Online Coil Inductance Calculators are Wrong.

[gary350]

Using several online coil inductance calculators they all give me the wrong answer.

My original coil is, 8 turns, 1" diameter, 1.5" long, wound with #10 copper wire. Wire diameter is .125". Coil diameter is measured center line to center line of the wire. My meter reads .799 uh which is what I want target is .8 uh. Online calculator is telling me, .820, .821, and .820 uh.

My new coil is, 8 turns, 1.250" diameter, 2.875" long, wire diameter is .250"copper tubing, coil diameter is measured center to center like before. My meter reads .850 uh. Online calculator says, .727, 727, and 727.

I am trusting my meter to be correct. I was trying to use online calculator to wind the correct coil on the first try and not have to wind a dozen wrong coils to get it right.

I assume the circuit will see any .8 uh coil the SAME even if the coils are different shapes or sizes?

 

[MikeMl]

Using this formula, I wouldn't expect that it would be better than +_10%. You came much closer than that...

 

[alec_t]

I assume the circuit will see any .8 uh coil the SAME even if the coils are different shapes or sizes?

I wouldn't assume that. I would expect parasitic resistance and capacitance to depend on coil geometry.

 

[DerStrom8]

The calculators depend on it being an ideal inductor. Unfortunately in the real world that won't be the case, so you will find some error there as well.

 

[AnalogKid]

Actually, the calculators are just fine. Your design data is wrong.

"wire diameter is .250"copper tubing"

That is not round solid copper wire. All calculators based on round solid copper wire will give what you consider to be inaccurate results because you are giving them incorrect information.

ak

 

[Tony Stewart]

Using several online coil inductance calculators they all give me the wrong answer.

My original coil is, 8 turns, 1" diameter, 1.5" long, wound with #10 copper wire. Wire diameter is .125". Coil diameter is measured center line to center line of the wire. My meter reads .799 uh which is what I want target is .8 uh. Online calculator is telling me, .820, .821, and .820 uh.

My new coil is, 8 turns, 1.250" diameter, 2.875" long, wire diameter is .250"copper tubing, coil diameter is measured center to center like before. My meter reads .850 uh. Online calculator says, .727, 727, and 727.

I am trusting my meter to be correct. I was trying to use online calculator to wind the correct coil on the first try and not have to wind a dozen wrong coils to get it right.

I assume the circuit will see any .8 uh coil the SAME even if the coils are different shapes or sizes?

Interwinding pitch and skin effects with frequency both affect the inductance.

Meter must be calibrated to zero nH on short circuit with probes.

Use this calc instead. https://hamwaves.com/antennas/inductance.html

Maximize your Q values and stay well below SRF as well.

 

[gary350]

The copper tubing is solid, I filled it full of solder so I could roll it into a coil. My meter shows .850 uh, wonder how much that will change when I heat it with a torch and all the lead runs out of the coil.

I can't wait until tomorrow to learn what happens when the lead is removed.

Tubing is hollow again meter now reads .834 uh. If I solder the tip ends shut meter reads .850 uh again. Very interesting. LOL.

 

[Tony Stewart]

Actually, the calculators are just fine. Your design data is wrong.

"wire diameter is .250"copper tubing"

That is not round solid copper wire. All calculators based on round solid copper wire will give what you consider to be inaccurate results because you are giving them incorrect information.

ak

Not all calculators are good.

THey must consider Skin effect and helicoil gap.
At these frequencies, hollow tubing is fine and my Calc link is closest one to NIST's formula.

 

[BobW]

The difference between thin tubing and solid wire is 50 nH per meter of wire. So, if you measure the actual length of wire (or tubing) used to wind the coil, and multiply the length by 50 nH, the inductance of the tube inductor will be smaller by that amount at low frequencies. At high frequencies, because of skin effect, the tubular conductor and solid conductor coils will converge to the same inductance value, because all current will flow on the surface of the wire regardless of whether it's solid or tubular.

The Hamwaves calculator uses Lundin's empirical formula which is much more accurate than Wheeler's empirical formula which is used on almost every other online calculator. My own online calculator uses the exact elliptic integral formula:
**broken link removed**

 

[mvs sarma]

The copper tubing is solid, I filled it full of solder so I could roll it into a coil. My meter shows .850 uh, wonder how much that will change when I heat it with a torch and all the lead runs out of the coil.

I can't wait until tomorrow to learn what happens when the lead is removed.

Tubing is hollow again meter now reads .834 uh. If I solder the tip ends shut meter reads .850 uh again. Very interesting. LOL.

" My meter" - how far that is accurate? !!! what is the brand and specification of the meter? many parameters differ.
say 8turns could be 7.9 or 8.1
former dia 1" could be 0.98 to 1.02
length 1.5 could be 1.55 to 1.65
turns spacing , also matters.

 

[Tony Stewart]

Sensitivity to precision of former and all dimensions are critical. Each tolerance adds up to a net error. Using 3 digit measurement accuracy in mm for better results.


Also Skin effect on 50KHz AWG8 is 10% so hollow tubing with higher spacing gives higher Q.

Consider thin 1/4" copper plumbing
50kHz skin depth is only 5%, so no conduction loss
Try >=10:1 former:wire diameter using a wrapping former. Fill with salt for bending
Look for a large industrial worn worm gear as a former with at least 10% gap and more at higher MHz

Nice job on the Calc. Bob
Tony