the saturation will hold it it steady, as it does on my water pump that comes with this unit standard. <p>And I did mention the minimum at about 1/4 amp. This is guess work based upon the size and shape of the coil.<p>[ November 29, 2002: Message edited by: Chris Smith ]</p>

[Ron H]

Edd, I believe a magnetically unbiased (no bias magnet nearby) reed switch is not sensitive to the polarity of the magnetic field.<p>A little research shows that reeds can switch in as little as 0.5 ms, INCLUDING bounce. I found one that was spec'ed at 3 ms.<p>Chris, I can see that, if the AC field is very strong, the dead time will be so short that the reed will stay closed during crossover. I can also see that, if the field is relatively weak, the reed will switch at 120 Hz.<p>Ron<p>[ November 29, 2002: Message edited by: RonH ]</p>

[L. Daniel Rosa]

Kim, I have some torriodal current transformers that can handle a 6 gauge wire thruogh the middle. The current ratio is 1000:1. If this fits your needs, let me know and I'll send a few to you.

[Chris Smith]

This is true Ron, Not every switch will respond at the HZ rate due to mass and Saturation. There are other ways such as small metal magnets in the proximity, alignment, etc.

[bodgy]

Elektor have had a number of current sensing circuits in their bumper issues. Max rating of 3A normally intended to sense the Amp being switched on and thereby switching feed to the peripherals.<p>Er that's the other way round.<p>Colin