Computing stuff tied to the physical world

Current clamp

In Hardware on Sep 5, 2012 at 00:01

To continue where I left off before the summer, let’s examine what a current clamp like this one does:

DSC 3354

You put it around a single wire in your AC mains cabling and it’ll generate a voltage proportional with the current going through it. This unit has a built-in burden resistor, which means you get a ± 1V (AC!) output when the current through the wire is 30 A. So let’s have some fun and look at a couple of different loads, eh?

Let’s start off with an old-fashioned 25W incandescent light-bulb, which is a resistive load:


Note the vertical scale – these voltage levels are tiny. The scope caclulates the Root Mean Square (RMS) voltage as being 3.52 mV in this case. That’s the voltage you’d need to draw as direct current to dissipate the same amount of power as this alternating current (let’s ignore phase shift and “reactive” vs “true” power for now).

Sure enough, a 75W light bulb draws three times as much power (note the different vertical scale):


Here’s a 2W LED light bulb, which uses an electronic circuit to pulse small amounts of energy from AC mains:


Again, note how this minute RMS reading corresponds quite accurately to the specified wattage.

Now let’s take a vacuum cleaner, which is an inductive load, and quite a lot beefier too:


The “blips” are switching artifacts from the TRIAC control included in this unit. From the RMS value, my estimate would be that it draws about 1500W. Here’s the same vacuum cleaner, with its power throttled back to minimum:


As with the LED light, you can see the electronics kicking in and pulsing AC mains to throttle power back to around 500W.

Conclusion: a current clamp is a safe way to measure current in an AC mains wire, and it more or less reproduces the measured current as a small output voltage. Very small in fact, for light loads. To accurately determine the RMS value of a load as small as our 2W LED light, we’re going to have to read out this signal in the sub-millivolt range, and do so at perhaps 1000 HZ to collect enough readings per 50 Hz cycle.

  1. Interesting plots! With loads like these, it’s a wonder that the power company manages to keep the mains supply waveform more-or-less sinusoidal.

  2. Could we sustain a JeeNode from a current clamp I wonder.

    • Probably not, because the clamp delivers about 500 millivolts at a 1500 watt load. You’d need to be continuously be drawing at least 2 kw continuously to be able to power it. A water kettle or a fridge might work in combination with a booster circuit, but I think it’s impossible to draw at least 20 mAh’s from the clamp. So that would be a no.

      I would like to see a “current clamp plug” for a JeeNode or something like it though!

  3. Does such a clamp also measure reverse current?

    (Solar cells connected to the mains)

    • @Mars: It measures the magnetic field generated by the current, so yeah, you’d get a measurement. The sign will flip due to the reverse current, but because it’s AC you won’t notice that.. Unless you also measure the phase of the voltage, I’d say.

      @JCW: Regarding the need for high sampling frequency: would it be possible to use a rectifier+low-pass filter, so you do the RMS calculation in the electric domain?

  4. @JohnO: maybe THIS current clamp cannot, but if you consider a straight mains wire to be a single winding of a transformer, I don’t see why one couldn’t make a secondary winding around a piece of 220/110V AC wire and harvest some energy for a node. To keep the node powered, it is necessary to have a load on the wire on a regular basis, of course. I lack the measurement tools and the formulas, but I can imagine that with 10 cm of mains wire, a power drill and some very thin wire, one can quickly create a spool to do some measurements.

    • I think your approach will work the wrong way around, up-transforming AC mains to even higher voltages (and very low currents) – since the secondary coil ends up being a multiple of the single-winding primary coil.

      I’ll see if I can set up an N-winding mains cable with a 1-winding secondary, and see what sort of energy could be harvested from that.

    • eljonco, what you describe is called a di/dt sensor and can be used for AC current measurement ( But power will be very low and using high number of turns will just increase the voltage (as JC has written already).

  5. @mars, it’s all in the phasing. Measuring current and then combining with knowledge of the corresponding voltage vector is the trick to determine the “direction” of power flow (just the voltage zero crossings will do).

    Interestingly, the old style moving disk power meters can do a fine job of sensing the power flow and agreggating total consumed from/total given to the network by simplying spinning “backwards” when putting power on the grid. Unfortunately, this ability is often suppressed by a simple mechanical oneway rotation device to work around a common cheating method (which I won’t fully describe).

    Now it is a considerable headache for electronic meter designers to handle reverse power flow correctly (and the Big Power politics that often say a kWh you generate is not the same price as a kWh you consume)

    A task the 19th century design handles just fine

    • Hmmm, I have no idea how to detect that voltage vector, but am glad that it should be possible. If I would measure all 6 groups here with such a clamp, reverse metering is a MUST have…

      And yes, my old and simple spinning disk has no problems rotating backwards! And because I still have this meter and not one of the digital ones, I get the full price for my solar kWh’s!

  6. a Kwh you generate is not the same price as a Kwh you consume…

    Although if the Kwh you generate is consumed by your next-door neighbor, you may have saved the utility company a bit more than one Khw, due to transmission line losses, and if it’s from solar, that’s usually peak-load hours, when electricity is most valuable.

  7. @JBeale, Ah – the politics of Big Power. Logically, micro and zonal power generation should be a significant part of supplying energy needs. As you point out, a kWh produced and consumed ‘locally’ is ‘better’ in many ways than production at a distance with the inevitable transmission losses.

    But we are so far from grid-free implementations that those huge towers and cables still need funding (and a bunch of office folk employed to ‘administer’ today’s grid and the megadollars spending soon on ‘upgrades’)

  8. When measuring small loads the output voltage from the clamp will indeed be low, but of I remember my physics correctly, you would double that voltage by winding the wire one turn through the clamp… Or do I remember that wrong? I am pretty sure I did something along those lines a couple of years back..

  9. @Göran, your recollection is correct – passing the sampled conductor through the centre of the clamp multiple times reduces the transformer ratio by n, increasing the secondary current by the same n and with a fixed burden resistor, increasing the voltage across the burden by n (to first order, ignoring factors like leakage inductance).

    It’s a good trick to increase the sensitivity of your current clamp meter, but of course it does not extend the range of measurable currents since the top end is limited by core saturation and that will be reached n times faster.

    • @martynj: right. Core saturation… Is this really a linear relationship? Not that it matters here though.. Going to spend some time with my old EE text books this weekend, I think. :)

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