Sensing AC voltage by non-contact means.
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Posted by David Collier on January 16, 2006, 6:40 am
Please log in for more thread options I need to detect live voltage in a mains cable, from outside the cable, without electrical contact. The cable is the one INTO the main fusebox, so no toucheee. It's obviously possible - you can buy a pen-shaped thing in any hardware shop which will do it. After some web research I found that the standard circuit to do it is to use an unloaded CMOS input, connected to a foil or wire, and pick up the local electrical field. I spend all my life avoiding unterminated CMOS inputs, and now I design one in? My question is - does anyone know of a ready-built sensor, maybe taking in power, gnd, and outputting a yes/no flag, which I can go and buy, rather than having to design, test and build my own? Seems easy to buy a clamp-on device which will measure current... but I can find nothing to detect just voltage. David Collier email can be sent to Dexdyne.com , under name from_usenet@ | ||||||||||||||||||||||||||||||||||||||||
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Posted by John Perry on January 16, 2006, 10:50 am
Please log in for more thread options David Collier wrote: You already have. The pen-shaped things do exactly what the CMOS circuit does -- sense the electric fields around the conductors, which propagate through the insulation just as well as magnetic fields from current do. You don't need current flow. So, attach a CMOS inverter to an insulated wire, which you'll then place next to the AC input. DON'T LEAVE THE INPUT WIRE BARE -- the circuit depends on capacitive coupling through the insulation. It should sense any voltage through the insulation. You should get a 60Hz (50Hz?) square wave out. Rectify the ouput of the input inverter, send the rectified voltage to a second gate, attach an LED to the output of the second gate, and you have a mains voltage sensor. I doubt that you really need to protect the sensing input -- very little energy can get through the insulation. Note that this is not a measurement tool, it is a sensing tool. Measurement depends on too many things to attempt it through two layers of insulation. It may also be too sensitive, so you may have to try some added grounding resistors (multimegohm, probably) to the sensing input. Note also that the pen gadgets are intended for internal unshielded wiring. At least where I live, the power company has individual conductors anchored to the house up high, then a shielded 2-conductor cable carrying the mains supply down to the meter. This circuit (or one of the pens) is unlikely to work through the cable shield. But it'll work fine on individual, insulated wires. John Perry | ||||||||||||||||||||||||||||||||||||||||
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Posted by David Collier on January 19, 2006, 6:15 am
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wrote: > > Seems easy to buy a clamp-on device which will measure current...
> > but I can find nothing to detect just voltage. > > >
> You already have. The pen-shaped things do exactly what the CMOS > circuit does -- sense the electric fields around the conductors, > which propagate through the insulation just as well as magnetic > fields from current do. You don't need current flow. > > So, attach a CMOS inverter to an insulated wire, which you'll then > place next to the AC input. DON'T LEAVE THE INPUT WIRE BARE -- the > circuit depends on capacitive coupling through the insulation. It > should sense any voltage through the insulation. You should get a > 60Hz (50Hz?) square wave out. Rectify the ouput of the input > inverter, send the rectified voltage to a second gate, attach an LED > to the output of the second gate, and you have a mains voltage > sensor. I doubt that you really need to protect the sensing input -- > very little energy can get through the insulation. > > Note that this is not a measurement tool, it is a sensing tool. > Measurement depends on too many things to attempt it through two > layers of insulation. It may also be too sensitive, so you may have > to try some added grounding resistors (multimegohm, probably) to the > sensing input. > > Note also that the pen gadgets are intended for internal unshielded > wiring. At least where I live, the power company has individual > conductors anchored to the house up high, then a shielded 2-conductor > cable carrying the mains supply down to the meter. This circuit (or > one of the pens) is unlikely to work through the cable shield. But > it'll work fine on individual, insulated wires. > > John Perry I'd give that answer an A* I am simply looking for a go/no-go so I can call out the right engineer, ( off a list of 3 ). It all saves money, and improves time - to - resume - service. I've got 8 hours of back-up battery to get it fixed, and I can't afford to call out all 3 engineers in succession, apart from the cost. This lot could be 50 miles from the nearest house, let alone depot. My only point is that I'd like to be able to BUY one of these things, rather than make one. Why can't I ( pout ). I am hoping that I can access the live tail just where it enters the board fuse - I appreciate it will be well shielded all the way to there, but I'm hoping the shielding stops short enough for my purposes. David Collier email can be sent to Dexdyne.com , under name from_usenet@ | ||||||||||||||||||||||||||||||||||||||||
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Posted by Joseph2k on January 23, 2006, 11:39 pm
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David Collier wrote: > wrote:
> >> > Seems easy to buy a clamp-on device which will measure current...
