AC Current sensor

[VictorPS]

Can I use optocoupler to sense AC current? (Shunt)

I have seen an article as above, but I can't find it again.

 

[akg]

Can I use optocoupler to sense AC current? (Shunt)

I have seen an article as above, but I can't find it again.

i presume ur talking abt main (220v/110v ac) supply. yes u can use opto for the sensing (not measuring) ac voltage .

 

[tkvenki]

AC current sensing can be done through,

1. A CT(current transformer
2. A rogowsky coil(Air core CT)
3. A di/dt sensor

THis information i got by my study of energy meters.
But i have not come accross using an opto-coupler for sensing ac current.

Please let me know about the details.

 

[hyedenny]

Hi,
how does one SENSE a/c with an optocoupler?!

 

[VictorPS]

Yes, I mean AC240V, measure current, not voltage, and I do not want to use CT which not easy to get and bulky.

There is such circuit, it use shunt resistor, voltage drop across shunt connect to optocouple input, but opto input need 1.2VDC, so there is a rectifier and pre bias voltage on optocouple input, using CTR (current transfer ratio) , we measure output of opto which is the current under measure.

This is what i remember, but I forgot exactly how the pre -bias circuit work.

BTW, what is di/dt sensor ?

 

[zevon8]

For the opto, you could use something like the H11AA3, an AC input opto, very common in telephone circuits.

 

[motion]

Can you sense current with an opto? Sure you can.

However, can you measure it? Opto-isolators have too high variations in transfer ratios that I don't think they're practical to use.

 

[Papabravo]

For my 36 VAC sensor I use a 1N4004 as a half wave rectifier. The output of the half wave rectifier goes to a capacitor. The capacitor charges to the peak voltage of the AC waveform. This goes to a voltage divider which supplies the LED current.

Now for higher voltages you probably want a divider in front of the capacitor since then you won't need a high voltage capacitor.

Let's see 240 * 1.414 is 340 volts
The first divider should be a divide by 10 with about 20 mAmps available to charge the capacitor to 34 volts. Now run a 2.7K from the capacitor to the anode of the opto and return the cathode to the other AC line.

This will be about 12 mA for the opto with a forward drop of 2.0 volts. If the CTR is only 30% then you should still get a little more than two TTL loads out the other side.

 

[falmeshon]

use a hall effect sensor wired up to an oscilloscope.

 

[Papabravo]

Never mind -- the power required to drop 340 volts down to a useable value for an opto is just too much for the average resistor to handle.

Twinkle twinkle little star
Power's equal I squared R

 

[VictorPS]

For my 36 VAC sensor I use a 1N4004 as a half wave rectifier. The output of the half wave rectifier goes to a capacitor. The capacitor charges to the peak voltage of the AC waveform. This goes to a voltage divider which supplies the LED current.

Now for higher voltages you probably want a divider in front of the capacitor since then you won't need a high voltage capacitor.

Let's see 240 * 1.414 is 340 volts
The first divider should be a divide by 10 with about 20 mAmps available to charge the capacitor to 34 volts. Now run a 2.7K from the capacitor to the anode of the opto and return the cathode to the other AC line.

This will be about 12 mA for the opto with a forward drop of 2.0 volts. If the CTR is only 30% then you should still get a little more than two TTL loads out the other side.

How about measure current but not voltage?

 

[VictorPS]

use a hall effect sensor wired up to an oscilloscope.

How to design the coil so it will detect small current <0.5A?

 

[Papabravo]

VictorPS

An opto isolator measures current not voltage, but has a forward voltage drop of 1.0 volts or so. Something must dissipate the current to drive the LED part of the opto times 337 volts. That's the sticky part.

 

[VictorPS]

Papabravo, thanks for the schematic.
The circuit is to sense AC voltage.

What I looking for is to measure/detect AC current of a load.
e.g: I have a 100watt bulb connect to AC240, I expect to measure a current flow of 0.4Aamp.