Low resistance problem

1. Apr 17, 2013

I want to generate magnetic field around a coil.. For this am giving input a sinusoidal wave form from function generator. But the coil(18AWG cu) wire is not drawing much current as the coil has low resistance. When i calculated X and Z at different frequencies they are like this

freq: X : Z
100 0.00771 0.0775
1Khz 0.07669 0.1088
100Khz 7.51 7.51
1Mhz 74.03 74.05

how can i eliminate the problem of low resistance, i think it is acting as short ciruit.. while measuring in LCR meter i even found that my capacitance values are negative how it will happen like this? how can i achieve good field around the coil?

2. Apr 18, 2013

Staff: Mentor

If it is low resistance then you should be drawing a lot of current. Do you know the resistance of the coil and the impedance of your voltage source? Can your voltage source act as a current source?

3. Apr 18, 2013

From the above measured values i can see that the resistance is almost negligible.... I am using function generator from agilent it has impedance 50 ohms

4. Apr 18, 2013

Staff: Mentor

Then you should make sure to match your coil to 50 Ohms also.

5. Apr 19, 2013

How can we make it match with 50 ohm i cant use impedance matching circuit like cc amplifier because i am using at 10MHz .. is there any other solution for that

6. Apr 19, 2013

Staff: Mentor

If you were to settle on using a particular frequency, then you could arrange for the coil to be the L in an LC tuned circuit resonant at that frequency. In a parallel resonant circuit, the current flowing in the inductor can be hundreds of times that drawn from the external circuit. Couple energy into the inductor using an impedance-matching tap partway up the coil. Keep safety in mind. Prepare insulation for high voltages; the voltage is magnified by the same factor as is the current. High quality, high voltage capacitors are required.

If you intend drawing a lot of energy from the magnetic field, then this will probably defeat what I've suggested.

As for your low current: if your signal source has an impedance of 50Ω, and voltages are around 10V, then it can't supply more than ⅕A anyway.

7. Apr 19, 2013

Staff: Mentor

An RF transformer.

8. Apr 19, 2013

My application is to transfer 1- 2watts power to a distance of atleast 1meter so do you think parallel resonant circuit works better for me..? and can you explain little more about impedance matching tap ... thing or can you provide any link where i can get an idea of it..?

9. Apr 19, 2013

Staff: Mentor

Here is a decent reference on this topic. The context is MRI receive coils, but it should work for your application too, just you are doing high power:

www.measurement.sk/Papers3/andris-2.pdf

And here is another reference with a little more background:
http://www.stanford.edu/~jbarral/CoilDesign.html [Broken]

Last edited by a moderator: May 6, 2017
10. Apr 19, 2013