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Wire wound Power Resister reactance to AF

C

Chris W

Jan 1, 1970
0
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.



http://www.newark.com/vishay-dale/rh0508r000fe02/power-resistor/dp/41K9178

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.


--
Chris W
KE5GIX

"Protect your digital freedom and privacy, eliminate DRM,
learn more at http://www.defectivebydesign.org/what_is_drm"

Ham Radio Repeater Database.
http://hrrdb.com
 
G

Greegor

Jan 1, 1970
0
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.
http://www.newark.com/vishay-dale/rh0508r000fe02/power-resistor/dp/41K9178

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.

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

http://www.defectivebydesign.org/what_is_drm
 
C

Chris W

Jan 1, 1970
0
If it's a dummy load to stand in for speakers,
why worry about inductive reactance when
speakers have it also?

Good point.

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?

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.



--
Chris W
KE5GIX

"Protect your digital freedom and privacy, eliminate DRM,
learn more at http://www.defectivebydesign.org/what_is_drm"

Ham Radio Repeater Database.
http://hrrdb.com
 
P

Phil Allison

Jan 1, 1970
0
"Chris W"
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.

http://www.newark.com/vishay-dale/rh0508r000fe02/power-resistor/dp/41K9178


** 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
 
P

Phil Allison

Jan 1, 1970
0
"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
 
J

Jasen Betts

Jan 1, 1970
0
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.

It will have less inductance than a loudspeaker.
 
P

Phil Allison

Jan 1, 1970
0
"Jasen Betts"
Chris W


It will have less inductance than a loudspeaker.


** 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
 
J

john jardine

Jan 1, 1970
0
Chris W said:
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.



http://www.newark.com/vishay-dale/rh0508r000fe02/power-resistor/dp/41K9178

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.

A friend was using some 22ohm versions of those resistors for the same
job. An impedance sweep ...
http://i44.tinypic.com/iggllx.jpg
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!.
 
P

Phil Allison

Jan 1, 1970
0
"john jardine"
A friend was using some 22ohm versions of those resistors for the same
job. An impedance sweep ...
http://i44.tinypic.com/iggllx.jpg
No problem.
My note say 9.8uH in series with 22ohms.

** Obviously wrong number for L.

Can't imagine the 8ohms part being much different. Incidentally, those
gimormous
300Watt versions, have only 1uH series inductance!.


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


...... Phil
 
P

Phil Allison

Jan 1, 1970
0
"The Phantom"
"Phil Allison"
A 50 watt 8 ohm in this style from my stock measures 130 nH @ 10kHz


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



...... Phil
 
J

john jardine

Jan 1, 1970
0
The Phantom said:
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.

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 ...
http://i43.tinypic.com/2d8fi2o.jpg
 
P

Phil Allison

Jan 1, 1970
0
"john jardine"
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.

http://i43.tinypic.com/2d8fi2o.jpg


** 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:

http://my.athenet.net/~multiplx/cgi-bin/airind.cgi

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
 
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