Secret Scotland

If it's secret, and in Scotland…

LNT – Strange case of the wonky DC current clamp adapter

I was caught out by a slightly unexpected problem after I threw together a small adapter intended to make DC current measurement a little easier.

I recently took possession of a clever new device, a DC current clamp meter which allows DC current to be measured with the same ease as AC current, using a clip, or clamp, which is placed around one of the current carrying conductors, and avoids the need to break into the circuit and insert an ammeter

In the past, this only worked with AC current as it depended on transformer action, with the conductor forming the primary, and a winding on the current clamp forming the secondary.

Now, magnetic (Hall Effect) sensors are used to pick up the steady magnetic field around a conductor carrying DC current, bringing the same non-invasive technique to this measurement.

I’ve been using this device for a while now, and was quickly impressed at how DC currents down to a few mA could be measured accurately, and matched simultaneous measurement made using a conventional meter in series with the same circuit under test.

One important thing to remember is that such meters have to be zeroed before taking a reading, The Earth’s magnetic field is detected by the sensor, causing a reading, and the orientation of the sensor in that field will also cause a reading. So, if the meter (sensor) is moved, it should be zeroed before taking a reading.

I also found it has a major advantage in that it does not load the circuit by adding a series resistor. In a normal multimeter, the current usually passes through a series resistor to develop a voltage which the meter shows as a current. Unfortunately, at low currents, this resistor has to be large enough to produce a volt drop across itself which the meter can display. This can alter the current significantly, and experienced techs will take this into account. As a real life example, in a battery operated set of 34 white LEDs, these showed about 40 mA with the clamp, but this figure dropped to about 20 mA when an ordinary multimeter was used (in series with the battery and LEDs), and they became significantly dimmer due to the increased load and reduced current. Inserting the conventional meter was therefore causing a significant error.

As it proved to be so good for low currents, I wanted to use it for battery powered devices, but these often hide the batteries inside a battery compartment, so have no leads the meter can be clipped around, or have a single moulded lead containing both conductors

I threw together the item shown below, where the loop of wire on the right is soldered to either side of a very thin (<0.25 mm) piece of double-sided fibreglass PCB material which can be pushed/placed between one of the battery terminals and its connector in a battery compartment.

DC current clamp adapter

DC current clamp adapter

I should have known better…

That things wouldn’t be that easy.

I’d carried out a number of tests to satisfy myself that there wasn’t any significant effect due to having the clamp near the other conductor (carrying the ‘return’ current in the opposite direction to the conductor which was clamped), of the direction of the current (relative to the clamp), and (within reason) how carefully, or not, the conductor passed through the clamp

The prototype adapter shown above apparently DOES produce anomalous readings.

I found that reversing the direction of the wire through the clamp, or just moving the wire, could cause a significant difference in reading. The reading in one direction would fall while that in the other direction rose. Very roughly, if the current was 40 mA, then I could find that the reading in one direction might rise to 60 mA (as I moved the wire), while it would fall to 20 mA if I reversed the direction of the wire through the clamp. I had suspected a possible variation (when I noted the unintended  small loop the prototype formed), hence the check – which proved to be worthwhile.

Rather than agonise over the ‘Why?’, I just cut the blue wire and extended it, to avoid having such a pronounced loop, ensuring the wire can be kept straighter as it passes through the clamp.

A quick check confirmed that the directional anomaly appeared to have been cured by this fix.

I suspect  combination of effects – the position of the sensor in the magnetic circuit within the clamp, and the shape of the magnetic field around that unintended small loop, which will produce an asymmetric magnetic field around the clamp. These two effects could result in reading that change depending on where that little loop lies around the clamp. This doesn’t happen when the conductor passes straight through the clamp, and produce a reasonably symmetric field around itself.

Oh well…

The real test had actually just been to see if that little piece of double-sided PCB would slide into the battery compartment – which it does perfectly.

Finding the anomalous readings was a bonus, which I can correct for when I make a tidier version of the adapter.

Historically this is nothing new

While i was poking around the archives, I was reminded that DC clip-on ammeters are nothing new, as seen by the wording on this Tong-test scale.

Note the correction figure shown at bottom left, and the patent date on the right, 1924.

