Looking for a drawing of what the kill a watt uses for it's current sensor.
It looks like a piece of wire forming a shunt resistor to me - pretty crude
Other low cost units use a Toroidal transformer for this. aka - AMwatt made by Reliance.
AMwatt could be a easier alternative to solder to - the LM358 is the surface mount type but at least it's on top of the board. ALso the toroid is more like a real current measurement circuit.
Kill a watt current circuit - does anybody have a drawing ?
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- randykarr
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Re: Kill a watt current circuit - does anybody have a drawing ?
a 'piece of wire' as a current sensing resistor is not crude at all....it is the most precise method of measuring current & its wikked cheap!
- amberwolf
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Re: Kill a watt current circuit - does anybody have a drawing ?
Exactly. Using the toroid probably means either they're:
--reading it's magnetic field with a hall sensor, and that will vary with temperature some (depends on the sensor), and uses more power (only a little, but still).
Or:
--inducing a current from one wire on the toroid into the other wire, and reading the voltage across the second one. Would have been simpler and cost less to just measure across a shunt wire.
EDIT: oh, and both of those methods require more calibration, since there's more inductance to deal with that might change (a tiny bit) due to slightly different lengths of wire on each toroid made, and possibly even the relative positions of wires on them (though this would not be very much, if even measureable).
--reading it's magnetic field with a hall sensor, and that will vary with temperature some (depends on the sensor), and uses more power (only a little, but still).
Or:
--inducing a current from one wire on the toroid into the other wire, and reading the voltage across the second one. Would have been simpler and cost less to just measure across a shunt wire.
EDIT: oh, and both of those methods require more calibration, since there's more inductance to deal with that might change (a tiny bit) due to slightly different lengths of wire on each toroid made, and possibly even the relative positions of wires on them (though this would not be very much, if even measureable).
- richms
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Re: Kill a watt current circuit - does anybody have a drawing ?
And you can calibrate the piece of wire by gloping more solder onto it or filing it off rather then putting up with dodgey potentiometers that will vary over time.
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Re: Kill a watt current circuit - does anybody have a drawin
So I realize this is a very old post but I happen to have one of these amwatt units laying around, can I use it? If pics are needed I can supply them.
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