project problem

Discussion in 'General Electronics Chat' started by xooxoxo, Mar 21, 2010.

  1. xooxoxo

    Thread Starter New Member

    Nov 19, 2009
    Hello everybody

    I have a problem in a project I'm working on these days.

    The thing I'm doing is, there is a current transformer that senses the leakage current,

    the current induced in the current transformer is low (100mA max), and I need to use

    this current to trigger voltage comparators (IC: LM311), three comparators each works

    at a different voltage.

    And for this I needed a current to voltage circuit, and what I did is that I used a

    transistor (2N2222), the sensed current (AC) goes into Base, and a DC voltage source

    applied at the Collector, and the Emmiter works as a common ground, R(Base) = 10

    ohms, R(Collector) = 100 ohms, when Base current increases there will be a voltage

    drop at the Collector, and I use this Collector voltage to apply on the voltage

    comparators, I tried this circuit with the current transformer and it didn't work. So I

    tested this method but used a function generator instead of the current transformer for

    testing, and found that there is no voltage drop till the base voltage reaches 1.3V, and

    the actual voltage that comes from the current transformer is less than 0.3V, even that I

    tried to make the current from the function generator the same as the current from the

    current transformer, it works but the voltage is higher than the voltage in the current


    Please help me to solve this problem, even if it mean to use any method.

    I tried to explain it as good as possible, and sorry for my English I'm not good.

    Thank you.
  2. SgtWookie


    Jul 17, 2007
    Since it is a current transformer that outputs up to 100mA, you can simply run that current through a resistor. A voltage will be dropped across the resistor. This is Ohm's Law.
    E = I * R, or Voltage = Current * Resistance
    Another way to look at the equation is:
    I = E/R, or Current = Voltage / Resistance.
    A current of 100mA flowing through a 10 Ohm resistor will cause a 1v drop across the resistor.
    The same 100mA current flowing through a 1 Ohm resistor will cause a 100mV drop across the resistor.
  3. Bychon


    Mar 12, 2010
    I think he needs to change the transformer current to a DC signal, then compare it. Of course, the first thing the output of the transformer needs is a resistor. That converts current to voltage. Then I'd use an op-amp as a precision rectifier and send that output to the comparators.
  4. russ_hensel

    Well-Known Member

    Jan 11, 2009
    And beware an open circuit on the output of a current transformer can give quite a shock at high voltage.