Help with inductor calculation

[mat350z1]

So Im trying to figure out the inductance of the primary of my flyback transformer so that i can calculate its current draw at certain frequencies. Im having trouble doing this. I seem to always get values that are way off the charts when it comes to inductance. does anyone know how to figure out the inductance of this coil on the flyback. there are seven turns of solid wire on the ferrite core which is about 1.1cm by 1.5cm. (its not round its a square core) length of the coil is about 2.5 cm. does anyone know what the inductance of this coil could be. with out measuring it??. I have attached a picture.

 

[ronsimpson]

So Im trying to figure out the inductance of the primary of my flyback transformer so that i can calculate its current draw at certain frequencies. Im having trouble doing this. I seem to always get values that are way off the charts when it comes to inductance.

So what inductance do you get? What do you think you should get? "Way off the wall" is a little vague.

 

[mat350z1]

i would get around 13ooo henrys. which at 30khz and 12 volts would only draw miliamps of current, i know this is wrong because i have had this flyback running before and it draws about 10 to 15 amps.

 

[ronsimpson]

i would get around 13ooo henrys.

Wow. 300uH is more than I would expect.
Wait.
I have 4 different inductance meters and none will measure 13,000 H. Have you slipped many decimal places?

 

[mat350z1]

No i don't think i have slipped a decimal point, i must be using the wrong values somewhere, just don't know where

 

[ronsimpson]

What meter are you using?

 

[mat350z1]

No i don't have a meter, that's the thing i want to calculate the theoretical inductance value and then build a driver circuit. I just want to know what my current requirements would be.

 

[ronsimpson]

i would get around 13ooo henrys.

No i don't have a meter

?

There is much I don't know.
Core material. Gap. Turns=7??
Pretty much all the important items.

 

[mat350z1]

sorry, i worded my first post wrong. should of mentioned that the values i was getting we from calculations i had done.
the gap in the ferrite is 1 millimeter. and i believe the material is manganese zinc.

 

[rumpfy]

This transformer looks like a flyback transformer from a television receiver. If so, it will be operating at about 15,70o hertz.
The core material for these devices is invariably similar to the Philips type 3C8 manganese zinc ferrite material. The material is designed for low power loss at a flux change of around 5 tesla.
Any of the ferrite materials suppliers will have a data sheet. Identify the core from its dimensions. The data will give the magnetic properties.

 

[mat350z1]

Thanks a lot. this is very helpful

 

[rumpfy]

I googled "epcos ferrite" and got started.
You would be looking for "U cores". The core looks like a pair of U cores.

 

[crutschow]

Do you have a signal generator and a scope?

 

[gary350]

You can use an online calculator like this. https://www.daycounter.com/Calculators/Air-Core-Inductor-Calculator.phtml

This one does not take into account you have a ferrite core.

You could buy a meter like this and test it, then you know what you have.

**broken link removed**

 

[atferrari]

Carl,
No matter he has them or not please explain how to do it. Definitely interested. Sig gen, scope and a LOT of cores here.

 

[dr pepper]

Build a simple resonant oscillator circuit, use a known capacitance across the primary you made about 100nF (use a xrated mains capacitor), then measure the resultant resonant freq, using the freq and the capacitance you can feed these into a lc calculator and work out the inductance.
Note it will change if you load the o/p.

I agree with rumpers on the core material, 3c8 is popular, 5 Tesla's may be possible but only with an air gap, the core material itself will only withstand around 0.25 Tesla.

https://www.daycounter.com/Calculators/LC-Resonance-Calculator.phtml

 

[crutschow]

Carl,
No matter he has them or not please explain how to do it. Definitely interested. Sig gen, scope and a LOT of cores here.

You could add a resistor in series with the inductor and apply a sine-wave signal. Adjust the frequency until the voltage across the resistance equals the voltage across the inductor so, at that point, the inductive reactance equals the resistance. Knowing the reactance and the frequency you can then calculate the inductance.
A good value for the resistance to use is probably in the neighborhood of a few hundred ohms to a few thousand ohms for typical small inductors.

 

[dr pepper]

Good idea of crut's, a simpler vrsion of what I was thinking.
Also if you were to put a lowvalue resistor inline with the inductance in the final circuit, say 0.1 ohm if the circuit pulls a few amps, you can monitor current into the inductor and see if saturation is occuring, this is the enemy of a switching supply, saturation causes high current drain, and exploded componnets.