Bifilar Coil Formulas: Calculate Capacitance & Impedance

[Jdo300]

Hi Everyone,

I read the Tesla patent about bifilar pancake coils here:

http://www.magnetricity.com/NeoG/Bifilar.php

and I read about how they are supposed to decrease the impedance and increase the capacitance of the coil. So I constructed one to try out the idea. To my surprise, when I tested the capacitance of the coil with my multimeter, it registered 5.45 uF, my coil is only about the size of a CD and made with 16 gauge magnet wire. I attached a picture of it to this post.

I am wondering if anyone out there knows of any formulas to predict the capacitance of the coil given the number of turns (in a flat spiral, not a solenoid), and given the size wire used.

Thanks,
Jason O

 

[berkeman]

5uF sounds way high for a 16AWG coil that size. Are you sure of your measurement? My inexpensive B&K LCR meter gives misleading results unless you're real careful...

 

[Jdo300]

5uF sounds way high for a 16AWG coil that size. Are you sure of your measurement? My inexpensive B&K LCR meter gives misleading results unless you're real careful...

Yes I'm quite sure of my measurements. I actually used the new multimeter I recently bought on eBay here:

http://cgi.ebay.com/New-Digital-LCR-Capacitance-Inductance-Meter-Multimeter_W0QQitemZ220001638302QQcategoryZ25421QQssPageNameZWDVWQQrdZ1QQcmdZViewItem

All I did was take some smaller gauge wire and stick it into the holes for Cx and set the dial to measure capacitance in the 20uF range. Then I connected the smaller wires stuck in the meter to the leads on my coil to get the measurement I observed. I also made sure I had the far right button on the meter pressed into measure Cx. I even tested some regular capacitors using the meter just to make sure that I had it setup properly to take the measurements and that seemed to check out fine. However, if you see anything wrong with my measurement method, please let me know and I'll gladly retest it.

Thanks,
Jason O

 

[berkeman]

Nice looking meter! But I do doubt that any handheld LCR meter will be able to tell you the capacitance of a coil like that accurately. The problem is that the meter assumes that you are measuring a capacitor, not measuring the parasitic capacitance of an inductor. You would need to use a full impedance analyzer like an HP 4194. You need to see the full complex impedance versus frequency in order to calculate the inductive and capacitive components (like Lp Cp on the 4194).

My little B&K meter has a D = 1/Q reading that it can make while measuring L or C, and that's helpful to see what the series resistance Rs is. But it doesn't have any way to help me measure the parasitic capacitance of a coil. I go use the 4194 for that.

You should be able to measure the inductance pretty well with the meter. Then you might be able to drive the Lp Cp resonance with a signal generator through a 10kOhm resistor or so, and get the Cp value from that. I'd guess it will be in the 100pF-500pF range or so from the size of your coil.

 

[Jdo300]

Hi,

Thanks for the insight. I'll take it over to my engineering college and have them check it out for me to see what the capacitance is since I'm getting questionable results with my meter.

Would you happen to know of any equations I can use to calculate the capacitance of the coils? Maybe I can at least predict what it would be.

Thanks,
Jason O

 

[berkeman]

Here's a calculator that may help. It's the 3rd one down the page. It also gives you the equation, so you can do the calc yourself to check it.

http://www.westbay.ndirect.co.uk/capacita.htm