Isolation in this case, is from the mains, and it transpires that finding something that will do the job safely is tricky.
Three isolated Hall-based current sensing modules are easy to obtain, as far as I can tell, based on:
- Allegro’s ACS712 or clones
- Allegro’s ACS758 or clones
- Winson’s WCS1800
All will work with a 5V MCU on the cool side.
A heath warning: Do not mess with the mains if you don’t know what you are doing.
In modules, any safety performance that these devices have is often ruined by poor pcb design.
WCS1800 is linear over ±25A and has a cable pass-through hole so its isolation is beyond question if mains insulated wire is used for the high-voltage circuit.
However, it is not factory-calibrated, and both gain (72±12mV/A, ~+2% ~-0.5% over -20 +40°C) and offset (±150mV, ±1mV/°C) are quite variable so calibration, possibly regularly, is necessary.
As it is has a hole, multiple turns can be passed through it to increase gain.
ACS758 (right) comes in a ±50A version, is factory-calibrated, and is rated for 4.8kVrms for a minute. Its isolation is rated for 1,358Vpeak basic, and 672Vpeak reinforced. Creepage and clearance are 6.9mm. Gain is 40mV/A and offset is ±5mV at 25°C. It has a built-in conductor, and is therefore limited to one turn.
Sinomags’ STK-600/M (data sheet is from DF Robot) is based on TMR and is similar to ASC758, comes in a ±50A version, and is rated for 4kVrms for a minute, with 8mm creepage and clearance.
Allegro’s ACS712 comes in a ±5A version, and so would be perfect, except that it is only rated for basic isolation at 297V, with 4mm creepage and clearance. It does not have a reinforced isolation rating, and so can only be used on the mains safely under limited circumstances. Gain is 180 to 190mV/A.
If only functional isolation was needed – for ground-loop breaking, for example – this seems to be a great part.
None of the above have a reference voltage output, and instead they need to be measured against Vdd/2.
Oh where is the reasonably-priced 5A sensor with reinforced isolation – does anyone know??
The application is rectified mains-fed motor armature and field current measurement.
A couple of ACS758 modules are winging their way, and if the boards are well designed and the chips are genuine, they might be the answer….
