Electronic – Capacitor for current transformer measurement circuit

capacitorcurrent measurementcurrent transformersensor

I want to measure an AC current with a current transformer and an arduino device. I found on a website (http://www.homautomation.org/2013/09/17/current-monitoring-with-non-invasive-sensor-and-arduino/) that measurement circuit:

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My question is, why do I need a capacitor here? The explanation from the website is:

The capacitor C1 (10uF) has a low reactance – a few hundred ohms – and
provides an alternative path for the alternating current to bypass the

But why do I need an alternative path for the alternating current?

Best Answer

The explanation is confusing, what RA, RB do is divide the 5 V of the Arduino's supply by a factor 2 to 2.5 V. This is used as a reference voltage because the sensor can generate positive and negative voltages.

Suppose the sensor makes -1 V, by itself the Arduino cannot handle this, you have to be between 0 V and 5 V. But if you "lift" that -1 V by 2,5 you get 2.5 V - 1 V = 1.5 V which is perfectly OK.

But a little bit of AC current will flow, disturbing the 2.5 V reference voltage. This is where the capacitor C1 comes in, it decouples that reference voltage. Indeed, the AC component will now travel (have a path through) the capacitor instead of the resistors. That's what they are saying.

But stating: "C1 decouples the 2.5 V reference voltage" would probably be easier to understand.