Contactless Voltage Detection

[Tim], builder of that sweet NES pad controlled Silvia espresso machine put up an interesting idea for voltage current monitoring – at least, for AC circuits. In a nutshell, he created a mini transformer by wrapping some wire around the outside of one wire of an insulated AC power cord. Espresso machines use 120/220VAC actuated solenoids, so that’s why he’s so interested. I love the idea, since the detection circuit is just a piece of wire.

17 thoughts on “Contactless Voltage Detection

  1. If you want to get fancy, there’s always the Hall-effect sensor version.

    You can get six Hall’s from a used (Sega brand only) Dreamcast controller which, even if you own a Dreamcast, is not very useful as a controller, anyway. ;)

    (Seriously, having used the Genesis, Game Gear, Saturn “Nights”, and DC pads, I’ve never seen a good 1st party Sega pad. What’s up with that?!)

  2. Yeah, you’d be a hater if you mentioned the awful temperature drift with that close of a coil plus different insulation, mentioned he’s measuring RF with low rejection (and no safety spark discharge points for the coil,) messing up his chance to make it a HPNA thing maybe, and included a colorcoded graph of stray currents around a pair of AC mains wires.

  3. The Hall sensor thing’s a good idea; Hall-Effect transistors order up about as cheap as wire does anyway. Of course the major advantage is that you get to strip (or tap) wires and design a field gap or gaps.

    Even better, you can probably put stuff in or by other field gaps to be more sensual with the capuccino machine; grab the sounds coming off, use an adsorber to detect aromacity, heck, play it like some kind of steam-powered Yngwie Malmsteen.

  4. Hi,
    I’m sorry but I think thats technically wrong. Thats not measuring… The Voltage he measures can be anything, but is not inducted by the current in the cable because the coil is vertical to the field created by the cable..
    see here:
    or look at this “device”:
    but as I commented on the page, it’s possible that I got something wrong…


  5. um,
    that shouldnt work. The current from the wire should create a magnetic field. this should require a toroid (something round with iron) to capture the field and a secondary winding to output the ac voltage, current transformer style

    without something short of slightly iron based wire, this shouldn’t work.

  6. Any live wire will induce energy into any metal nearby that is moved through its magnetic field (or in the case of AC, the field moves through the wire). The only time it wont work is if the wire is perfectly looped back on itself to cancel out the induction, or say when measuring phase, neutral is in too close a proximity and again cancels out the induction. The effect of the latter is reduced by wrapping the testing wire around one of the wires as he has done.

    And no it doesn’t have to be a ferrous material to work. Have a look at how magnetic brakes work and consider the fact that it works perfectly well with an aluminium disk.

  7. I’m an intern at NASA’s Wallops Flight Facility and we use similar things on our sounding rockets; namely the Pyro system. When it fires the pyro there’s a large spike in current. The wires being fed to the pyro are wrapped once or twice around a toroid, causing a voltage spike on the toroid. We then use a simple RC circuit to extend the pulse to something readable. The end result being a nice exponential decaying curve for half a second (or so). Obviously a scope is needed, or a much larger toroid (for a higher voltage spike).

    We also have very well calibrated current monitors from the 60’s that just clip on to the wire and can measure the current very well.

  8. We were just going over these in our Motors class. They’re called “current transformers”, CTs, or donuts. I believe you can get voltage varieties too. AEP uses them to step down voltages/currents to measurable levels.

  9. Actually this technique is used all the time, although the sensor portion is more usually a giant magnetoresistive think-film multilayer, just like those in hard drives. I think most newer electricity meters use this technique of passive sensing.

  10. “Current transformers are used extensively for measuring current and monitoring the operation of the power grid.”

    Take careful note;

    “Care must be taken that the secondary of a current transformer is not disconnected from its load while current is flowing in the primary, as this will produce a dangerously high voltage across the open secondary”

    A much more useful form can be made by simply passing one of the load wires through the spare “window space” of any small conventional (voltage) transformer.

    With a known turns ratio and a reasonable magnetic circuit a fairly accurate measurement of the load current can be made. I still make use of the one I knocked up years ago for measuring the power consumption of small appliances.

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