Multi-Conductor DC Clamp Meter

Question

Does a multi-conductor DC clamp meter exist? Possibly using hall effect sensors in a specific orientation?

I typically work with 24-28V DC solenoids valves & pressure transducers with 2 and 4-conductor configurations, respectively. The solenoid valves can have 50'+ of insulated 2-conductor SJOOW cable from where they are actuated using solid state relays to the valves themselves. Accessing the solid state relays can be a pain and actually fitting a clamp meter around the + excitation line is difficult. I'm trying to figure out a tool to easily measure if a DC circuit is powered or not. Ideally, this tool would be in a clamp meter configuration, where I can wrap it around any 2-conductor DC cable and be given some sort of audible confirmation that a load is present and even a rough current draw estimate (does not need to be accurate).

I know what I'm asking for seems impossible but I've referenced this previous forum several times now in regards to AC circuits and it gives me hope. Can single clamp measure total current from 3 wires?

An even more ideal feature would be if this clamp meter could also work on a 4-wire pressure transducer (+ Excitation, - Excitation, + Analog, - Analog). However I don't know how the hall sensors could be aligned for this application. Especially if the cable is shielded twisted 2-pair which we typically use (sounds even more impossible).

Any input is appreciated!

Answer 1

I'm trying to figure out a tool to easily measure if a DC circuitDC solenoid is powered or not.

Yeah. That tool is called a compass. Your phone has one :)

You're trying to measure current via a magnetic field emanating from a cable... when you've got this comparatively amazing current-to-magnetic field transducer... better known as a solenoid coil? Forget about the cable. Who cares. You want to know whether the solenoid is ON or OFF, not whether there's current in the cable - the latter is just a proxy for the solenoid state.

You can make a better bespoke tool by pairing some miniature Arduino board with a magnetometer and some LEDs/display to indicate field orientation and strength.

E.g. AdaFruit LIS3MDL (QT megnetometer) + Adafruit FeatherWing OLED STEMMA QT (display + QT connector) + Black Adafruit Feather RP2040 (RP2040 board with QT connector and LiPo charger) + two QT-to-QT cables + a LiPo battery. That can be wrapped together with Scotch tape (pretty much) and you have your bespoke solenoid activity sniffer. You can even calibrate it for the particular solenoid types you use, so that if you touch it to a certain location on the solenoid body, you can have readout directly in mA :)

If you have some solenoids available to calibrate such a device, then you should have it done in a couple of days at most. These days it's easy stuff. The whole thing can be done in nothing but MicroPython running on RP2040.

Use the buttons to select the solenoid type, effectively changing the calibration constant between the magnetic field magnitude and the current passed through the solenoid.

An even more ideal feature would be if this clamp meter could also work on a 4-wire pressure transducer (+ Excitation, - Excitation, + Analog, - Analog)

For that you'd need a suitable ADC and it cannot be clamp-on, it'd need to connect directly to the transducer (while said transducer is powered, or it could power it instead). AdaFruit's ADS1115 breakout would work well, since that converter is quite sufficient for direct 4-wire transducer conversion at the 8SPS data rate (with highest oversampling essentially) and PGA set to x16 gain. You'd use one channel to measure the excitation voltage (scaled down of course with external resistors), and another channel to measure the bridge output. The measured value would be the ratio of the two, in the units of V/V. You'd then apply the pressure sensor's calibration constant to convert that to pressure.

Answer 2

Can't remember the name but its the line integral of H dot dl around a closed loop = current enclosed, and since you (should) have the same magnitude current, but in opposite directions, the current enclosed is zero. That is on the conductor.

But ... I have had a problem at work with inaccuracies measuring DC current; when the DC clamp meter got too close to the coil of a 24VDC relay (designed to carry 400A), the reading on the meter would change. And then if I lined it up directly with the coil of the relay, I could get even bigger offset. It shouldn't because it has a metal shroud over the coil, but there it was. As I recall, the coil side was 1 < x < 3A.

Maybe you can poke around the area of the coil of the known working solenoid valve with your DC Clamp meter you are trying to analyze, assuming it is safe, and you have access to that point, see what happens.