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DDF 24th April 2013 03:12 AM

Power Resistor Inductance Measurements
 
1 Attachment(s)
I was asked about power resistor inductance and whether it's an issue in spaker crossovers or not. I ran the MLSSA through its paces on some items in my parts box in the past, I think the attached tells the tale.

Thought I'd share it here.

Dave

DUG 24th April 2013 03:30 AM

Interesting...any "non-inductive" wirewound available?

DDF 24th April 2013 03:43 AM

Quote:

Originally Posted by DUG (Post 3466202)
Interesting...any "non-inductive" wirewound available?

Yes indeed, for example the Dale NH series. Mills resistors claim non inductance as well.

Unfortunately I had neither available to measure.

Dave

DDF 24th April 2013 04:50 AM

1 Attachment(s)
Noticed a silly typo (gain vs loss) in the previous file, please use the attached. No changes to data or conclusions

system7 24th April 2013 06:43 AM

Very interesting, Dave. Nothing like actually doing it, eh? :)

Am I right in thinking the tale it tells is not to fret about inductance in wirewounds or metal film? This was something that bothered me when I used them in amplifiers.

A few uH is nothing compared to the typical 0.05mH Le inductance of a tweeter. :cool:

It makes sense. Resistors are designed to be used at much higher frequencies than we ever use in audio. The double winding technique is designed to cancel out most coil inductance. I had a look at some cables too, and again the self and mutual inductance was an order of magnitude lower than tweeter inductance even with 10 metre runs at audio frequencies.

AllenB 24th April 2013 02:05 PM

1 Attachment(s)
I like studies like this. When I remember that I used to trim my speaker cables to equal lengths :o

Thanks Dave.

DDF 24th April 2013 02:06 PM

Quote:

Originally Posted by system7 (Post 3466278)
Very interesting, Dave. Nothing like actually doing it, eh? :)

Am I right in thinking the tale it tells is not to fret about inductance in wirewounds or metal film? This was something that bothered me when I used them in amplifiers.

Hi Steve,

Interesting thing is that none of these even had the non-inductive wind (where the wire is folded back and then wound clock and counterclockwise simultaneously to canel inductance).

The way I like to look at it is that for woofers or mids, the self inductance of these has absolutely no quantifiable impact. For tweeters, at most its equivalent to changing toe in of the speaker maybe at most a degree or two.

I modeled the Vishay RH-25 inductance for the speakers I posted earlier in the week. It affected the response at 10kHz by at most 0.03 dB. I'm sleeping prettty soundly with the choice. :)

Dave

fpitas 24th April 2013 05:40 PM

Makes sense, but I'm sure someone can hear the effects of that extra inductance. Even if it's inaudible ;)

jneutron 24th April 2013 05:54 PM

Quote:

Originally Posted by DDF (Post 3466237)
Noticed a silly typo (gain vs loss) in the previous file, please use the attached. No changes to data or conclusions

A very nice effort, thank you for posting.

There is no such thing as inductance (external) of a straight piece of wire. A straight piece of wire will have 15 nH per foot internal inductance, but not external. What you measured was the inductance of the complete circuit loop including the wire being measured. A 3 foot length of any wire should have measured 45 nH, not 1270 nH.

More apropos would be the measurement of lengths of zip, as that should be in the 180 to 200 nH per foot range.

jn

DDF 24th April 2013 07:27 PM

Quote:

Originally Posted by jneutron (Post 3466895)
A very nice effort, thank you for posting.

There is no such thing as inductance (external) of a straight piece of wire. A straight piece of wire will have 15 nH per foot internal inductance, but not external. What you measured was the inductance of the complete circuit loop including the wire being measured. A 3 foot length of any wire should have measured 45 nH, not 1270 nH.

More apropos would be the measurement of lengths of zip, as that should be in the 180 to 200 nH per foot range.

jn

18 gauge diameter is 0.1024 cm, from
American wire gauge - Wikipedia, the free encyclopedia

Plugging that into a wire inductance calculator assuming approximately 90cm length:
Inductance of a Straight Wire: A Calculator

Expected result is 1334 nH. MLSSA measured 1270nH, pretty close.


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