# cheap AC current sensing for a microcontroller

May 6, 2011
5
Hi,

#### solosynergy

May 6, 2011
5
i am looking for a non instrument measurement technique. that is in a system. so it needs to be compact too.

#### poor mystic

Apr 8, 2011
1,074
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).

Last edited:

#### solosynergy

May 6, 2011
5
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

Apr 8, 2011
1,074
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

May 6, 2011
5
would that mean that i need to use an opamp and then sample the signal at the ADC

#### poor mystic

Apr 8, 2011
1,074
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.

May 6, 2011
5
thanks a lot.

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