Need help for current measurement...

[BHARGAVSHANKHALPARA]

Hello everyone...

i am new in current measuring concept, and i make one of control application type project...

actually i am controlling one ac induction motor using microcontroller (ATTINY88). and i want to add one extra function into it is over-current protection.(when current more than 6 amp, automatically gets turn off by microcontroller) for that i decide to use current transformer for current measurement. so i need to interface it with microcontroller.

i have current transformer with 1 (one) turn of primary winding and 350 turn of secondary winding. i know that to convert secondary current into voltage i need to connect burden resistor across it. i need to obtain voltage between 0-5v DC from that to apply microcontroller. also for that i need to signal conditioning for that before apply it to microcontroller.

problem is that...

i don't now that which value of burden resistor is best for my application and after obtaining well voltage signal condition circuit require which include (rectifier and amplifier).

after searching about signal conditioning circuit i found this document and circuit.

**broken link removed**

in this circuit i understand about all resistor and op-amp but initial two zener diodes and at last RC time constant make question in my mind that why its here and if here than what should be its value.

please help me in this project...

thank you...

 

[smijesh]

It is Transient Voltage Suppresser (TVS) diode for voltage reference application
RC for filter application. For 60Hz ,Time Period is 1/60 s, 10 times of 1/60 is 1/6
R(In Ω) x C(in F) =1/6s

 

[chuckey]

The burden resistor is to reflect a very low series impedance in the main current circuit. So 6 A in one turn = 6/350 A in the secondary, so with a 100 ohm burden , Vout = 6/350 X 100 ~ 1.6V, voltage drop on primary = 1.6 X ( 1/350) = ~50 mV. So 100 ohms looks reasonable, but could be lower( you could use it as a calibration control). Follow it with the IC rectifier circuit in your attachment.
Frank