Wire wound Power Resister reactance to AF

I am planing on making a dummy load for an audio amplifier to do some testing and was wondering if this resister would have a significant amount of reactance with frequencies in the range of 20 to 20,000hz? The spec sheet doesn't give any details on that.

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I will be using 8 of them in various configurations of series and parallel to get 2 4 8 and 16 ohm loads. They do offer a version that is supposed to have very low reactance but it is hard to find and is about triple the cost.

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Chris W
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Chris W
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If it's a dummy load to stand in for speakers, why worry about inductive reactance when speakers have it also?

Why are you trying to substitute a purely resistive load when the rated "ohms" of a speaker specifically is inductive reactance.

What brain fart made you think that

8 ohms inductive reactance is the same as 8 ohms of pure resistance in the first place?

And you're a licensed Ham?

I found the DRM information interesting though.

Protect your digital freedom and privacy, eliminate DRM, learn more at

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Reply to
Greegor

Good point.

I don't think that. But since the impedance curve for every speaker is different, I don't know what else to do and have seen other similar designs.

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Chris W
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Chris W

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No, it isn\'t.

The rated \'ohms\' of a speaker is its impedance, of which its inductive
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Reply to
John Fields

Massive Ego, compensating for a tiny weenie?

Reply to
default

"Chris W"

** Such resistors have very little inductive reactance, when used in values more than a few ohms.

You can expect their values to remains constant ( +/- 0.1%) over the audio range and way beyond too.

.... Phil

Reply to
Phil Allison

"Greegor"

If it's a dummy load to stand in for speakers, why worry about inductive reactance when speakers have it also?

** Yep - practically all loudspeakers have a rising impedance curve at high frequencies.

With bass drivers the rise begins above 500Hz and with tweeters it begins above 3 to 5 kHz.

Why are you trying to substitute a purely resistive load when the rated "ohms" of a speaker specifically is inductive reactance.

** There you are quite wrong.

The rated impedance for all bass drivers and most other drivers too is at taken at a frequency where the load impedance IS resistive.

For nearly any bass driver, this frequency is around 250 Hz and the impedance value is 10 to 25% greater than the DC resistance.

What brain fart made you think that

8 ohms inductive reactance is the same as 8 ohms of pure resistance in the first place?

** The OP never made any such silly assumption.

And you're a licensed Ham?

** Are you ?

..... Phil

Reply to
Phil Allison

It will have less inductance than a loudspeaker.

Reply to
Jasen Betts

"Jasen Betts"

** Says Jasen B, just like a sick little puppy with three legs

- hobbling along behind the rest of the pack.

Till a truck runs him over .....

Splaattt........

.... Phil

Reply to
Phil Allison

A friend was using some 22ohm versions of those resistors for the same job. An impedance sweep ...

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No problem. My note say 9.8uH in series with 22ohms. Can't imagine the 8ohms part being much different. Incidentally, those gimormous 300Watt versions, have only

1uH series inductance!.
Reply to
john jardine

"john jardine"

** Obviously wrong number for L.

** Proves that a smaller version must have less than 1uH of inductance.

..... Phil

Reply to
Phil Allison

A 50 watt 8 ohm in this style from my stock measures 130 nH @ 10kHz

A 250 watt 4 ohm unit I have measures 850 nH @ 10kHz

Reply to
The Phantom

"The Phantom" "Phil Allison"

** OK - what does a 63mm length of wire (ie the distance between the terminals of that resistor ) measure on your rig ??

..... Phil

Reply to
Phil Allison

The meter was zeroed with a heavy copper shorting plate, then:

A 63 mm piece of 16 gauge copper wire, pressed down as close to the fixture (to minimize loop area) as I can get it, measures 21 nH @ 10 kHz.

A 63 mm piece of 30 gauge copper wire, pressed down as close to the fixture as I can get it, measures 37 nH @ 10 kHz.

When I measured the resistor, I could only get the body of the resistor snug up against the fixture, not the actual current path, so there was more loop area than with the wire.

Of course, the OP, using these resistors for his load, would have much more loop area unless he exercised extreme care. The inductance of the resistor isn't going to be the main determinant of the inductance of his setup.

Reply to
The Phantom

Interesting!. Just measured a (to hand) 20ohm. Shows 5uH. Then measured a Welwyn 0.1ohm and it showed 60nH. Both seem reasonable numbers as I'd assumed all these types of resistor were wirewounds (observed via the flotsam of experimental disasters). From the figures you quote, it looks like the construction varies from maker to maker. In pursuit of geek science and with a milling machine to hand, I opened up the Welwyn 20ohm. A la ...

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Reply to
john jardine

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You'll notice that on the web page the OP linked:

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under description, it says "Resistor Element Material: Ceramic". I assume that would account for the low inductance.

Reply to
The Phantom

"john jardine"

** That pic shows a coil of presumably nichrome wire with about 66 turns on a 7.5mm diameter and about 50 mm long.

Using this on-line calculator:

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L = 4.6uH

A resistor of only 0.1 ohms is likely made with flat strip conductor and far fewer turns than the example in the pic.

8 turns gives about 60 nH.

....... Phil

Reply to
Phil Allison

"Phil Allison"

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.... Phil

Reply to
Phil Allison

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