# Toroidal inductors

I made some inductors with CF196T1305 ferrite cores ( http://www.cosmoferrites.com/Downloads/ProductSize/3defd392-c210-4f70-a6c6-70f5b60b1953_T1305%20_WS.pdf). According to the data sheet these should have an AL of about 1200 with +30/-20% tolerances. But the inductances of my inductors does not match the math.

Here are pictures of my windings.

• Turns = 20, $I_{math} = 480uH$ , $I_{actual} = 728uH$, about 50% more than calculated.

• Turns = 25, $I_{math} = 750uH$ , $I_{actual} = 1194uH$, about 59% more than calculated.

I do not posses a LC meter, so what I did is, I build a LC tank circuit, with a capacitor of some known valve. I give a square wave of fast rising and falling edges, now by watching the circuit resonate, I tried to calculate the incidence.

I don't know but such huge difference seen wrong. Are my windings OK? Or what could be the fault?

• I'm going to bet that your testing method uses a fairly small current compared to the rating. Toroids saturate at a point, and are often rated based on what inductance they would have at a certain operating current. – Sean Boddy Apr 9 '15 at 20:55
• Test method is all-important – Andy aka Apr 9 '15 at 21:21
• When measuring inductance you often get a large value just from the test leads. It's not like measuring resistance. You need to lay out your experiment, then short circuit the inductor under test with a short straight wire. Record that inductance, remove the short without moving any wires, and measure the extra inductance. – tomnexus Apr 10 '15 at 2:49
• Another way to check it is to measure the slope of the current ramp when you apply a known voltage. Obviously you have to limit the voltage duration to prevent blowing up the inductor. You can use a current probe if you have one, or measure voltage across a sense resistor if you don't. – mkeith Apr 10 '15 at 21:19
• @Andyaka I have updated my question to show my method of measurement. – Arjob Mukherjee Apr 11 '15 at 3:35