Current sense transformer from PCB traces (100-240V @ 15A)?

[jpyeron]

Thoughts and questions welcome - I feel there must be dragons around the corner or there are ways much "better".

Constraints are

• physical space
• assembly complexity - in house 
• BOM $ / availability

All the CT are eating up space we do not have. The ICs we find are for 100V and lower.

Wild thoughts were use thickest readily available center layers and loop traces on the surface layers. Obviously coils can be made: https://hackaday.com/2020/03/31/magnets-turn-flexible-pcb-into-electric-grasshopper/

When all set and done, the relay will turn a line off (NC) and the current will be reported to the bus for rough order of magnitude usage reporting to the control module. There will be 20-42 relay / CT modules per bus and 1 or 2 buses per control module.

Yes this is a PDU... Did not like what we found.

[T3sl4co1l]

Rogowski coils have been made in PCB before, if that's what you mean.  Poor gain and low turns count, but maybe that's good enough.

A proper transformer can easily be made with planar cores, if that's an option, but if you're working at mains frequency, you'll need way more turns than will fit that way.

Tim

[Wolfram]

For PCB Rogowski coils, this paper is pretty good: https://www.mdpi.com/2079-9292/12/5/1099/pdf

This example design attains a mutual inductance in the 20 nH ballpark. 15 A RMS at 50 Hz has a peak dI/dt of 15 * sqrt(2) * 2pi * 50 Hz = 6.6 kA/s, giving an output voltage of 20 nH * 6.6 kA/s = 130 microvolts peak, or just under 100 microvolts RMS. This low output level, combined with the increased sensitivity of the coil at higher frequency (rogowski coils have sensitivity linearly proportional to frequency) makes accurate measurement of mains frequency hard.

I wonder if a CT could be practically made with some finite and well controlled magnetizing inductance, but with a magnetic core to push the mutual inductance into a more interesting range for low frequency measurement, making it practical to implement for 50 Hz measurements in the form of a planar transformer. It would be constructed like a CT, but used like a Rogowski coil. The main challenge is that sensitivity would depend directly on magnetizing inductance, which depends strongly on the air gap, and also on core permeability, both of which are hard to do with fine tolerances and good temperature stability.

What are the exact requirements? Putting some numbers to this can make it simpler to find a suitable solution. You mention that you don't find ICs rated above 100 V, but there should be tons of options here, open loop hall effect sensors with internal (ACS712) or external (MXL91208) current conductors, closed loop hall (LEM LKSR-series) and flux-gate (LEM CKSR-series), GMR-based sensors (ACS70331), isolated amplifiers for current shunt measurements (AMC1200), even ones with integrated power isolation (AMC3306).

If I was to design the cheapest and most compact multi-channel current sensing system for a PDU, I would strongly consider resistive shunts. If you place them before the relays, then they can all have the input feed in common. This way, they can all be read by an ADC referenced to a single potential, making it cheap and easy to have a large number of channels. A multichannel ADC would sit at the mains potential and read all of the shunt voltages, and the interface can be done over isolated SPI for instance.

[voltsandjolts]

Suggestion:

https://www.allegromicro.com/en/products/sense/current-sensor-ics

[Terry Bites]

The magnetic coupling (k) will be poor and you'll be vulnerable to stray fields.
voltsandjolts  suggestion is a good one. Allegro and others provide a huge range of isolated sensors.
Rogowski coils are intended for high speed impulse measurements and will not be much use for steady state currents.
Coil design (massive pcb real estate) and support circuitry will cost you much more than a regular CT or Hall sensor.

If I needed to monitor many channels economically, I'd use PCB or resistor shunts and a multichannel isolated ADC.
16 Channel 10/ 12 bit ADCs are as cheap as chips. A few iCouplers a small iso DC-DC. Simple

[Alti]

Constraints are

What is the problem with a shunt? If that is mains voltage 15A, you can use difference amplifier but that is for rather low BW (maybe 1kHz). And if you only need approximate current value (10% error), you can use part of copper cabling as a shunt so no extra shunt needed.

Of course there are dedicated Allegro Hall sensors (as pointed above) but that is for 50kHz BW.

[coromonadalix]

you talk about tons of relays and can not accomodate an xformer or shunt or hall effect sensor ?

[jpyeron]

For PCB Rogowski coils, this paper is pretty good: https://www.mdpi.com/2079-9292/12/5/1099/pdf

This example design attains a mutual inductance in the 20 nH ballpark. 15 A RMS at 50 Hz has a peak dI/dt of 15 * sqrt(2) * 2pi * 50 Hz = 6.6 kA/s, giving an output voltage of 20 nH * 6.6 kA/s = 130 microvolts peak, or just under 100 microvolts RMS. This low output level, combined with the increased sensitivity of the coil at higher frequency (rogowski coils have sensitivity linearly proportional to frequency) makes accurate measurement of mains frequency hard.

Confirming that is a no go / waste of time.

... You mention that you don't find ICs rated above 100 V, but there should be tons of options here ...

The problem creeps in on stock availability:

If I was to design the cheapest and most compact multi-channel current sensing system for a PDU, I would strongly consider resistive shunts. If you place them before the relays, then they can all have the input feed in common. This way, they can all be read by an ADC referenced to a single potential, making it cheap and easy to have a large number of channels. A multichannel ADC would sit at the mains potential and read all of the shunt voltages, and the interface can be done over isolated SPI for instance.

and

What is the problem with a shunt? If that is mains voltage 15A, you can use difference amplifier but that is for rather low BW (maybe 1kHz). And if you only need approximate current value (10% error), you can use part of copper cabling as a shunt so no extra shunt needed.

I like - going to look this way.

you talk about tons of relays and can not accomodate an xformer or shunt or hall effect sensor ?

Correct, the socket and relay takes most of the volume. Fasteners are the second place consumer of space.

[Terry Bites]

Another thought. If you have limted space how about ACHS-719x for about $2 per channel, 400V rms working iso, no shunt needed.

[mikeselectricstuff]

What accuracy do you need ? No reason you can't do a resistive shunt with PCB track, but it will be subject to variations in the PCB process and temperature

[jpyeron]

What accuracy do you need ? No reason you can't do a resistive shunt with PCB track, but it will be subject to variations in the PCB process and temperature

Another thought. If you have limted space how about ACHS-719x for about $2 per channel, 400V rms working iso, no shunt needed.

both on my prototyping... Thanks.