# Current calculation

1. Apr 23, 2013

I am doing an experiment where am giving 5 volts sinusoidal input voltage to a coil and receiving 1.5 volts in other coil how can i calculate current flowing through it..? if i take current as the ratio of input voltage to impedance of circuit will it be correct...?

2. Apr 24, 2013

### Simon Bridge

Is that use consistent with what impedance is?

3. Apr 24, 2013

I didnt understand what you are asking for

4. Apr 24, 2013

### Simon Bridge

You say you are uncertain if it is OK to use impedance the way you describe to fine the current.
Well... what is the definition of impedance? How does the definition relate to voltage and current in a circuit?

5. Apr 24, 2013

### CWatters

Have you considered applying the law of conservation of energy?

6. Apr 24, 2013

No...

7. Apr 24, 2013

### nasu

With just the data given in the OP you cannot calculate the current.
Many different transformers can be built with the same ratio between input and output voltages and they will all have different currents through the coils.

8. Apr 24, 2013

What else information is required ..?

9. Apr 24, 2013

### nasu

It is not clear if you mean a theoretical calculation or you mean to measure things.
What is the purpose of your investigation?
The easiest way will be to measure the current itself.

10. Apr 24, 2013

I am telling about practical case.. right now i do not have a device to measure it so, am asking what is the other way through which i can proceed to calculate it

11. Apr 24, 2013

### nasu

Well, you need to measure something. How would you measure the impedance?
Is there anything that you can measure, then? Measuring just the voltages is not enough.
At least from what you say. The description of your setup is very vague.
You don't even mention what current do you want to calculate. In which coil?

12. Apr 24, 2013

I am operating at 7 MHz, 5v frequency sinusoidal voltage, which is given to primary coil from function generator. I am receiving 1.5 volts in the secondary coil i need to calculate the power efficiency for which i am thinking of calculating current... now you got my problem?

13. Apr 24, 2013

### nasu

The power efficiency of what? Of the transfer between the primary and secondary coil?
This will depend on the load in the secondary, right?
If the secondary is open circuit, there is no power transferred, is it?
But you may be looking at something else. Who knows?

14. Apr 24, 2013

### davenn

You are still not quite understanding that you cannot measure the current when only given the voltage

to measure current you need to know 2 other values Resistance and Voltage ( for a DC circuit) or in your case, an AC circuit, Impedance and Voltage.
You are still giving only one of those required values, the 5V AC.
You need to work out the impedance of the primary coil ( its AC resistance), then you can work out the AC current flowing in the primary coil. From there you will be able to move on to working out the current flowing in the secondary coil with your given Voltage (1.5V) across what ever resistance of load you have across the secondary coil.

regards
Dave

15. Apr 24, 2013

### davenn

Hi nasu

hopefully the "penny will drop" for him soon :)

D

16. Apr 24, 2013

Yeah you are correct that i gave only one value but my question was if we can calculate with impedance and voltage. I measured the values of L, C, R, Z with LCR meter and i found that all values are changing with frequency. at 7 Mhz i got the impedance as 740 ohms. so with this information i hope i can solve for current according to you. One more doubt i have is why L,C, R values are changing with frequency?

17. Apr 24, 2013

### davenn

because inductive and capacitive reactance change with frequency. That's the way it is :)

that is .... the value of the capacitor etc doesnt change ... its reactance changes for a given freq.

and as a result, your impedance will change with freq. So in a given circuit where you want a fixed input and output impedance, you need to calculate those reactances ( the resistor, capacitor and inductor) values for the freq of interest to maintain your required impedance

Dave

18. Apr 24, 2013

I am not sure of if am doing correct or not, i am checking the output from oscilloscope am not connecting it to any load. I just want to know how much power i am transmitting from primary coil and how much am receiving at the secondary coil. So can you tell if am going in proper way to get what am willing to get?

19. Apr 24, 2013

### davenn

As nasu and I have said, you are not going to have a current flowing in the secondary coil unless you have a load across it ..... so no load = no current flow = no power to measure

Dave

20. Apr 24, 2013