Inductors in Xovers

[A Stuart]

I presume you can measure AC volts and AC amps reasonably accurately. Can you measure mains frequency if you want the last few % accuracy?

The simplest method would be, if you have two meters and a transformer, to apply a measurable AC voltage to your ammeter and the inductor in series and measure volts across the inductor. Volts over Amps = Reactance = 2 Pi f L

If you only have one meter -
The large 50H chokes should, if I calculate correctly, have a reactance at 50Hz of 15700 ohms. So if you fed a 50Hz AC voltage to the 50H inductor and a 15700 ohm resistance in series, the voltage would be equally divided between them. eg 10V rms total would read 7.07V rms across each ('tis magic, as Paul Daniels would say).

Also however, to cancel the effects of the DC resistance of the inductor, I reckon you would need to add (asuming you already have known inductors) 1H to the resistor, in series, for every 314 ohms of DCR of the 50H inductor (and subtract the DCR of this inductor from the resistance).
The object of the above being to create a reactance vector for the "resistor" mirroring that of the the 50H inductor (plus its internal resistance) the other side of a 45 degree line, so that the total voltage would be spread half and half on the 45 degree line.
If the voltage does not divide equally with 15700 ohms, try different resistances until it does and calculate the true inductance from that.

On further thought - even simpler , if you have an accurately known inductor - make a simple voltage divider between the two inductors, measure and calculate the ratio, adding resistance as required to allow for differing DC Resistance:Inductance ratios of each component. Make sure the fields of two inductors do not interact.

I have not worked out how to allow for the unknown interwinding capacitance in the inductor but if you are designing speaker crossovers you are probably better placed than me to do this .

Beginning to wish I hadn't started this answer! Depending on postage costs, Greg's offer to have them measured sounds more attractive.

Alastair

[Paul Barker]

You're absolutely right in all these things, but you may have bored Andrew along the way.

One of your methods is frequently used using a pot and the unknown inductor voltage devide down the middle adjusting the pot to get equality, measure value of pot apply formula. The measured value of the adjusted pot being the reactance of the inductor known as XL then a little algebra leeds us to L in henries with the formula XL = 2.pi.f.L where f = 50 in the UK.

The extra mile you weant to arrive at a more acurate figure is probably very helpful in Andrews situation where he is measuring such low value air cored inductors with many turns, but in the case of average audio chokes and transformers not essential.

For LF chokes and SE OPT's I prefer to use real life dc conditions so load the item with a current sink place it as a component in a pi filter and measure it's ripple reduction capabilities to derive true inductance in working conditions.

But an iLCR meter is swifter and easier, and a good enough guide. That's all static inductance is anyway, a guide, says nothing about real audio performance.

[A Stuart]

Apologies for boring all and sundry.
I started off thinking I could provide a calculation method in about three lines.

Glad I got it tolerably correct as it was all scribbled with pen & paper, not copied from as book.
Should I resurrect the Grumpy Old Men thread with the downfall of society resulting from the Yoof Of Today typing figures into a computer, and despising pen and paper?
I do not of course know how yoof-ful Andrew may or may not be, and would not stereotype or stigmatise or otherwise discriminate against him even if he was! Better shut up now .

Alastair

[andrew ivimey]

I fell asleep Mr Stuart, I confess.

I understood the maths but, unlike some I respect and like, it doesn't 'hit the spot' for me, so to speak. Statistics, I confess, fascinate me, so there you go.

I sincerely appreciate anyone's attempt to help / advice etc so many thanks!

I bought one of those Hongkers meters and ... Me!!!, not experiment! not suck it and see!? Ha!

My next birthday is my 21st - yeah right.

[andrew ivimey]

Four days after I shelled out a little under £25, the meter arrives from Hongkers - Hooray! All the instructions are in Chinese but who cares! - marvellous! Now where did I put all those inductors?

[simon2a3]

It would be very interesting to hear how "known" chokes measure up Andrew. Quite fancy one myself.

[andrew ivimey]

yes, it worries me too. Just how accurate is the meter and just how accurate are other chokes etc I already have. Just becasue e.g. Mr Kef,Spendor or Mr Woodman says x crossover choke is 1.2mH ... well I wonder.

But rough and ready, perhaps, I am prepared to run with this.

At present I'm trying to see what the inductance is of my telephone handset... perhaps my enthusiasm is getting a little ahead of me.

[A Stuart]

"Just how accurate is the meter and just how accurate are other chokes etc "

If you can measure mains frequency or generate your own frequency you can discover exactly (let's say a few of % accuracy) that - see above.
Alastair