Measuring low AC current (ACS712?)

[gregmcc]

I"m busy with a project which will measure the AC current and AC voltage and then display the values on a LCD.


I've got a ACS712, the 5A version .The problem is I've connected it all up and don't detect any voltage increase on the output pin. It sits at 2.5V.

The problem is the input current is too low - I've plugged my soldering iron in and it only draws 70mA (According to my DVM) From reading the specs of the ACS712 this current is too low for it.

Any recommendations on what I can use to measure these low currents. Are there any other sensors I can use for this?

I also tried plugging in a fan - still no movement on the output pin. How much current do I have to put through the thing to get a change on the output pin?

 

[MrAl]

Hi,

I think you need a higher resolution device to measure much lower currents like that.

You can measure low currents with a resistor pretty easy, as long as it doesnt eat up too much voltage.

What is the range of currents you need to measure?

 

[gregmcc]

Probably to a max of 1A - btw its mains current. 220V AC.

 

[MrAl]

Hi,


And what did you intend to measure the output with, a DMM?
If so, for example if you use a 1 ohm 5 watt resistor you can measure the voltage across it. If it measures 1v that means there is 1 amp flowing, 0.5v then 0.5 amp flowing, 0.2v then 0.2 amp flowing, etc.

 

[gregmcc]

I'm going to connect it to a microcontroller and sample the V and I . Then get the PF and work out the Kwh and cost.

 

[ronsimpson]

What value of capacitor do you have no the filter pin? Pin-6

How are you measuring the voltage on the output? With a DC meter or a AC meter? I believe the output voltage will have a average of 2.5 volts. There might be 1 volts of A.C. on that pin but a DC meter will still measure 2.5 volts.

 

[gregmcc]

I'm not sure what the cap value is - its a smd cap that's on the breakout board.

I'm measuring the voltage using a scope. The output sits at 2.5V DC. I've looked at the AC on that pin as well - nothing.

 

[MrAl]

Hi again,


Yes the resolution on those devices isnt that good really. Have a look on their website. Those sensors are really made for currents a bit higher than 70ma, like maybe twice that minimum.

If you are going to use a uC to measure the current and you want to use a resistor in series with the power device (such as soldering iron) then you have to think about safety also. With the resistor connected to the line and the uC connected to the resistor, that means the whole uC circuit is live and certain extra precautions have to be thought about such as using a plastic case and other things. Im not sure if you want to do it this way or not. There are also current transformers that can measure the current and also isolate the line from your uC circuit. With a current transformer you have the option to wind more than 1 primary turn around the core which gives you better resolution which means the ability to measure low level currents like 50ma.

If you do want to use a resistor like a 1 ohm 5 watt resistor, then to measure 50ma you have to also use an op amp to boost that 50mv up to some reasonable level that the uC ADC can measure. For example, if you boost it by 10 the ADC will see 0.5v, and if you boost it by 50 the ADC will see 2.5 volts.
If you boost it by 10 you will get a range from 0 to 0.5 amps, but if you boost it by 50 you'll have to include a way to switch ranges so you can measure higher currents too. This means making the uC switch the amplifier from a 10x to a 50x, or from 10x to 100x, or whatever works best for you.
If you boost by 5x constantly, then for 50ma the ADC will see 250mv, and for 500ma the ADC will see 2.5v, and for 1 amp the ADC will see 5.0v. That means the ADC will have less accuracy at 50ma and more at 1 amp, but sometimes it doesnt have to be super accurate anyway.

 

[KeepItSimpleStupid]

Is this an exercise or something that you rally want to do? You might check out: https://en.wikipedia.org/wiki/Kill_A_Watt

The current waveform could be non-sinusoidal depending on the controller.

Ass for another sensor look for: allegro hall effect current sensors

 

[gregmcc]

Yip - thats it. I thought about buying one of those but its more of a hobby making it

 

[ronsimpson]

If you only want AC current and there is no DC then use a current transformer.