Brief sum up (you can go directly to the end if you already read the sum up in the other question)

For a home project I'm making a controller to drive a two-direction 230V motor. For those unfamiliar with this, it is a motor with two windings; when you power one of them the motor turns clockwise, when you power the second it turns counterclockwise.

Excluding the power earth, the motor consequently has three wires. The neutral is always connected to the mains N, while the other two get connected to the L 230V wire when the motor should be turned. Only one winding should be connected to the L at a time, the other is left unconnected.

For this project, space is very critical; please keep this in mind when replying.

Now, in this project I need to monitor the voltage on the two wires, in order to see whether a command is sent, and to monitor the current flowing in the motor, in order to understand when it finished the movement. This question is about the second problem. You can read about the first here.


Ok, so I need to detect the current inside the motor. What I thought of is detecting the current through the neutral wire (I don't need to know which winding is absorbing).

The current in the motor is about 500mA - 1A.

I need a simple on-off information (with a threshold which has a fairly high margin).

What I came up with is using something like the ACS712, which gives me the current through the terminals, but it seems quite overkill to me (I don't need to measure it, just an on-off information is fine).

Using a shunt can be ok, but insulation between the 230V and the low power should be enforced.

And... space costraints!

Do you know any technique which is suitable in a small space?

Best regards

  • \$\begingroup\$ "Space is critical" could be better defined. In many of the things I work on that means a part no larger than 1mm x 1mm x 0.65mm. In other "space critical" things a TO-246 would be fine. I'd look at some type of hall sensor for a current present/absent sensor. If it's 230VAC you would need some type of interface circuitry on the output to figure out when the current went away as opposed to an AC zero crossing. \$\endgroup\$
    – John D
    Aug 18 '18 at 20:13
  • \$\begingroup\$ @JohnD consider that I have a volume of roughly 20x30x20mm, and I have already started to mount pcbs in "3d" to be able to get more space. I don't have a defined space, but solutions like using a current TA like this are not feasible for me \$\endgroup\$
    – frarugi87
    Aug 19 '18 at 5:14

Current sensor with hall effect solves both detection and insulation problems.

I don't need to know which winding is absorbing

However, in your other question:

in this project I need to monitor the voltage on the two wires, in order to see whether a command is sent

So, why don't you use two hall effect current sensors on two lines and kill two birds with one stone - get current and direction simultaneously.

This also takes care of your third stated goal of minimal space.

  • \$\begingroup\$ The main problem with sensing the 230v with the hall sensor (so using two) is that the motor stops itself at the endstop, so voltage is present but current is not flowing. Or at least this is what I thought, but in the other question someone pointed out that there is possibly a capacitor to get the third phase. If the circuit absorbs enough current, maybe I will be able to switch to this solution. Do you know any cheap and small sensor which is suitable for this? At present, I only know the APS712 5A.. \$\endgroup\$
    – frarugi87
    Aug 19 '18 at 5:19
  • \$\begingroup\$ Sorry, ACS712... \$\endgroup\$
    – frarugi87
    Aug 19 '18 at 5:33
  • \$\begingroup\$ @frarugi87 - there's also the KEMET AC-series detectors... might be more useful for smaller currents? \$\endgroup\$
    – Jules
    Aug 19 '18 at 7:55
  • \$\begingroup\$ @Jules the main issue with those is that I'll need to mount them externally, since they are quite big... \$\endgroup\$
    – frarugi87
    Aug 19 '18 at 8:03
  • \$\begingroup\$ @frarugi87 those Kemet transducers are 17x17mm. I would not call them big. Anyway, google for "current sensor" and you'll find hundreds if not thousands of them. Allegro chips are probably most popular and can be found everywhere for a few bucks already mounted on breakout board. \$\endgroup\$
    – Maple
    Aug 19 '18 at 9:39

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