How do you figure out the value of a torroidal inductor?

[raspberrypi]

Since torroids are hard to find I salvaged one out of an old computer monitor. What the math equation to figure out what the value is?

Its a magnetic ferite? Its just a gray color, looks like if you dropped it would shatter, torroid (sorry for non metric I can't find one metric measure in the whole house!):
7/16" thick
15/16" diameter
3/16" core
21 turns of stranded copper 20ga wire

My goal is 350 uH or at least 275

My other option is 30ga wire wrap wire and lots of spare time.

What equipment would you use to build an inductor?

I actually found a great starting point on Tube of You:

[bktemp]

What the math equation to figure out what the value is?

L=n²*Al
Since you don't know Al or any other properties of the material used, you can't calculate anything, you need to measure the values.

What equipment would you use to build an inductor?

At least an LCR meter for measuring the inductance using a couple of turns and then calculating the Al value.
Then you can calculate the required number of turns.

If you want to use the inductor for a SMPS you also need this (because you need to know how much current you can put through the inductor until the core goes into saturation):
http://elm-chan.org/works/lchk/report.html

[chris_leyson]

You could measure the inductance using a sig gen and scope if you don't have any other means, http://www.dos4ever.com/inductor/inductor.html I you've got 21 turns then Al = measured inductance/21²

[HighVoltage]

If you have a way of measuring the inductor current with a scope, you can just use a switch, apply some DC voltage and look at the rise time "t"

Then use this formula to calculate the inductance "L"
I = E/R(1 – e-t/(L/R))

This is a poor mans way, but it works really well.

[ali6x944]

The way I would do is to create a resonance circuit and measure the frequency of the oscillation from the LC parallel resonance circuit and use this formula to find the inductance:

L=1/(2*pi*f)²C

Were L is inductance in Henry,f is frequency in hertz,C is capacitance in farads.
 See circuit here:
https://plus.google.com/115108013327841958420/posts/LsGVzzE9oXo

[T3sl4co1l]

Since it is most likely a ferrite material, it will saturate quickly without storing much inductive energy.

It will, however, make a fine transformer.

What was it doing, in the monitor?  Was it looped around a cable, or something like that..?

Tim

[Zero999]

If you have a way of measuring the inductor current with a scope, you can just use a switch, apply some DC voltage and look at the rise time "t"

Then use this formula to calculate the inductance "L"
I = E/R(1 – e-t/(L/R))

This is a poor mans way, but it works really well.

That would work in theory. In practise though, switches bounce, which will make it difficult. You could use a push button to trigger a thyristor to swich the LR circuit and connect it to a 'scope set to single shot mode.

[HighVoltage]

If you have a way of measuring the inductor current with a scope, you can just use a switch, apply some DC voltage and look at the rise time "t"

Then use this formula to calculate the inductance "L"
I = E/R(1 – e-t/(L/R))

This is a poor mans way, but it works really well.

That would work in theory. In practise though, switches bounce, which will make it difficult. You could use a push button to trigger a thyristor to swich the LR circuit and connect it to a 'scope set to single shot mode.

I used to do this a lot, long long time ago, before I had any LCR meter and it works kind of surprisingly well.
As a switch I used an old Morse Key (like the one in the picture) and had no bouncing at all.
If you trigger the scope for a single shot it is pretty repeatable even.
 
well

[K2CK]

I find an LCR meter invaluable for determining the inductance of wound torroids.  Even an inexpensive model should get you into the ballpark.

[danadak]

You can always build your own LC meter for LF work -


http://www.sillanumsoft.org/ZRLC.htm


Regards, Dana.

[raspberrypi]

Since it is most likely a ferrite material, it will saturate quickly without storing much inductive energy.

It will, however, make a fine transformer.

What was it doing, in the monitor?  Was it looped around a cable, or something like that..?

Tim

Whatwas it doing in the monitor? Just kind of sitting there. Actually it had a power cable going through it.

I have no equipment other then a multimeter and an SDR radio and alot of creativity. It's going into an LC circuit that will block the AM band for an SDR radio. Basically I just keep messing around with the circuit then look at how the SDR behaves.