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Simplest way to know if current is flowing in trace

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HSPalm:
Hello,
I am interested in finding a simple solution to have a digital signal to toggle when current is flowing or not in a wire

- The wire is conducting 230VAC
- The current will be around 90mA, but I'd like to widen the ranges of detection as much as possible
- The detection circuit must sit "on the safe side", meaning galvanic isolation

There are lots of specialized hall effect or CT based ICs to measure the amount of current for an analog reading. Yes I can put this analog reading into a comparator, but looking for a simpler and cheaper solution.

1. How much inductance would I need from a series inductor to create a magnetic field to be picked up by close by digital ouput hall effect sensor?
2. Can I drop some voltage over a series resistor, and have the voltage drop power an AC photocoupler with digital output? I don't like putting series resistor, because I'm afraid of short circuit events.
3. Those "wireless" voltage detectors that electricians use, how do they work? Must current be flowing for it to work?

Thank you for chiming in with your thoughts!

CaptDon:
Those 'voltage detectors' do not need current flowing in the wire, only voltage to be present. You can use them to trace the hot lead of a wall receptacle even if there is nothing plugged in to the receptacle. I also like the two piece 'tracer' units. You can plug the little generator source into a receptacle and then use the detector part to sniff the fuse or breaker panel and find which circuit the receptacle is on. There are hall effect sensors made for PCB mount that have the current conductor built into them. We used some rated to over 30 amps but there are also more sensitive versions. They are available unipolar and bipolar. Of course bipolar is best for A.C., but then you also need a bipolar supply.

bob91343:
I don't know what speed of response you are looking for, but consider a thermistor.  Put a small series sensing resistor (if you even need it) next to a thermistor and see it change resistance with current.  You can also put a second one in an area of no current and look at the differential.

I have a DVM, HP3456A, that has temp readout with a 5k thermistor.  I can set it for 6.5 digit temp readout and watch the millidegree change.  This would work as a probe to look for traces that are carrying current, or not.

Berni:
90mA is not going to heat up a wire all that much. But temperature is a way of doing it. That youtube woodworker Mathias made a dataloger for a hot water heater using a I2C thermometer strapped to its mains cable. There the currents are large enough to make a easily detectable temperature difference.

The easiest way to do this is a current transformer. You can buy them and they are just simply a toroidal inductor with a lot of turns. You pass the wire trough the middle of the toroid and this turns it into a transformer where your sensing wire is a single turn primary. If the toroid has 1000 turns this makes it a 1:1000 transformer so passing 1A trough the sense wire gives you 1mA on the output. This 1mA can then be sent into a 1 KOhm resistor to produce 1V. So you now have a completely isolated voltage that represents the current in the wire. All you need then is a comparator to detect it being above it. If you don't want to use a comparator then you can build your own out of a adjustable voltage reference like the TL431, or you can simply use a NPN transistor as that will turn on when you place about 0.6V on the base (Tho not quite as temperature stable as a reference).

fourfathom:

--- Quote from: Berni on December 03, 2021, 06:53:25 am ---90mA is not going to heat up a wire all that much. But temperature is a way of doing it. That youtube woodworker Mathias made a dataloger for a hot water heater using a I2C thermometer strapped to its mains cable. There the currents are large enough to make a easily detectable temperature difference.

The easiest way to do this is a current transformer. You can buy them and they are just simply a toroidal inductor with a lot of turns. You pass the wire trough the middle of the toroid and this turns it into a transformer where your sensing wire is a single turn primary. If the toroid has 1000 turns this makes it a 1:1000 transformer so passing 1A trough the sense wire gives you 1mA on the output. This 1mA can then be sent into a 1 KOhm resistor to produce 1V. So you now have a completely isolated voltage that represents the current in the wire. All you need then is a comparator to detect it being above it. If you don't want to use a comparator then you can build your own out of a adjustable voltage reference like the TL431, or you can simply use a NPN transistor as that will turn on when you place about 0.6V on the base (Tho not quite as temperature stable as a reference).

--- End quote ---

Yes the current transformer is one way to go for good isolation, but of course you're going to get an AC voltage output which you will need to detect.  The 50/60 Hz ones are fairly bulky as well.

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