Computing stuff tied to the physical world

AC Mains measurements

In Hardware on Nov 17, 2011 at 00:01

One of the things I ordered recently was a High-voltage Differential Probe. The reason for this is that I want to be able to view signals which are tied directly to 230V AC mains, both isolated and directly connected.

That’s – d a n g e r o u s – stuff, unfortunately.

And I intend to do this absolutely safely. My own safety first, of course. But also the safety of all the circuits I’m messing about with. One important trick is to simplify: the shorter the checklist of things I need to deal with, the more concentrated I can stay on the task and its risks.

With a multimeter, measuring 230V stuff goes as follows: disconnect all power, attach the multimeter, put it in the right mode, check that nothing touches anything it shouldn’t (and that all connections are solid!), stand back, turn on the power, read out the measurement, and power down again.

That’s already more than enough to worry about. Now imagine hooking up an oscilloscope and adjusting its knobs to get a good readout. Too many risks, potential for mistakes, and wires going all over the place!

I use the isolation transformer as much as possible. The voltages are still just as high between the two mains pins, but at least an accidental single connection to me or anything else isn’t a problem. The worst that can happen is a blown fuse, which in the case of the isolation setup is resettable and is set to go off at 4 amps.

Trouble is, an oscilloscope normally measures between its probe input pin and ground. Yes, ground!

One way to deal with that is to power the scope through an isolation transformer. But that’s actually one of the most dangerous “solutions” one could think of, because that means you lose all safety nets of having a safe ground attached to the instrument, its case, its probe’s BNC connectors, etc. I want more safety, not less!

Luckily, there’s a much better way to do this – but at €200 it’s not cheap:

DSC 2776

This is an isolated differential probe, which I got from BitScope. The box is 20 cm long, so it’s quite large.

Looks like there’s a fair bit of circuitry in there, too:

DSC 2773

It’ll run off an internal battery (which I just added) or off the included 9V power brick.

The point of all this is that you can put any voltage up to 1000 V between the two input pins, and that both of the pins can be up to 600 V “away” from ground potential. Yet the output will still stay within 6.5V of the scope’s ground level. The differential aspect is that it doesn’t care what the common-mode voltage excursions are, i.e. it’ll ignore any voltage which happens to be present on both inputs – only the difference is passed through.

With this probe, all you need to think about is the high voltage on the circuit and on the test leads. And as you can see, it comes with some very well-isolated cables, (huge!) clips, and test hooks. The specs are as follows:

Screen Shot 2011 11 15 at 23 05 28

The 20x range is nice for low-voltage measurements on AC-connected stuff, such as the output of a switching regulator: a scope set to 1 mV/div will be able to display signals from this probe at 20 mV/div, which is enough to view power supply ripple, for example.

Here’s a first test, simply viewing the 230V mains:

DSC 2775

Note that the vertical scale is 200x higher than indicated on this snapshot, i.e. 100 V/div (230 VAC RMS ≈ 325 VAC peak). The voltage shown here is definitely not a 100% pure sine wave – I have no idea why.

Onwards! (with caution)

  1. Nice device! Does it have an auto OFF? Always handy for battery powered and easy to design in day 1.

    The mains waveform is puzzling – was that through your isolation box? An outlet distant from the incoming feed to the main fuseboard/distribution box?

    • No mention of Auto-OFF anywhere, probably not. But at least it comes with a mains adapter.

      The sine wave comes from a direct connection to mains. On a power strip with heavy cabling to a mains outlet – not near the distribution box, but no heavy loads on anywhere at the time either. Maybe an FFT will explain it, I’ll check that later this month.

    • Hm – maybe there’s an auto-off after all – see the manufacturer’s site. And here is their comparison chart.

  2. Try the Rigol in Slope trigger mode (both +ve & -ve) with persistence – this will give a classic “eye” diagram of the mains waveform. You may see spikes, frequency jitter and X10 fuzz around the zero crossing.

    If that peak lopping is consistent, it points at a significant percentage of loading in the locale is from uncorrected switchers. The FFT will show odd number harmonics, tapering slowly as the order increases.

    This type of load really annoys the power utility – the harmonics (especially 3rd) require over-rating of cables and transformers to cope. The currents are real enough, but the harmonic power is virtual so they can’t charge for it ! Ultimately this becomes an additional burden on the consumer to pay for upsizing the transmission/distribution equipment.

    Does this make sense for your locale? Suburban, space heating by gas, proliferation of PC’s, CFL’s?

    • Now about this virtual harmonic power they can’t charge for… How can I use more of that, and less of the regular stuff! ;-)

  3. Suburban, space heating by gas, proliferation of PC’s, CFL’s?

    Yep, that’s here. I’ll do a daytime “eye” + FFT check once my scope arrives (the older RIgol can do neither).

  4. This waveform (clamped peaks) looks fairly typical for what one would get from a diode bridge + capacitor load (i.e. input of virtually any switched-mode power supply). Regulatory agencies usually require power factor conversion circuitry before bridge/capacitor to prevent this and get power factor closer to 1, but it’s becoming progressively more expensive when approaching 1.

    Inductive load (i.e. motors and those bulky CFL lamp ballast) also have power factor lower than 1, but waveform is different.

    • Ah – that makes a lot of sense for a bridge rectifier + caps, which always draw current near the peaks.

      Thanks for the (hidden) Wikipedia link – extremely informative page (it also reminded me to donate to Wikipedia once again – I’m sooo happy with its uncluttered ad-free content).

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