# CURRENT TRANSFORMER 30A / 5A Interfacing with AVR Atmega32 ADC

I am trying to Calculate Current of 1HP Motor uses with Motor Starter Box. Initially when Motor is started i can see current shoots upto 20-22A on analogue meter in starter box.

I want to measure this Current Continously while the motor is in operation.

For that purpose i have purchased 30A / 5A (CT) (12 Turns wound on CT) AC Current Transformer 30/5 A (Two turn Primary)

on 30A AC load it will provide me 5A AC output on secondary (CT output). Burden Capacity is 2.5VA. I am confused with burden resistor calculator and how can i interface CT with ADC of atemga32.

• A data sheet is needed to answer this with certainty. Dec 27, 2015 at 11:46

Most current transformer use a resister in parallel to convert the current into voltage. This is because most ADC's are Voltage ADC's. The resistor should be chosen such that the peak expected current produces the maximum voltage that your ADC can read. There is one caveat. Your transformer says that it can/should only dissipate 2.5VA (watts) into the load.

Calculation for the resistor is: V_adc_max/I_current_transformer_max = Resistance. Also 2.5 >= (I_current_transformer_max)^2 * Resistance.

You have about 24A which gets you about 4A on the secondary

you're allowed 2.5VA so the max RMS secondary voltage is 2.5/4 = 0.625 volts RMS (I assume about 1V peak) which seems not far from ideal in this application.

To get that voltage you want a resistor 0.625/4 or about 0.15 ohms.

That's a E12 size so should be readily available.

Be sure to get a 2W or larger part. (2W should be plenty as the 22A surge is presumably only brief)

For interfacing bias one end of the burden resistor to about 1/2 VCC and connect the other end to the DAC input