# Measuring AC Voltage and Currency with SCTs

Hi,

I want to know is it possible to measure voltage with SCT-013-000 device or it is only for current measurement. Second question is Currently I need to measure home electricity current and voltage standard is 240v-250v in my region and our maximum is 6KW. I want to know which version do I need to choice between SCTs: http://www.seeedstudio.com/depot/images/product/2009511142810295.gif

There are Current/33ma and Voltage/1v which is I am not sure what do this means. Please advise.

Thanks beforehand.

Regards, Turkel.

Firstly, you can not use a current transformer to measure voltage.

When you use a current transformer you need to connect a "burden resistor" across the output.

If you do not have a load connected across the output, then the output voltage of the current transformer will rise to dangerous levels, as the transformer tries to deliver current into an open circuit.

The devices shown as having a voltage output have a burden resistor already fitted. The variant called current output does not have a resistor fitted, you can choose a value to fit externally, to give you the voltage that you want. This type has back to back zener diodes fitted to prevent the output voltage from reaching a dangerous value.

There is lots of useful information about current transformers at openenergymonitor.

The following is a quote from that site:

Safety In general, a CT must never be open-circuited once it's attached to a current-carrying conductor. A CT is potentially dangerous if open-circuited.

If open-circuited with current flowing in the primary, the transformer secondary will attempt to continue driving current into what is effectively an infinite impedance. This will produce a high and potentially dangerous voltage across the secondary [1]

Some CT's have built-in protection. Some have protective Zener diodes as is the case with the SCT-013-000 recommended for use in this project, or if the CT is of the 'voltage output' type, it has a built in burden resistor, thus, it cannot be open-circuited.

20A voltage output type means

1) It has an inbuilt burden resistor, so it won't generate high voltages and be a hazzard. 2) I presume its 1V full scale, so some overhead for handling > 20A (which is only 5kW)

Note that the output is AC, with 1V rms representing 20A rms. The output amplitude is 1.414V for 20A of course as 250V mains has amplitude sqrt(2) times the rms value.

You have to arrange that output is superimposed onto 2.5V via a resistive divider to one side of the sensor, other side to analog pin. If the resistor divider has about 10k impedance that will protect the pin from overload as well. Add 1--10nF cap to ground at the analog pin to reduce high frequency noise if you like