>> > but I can find nothing to detect just voltage. >> > >>
>> You already have. The pen-shaped things do exactly what the CMOS >> circuit does -- sense the electric fields around the conductors, >> which propagate through the insulation just as well as magnetic >> fields from current do. You don't need current flow. >> >> So, attach a CMOS inverter to an insulated wire, which you'll then >> place next to the AC input. DON'T LEAVE THE INPUT WIRE BARE -- the >> circuit depends on capacitive coupling through the insulation. It >> should sense any voltage through the insulation. You should get a >> 60Hz (50Hz?) square wave out. Rectify the ouput of the input >> inverter, send the rectified voltage to a second gate, attach an LED >> to the output of the second gate, and you have a mains voltage >> sensor. I doubt that you really need to protect the sensing input -- >> very little energy can get through the insulation. >> >> Note that this is not a measurement tool, it is a sensing tool. >> Measurement depends on too many things to attempt it through two >> layers of insulation. It may also be too sensitive, so you may have >> to try some added grounding resistors (multimegohm, probably) to the >> sensing input. >> >> Note also that the pen gadgets are intended for internal unshielded >> wiring. At least where I live, the power company has individual >> conductors anchored to the house up high, then a shielded 2-conductor >> cable carrying the mains supply down to the meter. This circuit (or >> one of the pens) is unlikely to work through the cable shield. But >> it'll work fine on individual, insulated wires. >> >> John Perry >
Oh crud, they are cheap and easy to find in Los Angeles and any other city
> > I'd give that answer an A* > > I am simply looking for a go/no-go so I can call out the right engineer, > ( off a list of 3 ). It all saves money, and improves time - to - resume > - service. I've got 8 hours of back-up battery to get it fixed, and I > can't afford to call out all 3 engineers in succession, apart from the > cost. This lot could be 50 miles from the nearest house, let alone depot. > > My only point is that I'd like to be able to BUY one of these things, > rather than make one. Why can't I ( pout ). > > I am hoping that I can access the live tail just where it enters the > board fuse - I appreciate it will be well shielded all the way to there, > but I'm hoping the shielding stops short enough for my purposes. > > David Collier > > email can be sent to Dexdyne.com , under name from_usenet@ where there are established IBEW locals. I have one. I bet i can find another. If you really want one, send your address to joseph_barrett "at" sbcglobal.net I will even send you a copy of the receipt. -- JosephKK | ||||||||||||||||||||||||||||||||||||||||
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Posted by Phil Allison on January 16, 2006, 11:01 pm
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"David Collier" >
> I need to detect live voltage in a mains cable, from outside the cable, > without electrical contact. The cable is the one INTO the main fusebox, > so no toucheee. > ** Stick a fine sewing needle through the insulation. You can use any convenient AC voltmeter then. ........ Phil | ||||||||||||||||||||||||||||||||||||||||
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> It's obviously possible - you can buy a pen-shaped thing in any hardware
> shop which will do it.
>
> After some web research I found that the standard circuit to do it is to
> use an unloaded CMOS input, connected to a foil or wire, and pick up the
> local electrical field.
>
> I spend all my life avoiding unterminated CMOS inputs, and now I design
> one in?
>
> My question is - does anyone know of a ready-built sensor, maybe taking
> in power, gnd, and outputting a yes/no flag, which I can go and buy,
> rather than having to design, test and build my own?
>
> Seems easy to buy a clamp-on device which will measure current... but I
> can find nothing to detect just voltage.
>