cheap AC current sensing for a microcontroller

[solosynergy]

Hi,

1.I am looking for an AC current sensing scheme which is under $2.

2. I have come across various sensing schemes like hall effect, delta sigma true RMS and CT's with rectifiers fed to a microcontroller.

3. All of these seem to be going beyond my budget.

4.is there any way to do the current measurement without a CT

5.In case i use a CT then will i not have to use an opamp to condition the signal to offset it into the positive region and amplify the signal.

6. My current measurement range is 0-5A with an accuracy of .1A

 

[(*steve*)]

have a look at how AC clamp meters work. It's pretty simple and for limited accuracy you should be able to make something pretty simple.

under $2? depends on what you already have on hand.

 

[solosynergy]

i am looking for a non instrument measurement technique. that is in a system. so it needs to be compact too.

 

[poor mystic]

How much current do you want to measure please, and what frequency will it be at?

Some kinds of measurement are easier than others. If you only want to know when current goes above some value perhaps even a reed relay could help you.

Every conductor has resistance. A voltage is developed along every conductor according to the current it carries.

Is there anything stopping you from using a ferrite ring with several turns of sensing wire on it? You'd pass the current-carrying cable through the ring during installation, and otherwise all you may need is a potential divider (series resistors).

 

[solosynergy]

I will be measuring 0-5A @ 50Hz. I have thought about the ferrite ring but then will i need to add extra hardware to rectify the output from the ring.

As far as the resistance technique goes i am able to measure the current based on the voltage drop across the shunt resistor but then i am stuck at as to how to rectify the measured voltage drop.

 

[poor mystic]

You do not have to rectify the signal.
All you need do is bias the signal received by the A-D converter so that the ADC always sees a positive at its input.
The rest can be done in software.

 

[solosynergy]

would that mean that i need to use an opamp and then sample the signal at the ADC

 

[poor mystic]

hi solosynergy
All you need is 3 resistors - a potential divider between +V and ground, with the divided potential fed to the ADC, and a 3rd resistor from the ADC to the shunt resistor from which you wish to sample a voltage.
So the ADC is connected to +V, ground, and the sampled point, by resistors.

 

[solosynergy]

thanks a lot.