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Most inexpensive method for detecting if some current is flowing in 120VAC or 240VAC PCB Trace

Mahonroy

Oct 21, 2014
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Hello,
I am trying to figure out what is the most inexpensive method for detecting if current is flowing in a 120VAC or 240VAC PCB trace.
I came across the ACS730, however this gives me the exact current that is flowing, and they are expensive. A simple yes or no would be preferable and I would assume less expensive?

The PCB trace will generally be flowing around 10-15 amps, but it can go up to as much as 30 amps. If I had to put a threshold on it, I would say anything less than 100mA and we could say no current is flowing... anything above 100mA and we could say yes current is flowing.

Eventually I need to get this signal back to a microcontroller pin, which is 0-3.3V. Because of this, I'm assuming the circuit would contain an optocoupler.

Can u guys advise me on how to go about this? Can this be accomplished with a low ohm resistor somehow? Or is there some inexpensive hall effect sensor that I could locate near the trace? Like I mentioned earlier, I don't need to know an exact current value, just a yes or no.

Thanks and any help/advice is greatly appreciated!
 

Minder

Apr 24, 2015
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Would the ACS712 work for you? They are found on ebay for $1.00 complete with circuit board
M..
 

Mahonroy

Oct 21, 2014
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Would the ACS712 work for you? They are found on ebay for $1.00 complete with circuit board
M..
I looked up the part on digikey and they are labeled as "Not for new designs", they also start at $5, and plateau around $2.50. Since this is something I plan on getting assembled it has to be a legit part that can be sourced.
 

hevans1944

Hop - AC8NS
Jun 21, 2012
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An inexpensive Hall Effect sensor is probably your better alternative to a low-resistance current-sensing resistor. You can also measure the voltage drop along a length of PCB trace (will be in the low millivolts to microvolts range, depending on geometry of trace and current through it) without opening the trace to insert a low-resistance current-sensing resistor, but that requires a differential measurement with a high common-mode voltage to rejection, a really expensive approach.

Why are you being so cheap about spending a few bux since you "plan on getting assembled it has to be a legit part that can be sourced"? Are you putting together a large quantity and need the cheapest solution possible? There is a thread here on this forum on how to do AC current sensing with Hall Effect sensors for motor control of a dust collector system used in woodworking. Several motors can be sensed simultaneously to control the motor of a single dust collector servicing all the woodworking tools.
 

Minder

Apr 24, 2015
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An inexpensive Hall Effect sensor is probably your better alternative to a low-resistance current-sensing resistor. .

The ACS series IS a Hall effect device and offers galvanic isolation ;)
M.
 

duke37

Jan 9, 2011
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A current transformer could be used if the wire can be broken. You will need a little transformer with a large turns ratio and a load resistor to give a voltage output. There will be galvanic isolation.
The transformer may cost more than than the Hall effect device.
 

Mahonroy

Oct 21, 2014
69
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So it sounds like the only feasible option is to cut the trace, and insert in an ACS730? (this one being a 30 amp version).

The reason why I was being cheap with it was because I didn't need all the features of the ACS730, so it seemed like surely there was a cheaper option with less features.

Speaking of which... would it be possible for me to run a thin trace right up next to the 240VAC thick trace, so that if current is being drawn it would generate interference onto the small wire, which I could detect and relay to the microcontroller? Like I mentioned earlier, I don't need to calculate an actual amperage value, I just need a boolean yes or no.
 

Minder

Apr 24, 2015
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A better way may be a small hand wound coil of a few turns around the 240v trace.
M.
 

Alec_t

Jul 7, 2015
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would it be possible for me to run a thin trace right up next to the 240VAC thick trace
That would be dangerous if the thin trace is supposed to be at a safe low voltage. Design rules specify a minimum clearance between traces with a high voltage difference.
 

hevans1944

Hop - AC8NS
Jun 21, 2012
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So it sounds like the only feasible option is to cut the trace, and insert in an ACS730? (this one being a 30 amp version). ,,,
No, there are other options available to sense the AC current in a PCB trace. All that I am aware of create a small AC analog voltage that you must then amplify, condition, and use to set a threshold on/off current. The Allegra device integrates all that and provides galvanic isolation so its output can be connected directly to a microprocessor or other low-voltage electronic circuit. Not bad for a price of less than six bux in onesie-twosie prototyping quantities.

...The reason why I was being cheap with it was because I didn't need all the features of the ACS730, so it seemed like surely there was a cheaper option with less features. ...
Exactly what features of the ACS730 do you not need? What "less features" do you need? It is a simple device with almost no features, other than an analog output signal that follows the amplitude and polarity of the input current. It's output is half of the supply voltage when no current is being sensed. When an AC current is present, the output swings above and below that value (nominally +2.5 V DC with +5 V DC supply) until the peak swings approach ground or the positive supply rail, at near the rated current. However, even if the rated maximum current is exceeded, the device still works, just not as accurately. I suppose Allegra could have built in a "self destruct" feature if the rated current was exceeded, forcing the end user to swap out for a higher rated device under this "failure condition". But they didn't. It degrades gracefully when the maximum rated current is exceeded.

... Speaking of which... would it be possible for me to run a thin trace right up next to the 240VAC thick trace, so that if current is being drawn it would generate interference onto the small wire, which I could detect and relay to the microcontroller? Like I mentioned earlier, I don't need to calculate an actual amperage value, I just need a boolean yes or no.
Yeah, that would be a one-turn air-core transformer and the signal introduced into the parallel "thin" trace is not interference, it is an inductively coupled potential proportional to the current in the "thick" trace. Unfortunately, the potential induced is quite small, on the order of microvolts or less, so you have your work cut out for you getting that up to a level you can use to sense an on/off threshold. Good luck doing that inexpensively. Same applies if you pass the "thick" trace, or the wire connecting to it, through the core of a current transformer. Larger signal to work with, but still some signal conditioning and further processing required. And most CTs that are not hand-wound by hobbyists on toroid magnetic cores, are expensive.

Are you trying to modify existing equipment by adding a microcontroller to sense the presence of AC current? Or are you designing something from scratch? Please tell us what you are trying to DO, less about how you think it should be done. Someone here may have already spun that wheel for you.
 
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