AC DC Tong-test scale

AC DC Tong-test scale

These used to be popular for measuring DC current of rechargeable cells in factories – not much chance of measure 20 mA with them though.

This type used moving iron meter movements, which relied on the magnetic field produced in soft iron by the current, and could operate on AC or DC circuits.

They also achieved multi range operation by swapping out the entire movement, complete with scale, and you can see the clips on the side of the instrument, which held each movement in place within the magnetic circuit of the clamp behind. The moving iron part (at the base of the pointer) slid into a hole in the centre of the clamp.

AC DC Tong-test view

AC DC Tong-test view

Not exactly a handy pocket meter – they came in a carrying case, holding the clamp plus a selection of scales.

And finally – SAFETY!

I have to be honest, and admit I wasn’t wearing the proper safety gear when I carried out the tests mentioned above 😦

No hard hat…

Or full face visor.

Or safety glasses under the visor.

Or full face mask.

Or overalls.

Or high voltage rubber gloves.

Or gloves to protect those gloves (from piercing).

Or, out of sight, insulated footwear.

Or a rubber mat.

Using a clampmeter safely

Using a clamp meter safely

It’s not a bad idea (I’m having fun, not mocking).

I used to have a 1 cm length of wire embedded in the clear plastic face of one of my multimeters, blown there and melted into place as it cooled from white-hot to red, after a fault developed in ordinary mains as I was carrying out some tests. I left it there for years, and only removed it recently as it became too irritating where it blocked visibility of the scale.

When I was called in to carry out tests in a factory, I was intrigued when the sparky there had everyone stand behind a brick wall each time we powered up a suspect control panel. It made a lot of noise, but never blew up. Having seen such an event in videos, I know I don’t want to be standing beside such an event. Safety gear wouldn’t really do a lot of good where industrial power is concerned and there’s a serious failure.

14/09/2019 Posted by | Civilian, photography, Uncategorized | , | Leave a comment

Ayr’s Water On Tap (Not)

I came across the animals gathering at a new watering hole in Ayr a while ago.

Passing the same spot recently, it seems this has already dried up.

Ayr Water On Tap NOT

Ayr Water On Tap NOT

How it used to be.

Ayrshire gorilla leaving watering hole

Ayrshire gorilla leaving watering hole


14/09/2019 Posted by | Civilian, council, photography | , , , , , | Leave a comment

Glasgow bus story starts on subject of stopping a bus – then ends on saying ‘Thank You’ to the driver.

I’m sometimes guilty of losing the thread when I start writing about one thing, but then forgetting where I started and becoming diverted onto something else.

I don’t think I ever managed to include content I didn’t even refer to in the text.

I looked at this article since I’d recently caught a bus which stopped for me simply because I was standing at the bus stop. I’d have missed it otherwise.

We asked Glaswegians if you always need to flag down a bus- here’s what you said

I’m not sure what was wrong with its display, normally a very bright and readable yellow, this one was more like a dim white I couldn’t read from a distance (or even closer) and thought the bus wasn’t in service, so hadn’t put my hand out. But it stopped anyway (and, no, nobody was getting off).


I may be wrong, but while the article concluded you probably want to make sure you get that hand out there and let the driver know you want the bus to stop, it actually contained a poll about whether you should say ‘Thank You’ to the driver.

Bus driver Thank You poll

Bus driver Thank You poll

As someone taking up regular bus journeys recently (having tended towards trains in the past), the one thing that struck me was just how polite the vast majority of passengers were to the driver, and how prevalent an offer of ‘Thanks’ at the end of a journey had become.

Sometimes it almost like a parting of old friends when a group gets off, and they all offer a ‘Thank You Driver’.

Unfortunately, that warmth is matched by the abuse given to the driver by the few wasters who want to argue with the driver, or dispute their tickets in some way. The most recent being one individual who got on the bus and apparently ignored the driver’s request to examine his ticket more closely, and planted himself at the back of the bus until the driver stopped the engine and began to phone for assistance. This character only moved and took his ticket to the front after the passengers started to growl at him, and told him to stop swearing. And, no, he wasn’t drunk, but just seemed to be spoiling for a fight/argument.

I don’t know what his problem was, since the driver was happy with the ticket once he was given the chance to look at it.

First Bus

First Bus

14/09/2019 Posted by | Civilian, Transport | | Leave a comment


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