Online Inductance Calculator Gives Wrong Answers

Thread Starter

Ohmlandia

Joined Mar 2, 2020
32
I wanted to design an inductor, so I googled the web for ages and found a few equations. But I am a sceptical old timer who trusts no-one! So I made a spreadsheet and entered 4 equations I found, and tested them with my data. And was pleasantly surprised that they all gave the same answer (give or take a few percent). But guess what: the online calculator at https://www.allaboutcircuits.com/tools/coil-inductance-calculator/ ... yes that's right, the one on this website, gives a remarkably different answer to the consensus of alternative opinion, being 10 times or so what it should be.

Can anyone confirm or deny this? My inductor is 133 turns, 50 mm diameter, 150 mm long, air cored. The majority place it around 250 to 300 uH, but not our calculator!

BTW I couldn't find our calculator in the navigation system, only by googling it. Is that because it isn't supposed to be available?
 

wayneh

Joined Sep 9, 2010
16,398
I wanted to design an inductor, so I googled the web for ages and found a few equations. But I am a sceptical old timer who trusts no-one! So I made a spreadsheet and entered 4 equations I found, and tested them with my data. And was pleasantly surprised that they all gave the same answer (give or take a few percent). But guess what: the online calculator at https://www.allaboutcircuits.com/tools/coil-inductance-calculator/ ... yes that's right, the one on this website, gives a remarkably different answer to the consensus of alternative opinion, being 10 times or so what it should be.

Can anyone confirm or deny this? My inductor is 133 turns, 50 mm diameter, 150 mm long, air cored. The majority place it around 250 to 300 uH, but not our calculator!

BTW I couldn't find our calculator in the navigation system, only by googling it. Is that because it isn't supposed to be available?
I’ll take a look soon. I’ve made several inductance models myself and can check against those. In the meantime, did you carefully check the units of the input and output?
 

Papabravo

Joined Feb 24, 2006
13,924
I see both μο and μr in the equation and the usual value is given for μο but what value is used for μr? The units given for μο are given as Henries per meter. Other length units should be converted to meters.
 
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Thread Starter

Ohmlandia

Joined Mar 2, 2020
32
Yep ... I checked my units very carefully. For μr, I assumed that 1 was the correct value for an air core. The various boxes suggest that the input value conversions between units will be done for you.
 

wayneh

Joined Sep 9, 2010
16,398
Yep ... I checked my units very carefully. For μr, I assumed that 1 was the correct value for an air core. The various boxes suggest that the input value conversions between units will be done for you.
I get 185µH using the single-layer Wheeler formula, which I think is appropriate for your coil. If it's multi-layer, It's a little lower but more details would be needed to calculate an estimate.

The online calculator is poorly documented. The formula is provided but not the units. I can't make heads or tails of it.
 
Last edited:

Papabravo

Joined Feb 24, 2006
13,924
I get 185µH using the single-layer Wheeler formula, which I think is appropriate for your coil. If it's multi-layer, It's a little lower but more details would be needed to calculate an estimate.

The online calculator is poorly documented. The formula is provided but not the units. I can't make heads or tails of it.
Sounds like a task for..... @WBahn. If anybody can suss out what is going on...he can.
 

wayneh

Joined Sep 9, 2010
16,398
Sounds like a task for..... @WBahn. If anybody can suss out what is going on...he can.
Perhaps.

It's unfortunate that the source of the formula isn't given. Whoever wrote that up basically plagiarized someone else's work or made up their own formula. Not cool either way. I thought I had seen every coil inductance formula out there but not this one.

If you use a coil diameter in meters to work the formula, you'll get the same result given by the calculator. But that's ~10X larger than it should be. Several of the comments on that page have pointed out the error.
 

Thread Starter

Ohmlandia

Joined Mar 2, 2020
32
OK, thanks everyone. Maybe I'm not going mad or stupid after all! Our website inductance calculator is not to be trusted or believed. Who is going to fix it?
 

Papabravo

Joined Feb 24, 2006
13,924
OK, thanks everyone. Maybe I'm not going mad or stupid after all! Our website inductance calculator is not to be trusted or believed. Who is going to fix it?
Maybe one of the enthusiastic marketing guys. What was the name of that earnest young fellow brimming with enthusiasm?
 

Tesla23

Joined May 10, 2009
403
This calculator is clearly wrong. It seems to be based on the approximation for the inductance for a single loop that is actually shown correctly here:
https://www.allaboutcircuits.com/tools/wire-loop-inductance-calculator/

This formula has been around for a long time (dates back to Maxwell I think) - it's in Terman: https://electrooptical.net/OldBooks/Terman-RadioEngineersHandbook_1943.pdf (p52, eqn 29)

But to assume that you can get the inductance of N-turns by simply multiplying by N^2 is clearly wrong - it will give an approximate inductance for a zero-length coil, but the coil shown on the calculator page clearly has separated turns.

For solenoids like this use Wheeler's formula: L(uH) = n^2 r^2 / (9r + 10L), r and L in inches. I get 252uH for your coil.

There's reasonable feedback on the calculator page that it is wrong - clearly nobody pays attention to that.
 
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