Measure a choke

[martin manning]

Yes "r" is just the measured DC resistance of the choke. Sorry if the grounds are confusing, all you really do is put the choke and the known resistance R in series, and apply the AC source across the pair. Measure voltage across the pair (which is across the source) to get Vs, and measure across R to get Vo. That's it... Plug in the numbers and get L.

R was chosen to get roughly equal voltage drop across the choke and R for good accuracy. I did this last night with a filament trans and a little 4H Deluxe Reverb choke. The test said 4.3H.

Phil, I see that the equation in the method you posted is wrong. It ignores the internal series resistance and also there should be a sqrt(3) in the numerator.

PS Yea Miles, an AC wall wart is good too.

[vibratoking]

I am a tauist. I will just use L=R/(τ*Freq)

Be careful there. I'm not sure where you got that equation or what τ is supposed to equal in that equation, but that is VERY non-standard. τ almost always refers to the time constant of the circuit. So τ = L/R for a single time constant LR circuit. You might have meant the following: L = R/(2*pi*fc), where fc is the -3dB cuttoff frequency in Hz?

Martin's method will work as long as you are not shooting for a result that is extremely accurate. For the values Miles listed, you should be able to tell them all apart, but I would not make the mistake of thinking the value you measure is the exact value of the device. Those methods have many possible error sources.

[Reeltarded]

Tau is 2*Pi. Not meaning to complicate things but I see where it could be misunderstood.

I was being cheeky about Pi.

[Phil_S]

Phil, I see that the equation in the method you posted is wrong. It ignores the internal series resistance and also there should be a sqrt(3) in the numerator.

Cripes! You'd think it if is posted on the internet, it has to be correct! I'm going to return that one to where I got it.

Thanks, as always, Martin, for sharing your seemingly unlimited expertise and for making this clear to me.

Maybe I'll snip an AC wall wart, but I've already got a filament tx on the bench that supplies 6.3 and 25.2.

Phil

[martin manning]

What would be interesting is if someone with access to an accurate LCR meter could run the simple test with a filament transformer supplying the signal and compare the results. I got pretty close (within 10%) of the marked value on the choke I tested, but I don't know what the actual value of that part is.

[Phil_S]

...the equation in the method you posted is wrong. It ignores the internal series resistance and also there should be a sqrt(3) in the numerator.

I'm interested in sorting this out, so I went back to AX84. The individual who sent me to the link https://dl.dropbox.com/u/1326040/Induct ... ef%20R.jpg says as I suspected, the 330K swamps what little DCR is added by the choke. He agrees it is an error, it is a very small error.

Where does sqrt(3) come into it? Or is that a typo? Sqrt(3) is a pretty weird number ~1.73.

[martin manning]

The significance of the DCR depends upon the other variables. In the example I used with R = 1k, neglecting the 179 ohms r incurs a 7% error. Using a 60Hz source requires the low R value so as to roughly match the voltage drops, and the need to account for the DCR. Re the root 3, if you start with the equation I gave, set Vs/Vo=2 (equal voltage across L and R), set r=0, and simplify, you will get (sqrt(3) x R)/(2pi f). This means a value 73% larger than the other equation.

[Phil_S]

Martin, I'm dazzled! Never doubted you, just couldn't parse what I found. Thanks.

[Kagliostro]

I've this one

http://shop.paolettiferrero.it/product. ... anza~.html

measure till 200H

K

[Reeltarded]

I don't have a meter that is good enough to do anything small...

SO, I just started unwinding an 80s Strat pickup on spindles I made from lengths of plastic tube with a split on one side. The contraption I rigged up as the feed reel was a total waste of time.

I was interested in using inductors on mixers and cathodes after reading the Manzamp thread. Cool amp!

The little ones made not much difference in this amp. I decided it was hammer time. I yanked the magnets out of what was left of that strat pickup and duct taped it up with the tails sticking out then started pinning it over stuff. Hey! It works! You can hear that! About 60% of the windings left on the bobbin. The coolest thing I found was over the 2nd stage mixer of a high gain amp where it suppressed highs without being dull.

NOISY AS ALL HELL

I need a much finer meter, and maybe I should wrap it in another layer of tape and foil... or just remember the fun and move on.