# project problem

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

1. ### xooxoxo Thread Starter New Member

Nov 19, 2009
2
0
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

transformer.

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

Thank you.

2. ### SgtWookie Expert

Jul 17, 2007
22,202
1,791
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 Member

Mar 12, 2010
469
41
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 Distinguished Member

Jan 11, 2009
825
57
And beware an open circuit on the output of a current transformer can give quite a shock at high voltage.