Is the wattage linear on a wirewound adjustible resistor?

While looking at resistors on ebay, to use as an 8 ohm speaker load, I
developed a question.

There is a 25 ohm 25 watt wirewound resistor with adjustible slider. In
other words, the two ends of the resistor are 25 ohms, but there slider
can be adjusted to get any resitance between 1 and 25.

So, if I had one of these, and put the slider at 8 ohms, to use for a
speaker load, would it still be rated at 25 watts? -OR- does the allowed
wattage drop when only part of the resistor is being used?

I'm not sure how this works????? (In this case, I'd be using about 1/3
of the entire resistor).

 

[Phil Allison]

mike wrote:

If you tie the ends together, you get a 12.5 ohm resistor.

** Wrong - you get a 6.25ohm resistor IF the slider in centralised.

There are then two, 12.5 ohm sections in parallel.



..... Phil

 

[Foxs Mercantile]

On 2/26/2017 1:51 PM, oldschool@tubes.com wrote:

While looking at resistors on ebay, to use as an 8 ohm speaker load, I
developed a question.

There is a 25 ohm 25 watt wirewound resistor with adjustible slider. In
other words, the two ends of the resistor are 25 ohms, but there slider
can be adjusted to get any resitance between 1 and 25.

So, if I had one of these, and put the slider at 8 ohms, to use for a
speaker load, would it still be rated at 25 watts? -OR- does the allowed
wattage drop when only part of the resistor is being used?

I'm not sure how this works????? (In this case, I'd be using about 1/3
of the entire resistor).

Wire wound resistors, are rated at total watts across the entire
resistor.

For less than the total amount you are limited to what would be the
current through the entire resistor.

SQRT(25/25) = 1 amp.
Dialing it back to 8 ohms, 1^2 * 8 = 8 watts.

--
Jeff-1.0
wa6fwi
http://www.foxsmercantile.com

 

[MOP CAP]

On 2017-02-26 19:51:07 +0000, oldschool@tubes.com said:

While looking at resistors on ebay, to use as an 8 ohm speaker load, I
developed a question.

There is a 25 ohm 25 watt wirewound resistor with adjustible slider. In
other words, the two ends of the resistor are 25 ohms, but there slider
can be adjusted to get any resitance between 1 and 25.

So, if I had one of these, and put the slider at 8 ohms, to use for a
speaker load, would it still be rated at 25 watts? -OR- does the allowed
wattage drop when only part of the resistor is being used?

I'm not sure how this works????? (In this case, I'd be using about 1/3
of the entire resistor).

The 25 watt rating is for the entire resistor. If you tried to put
25watts into only 1/3 of the R it would overload both the R wire as
well as the ceramic core. I have never seen a formula for this
calculation.
If you put only 25/3 watts in the 8 ohms you might also damage the
resistor due to uneven thermal expansion. No definate answer, but go
easy. CP

 

On Sun, 26 Feb 2017 14:08:57 -0600, Foxs Mercantile <jdangus@att.net>
wrote:

On 2/26/2017 1:51 PM, oldschool@tubes.com wrote:
While looking at resistors on ebay, to use as an 8 ohm speaker load, I
developed a question.

There is a 25 ohm 25 watt wirewound resistor with adjustible slider. In
other words, the two ends of the resistor are 25 ohms, but there slider
can be adjusted to get any resitance between 1 and 25.

So, if I had one of these, and put the slider at 8 ohms, to use for a
speaker load, would it still be rated at 25 watts? -OR- does the allowed
wattage drop when only part of the resistor is being used?

I'm not sure how this works????? (In this case, I'd be using about 1/3
of the entire resistor).




Wire wound resistors, are rated at total watts across the entire
resistor.

For less than the total amount you are limited to what would be the
current through the entire resistor.

SQRT(25/25) = 1 amp.
Dialing it back to 8 ohms, 1^2 * 8 = 8 watts.

That's kind of what I was thinking, but I had to ask to be sure.

I've never been good at math, but it makes sense that if I'm using 1/3
of the resistor, I am getting 1/3 of the wattage (about 8W).

I dont need to do this, because I have a pair of 8 ohm 100W fixed
resistors, but I was just curious.

 

[Foxs Mercantile]

On 2/26/2017 2:42 PM, oldschool@tubes.com wrote:

That's kind of what I was thinking, but I had to ask to be sure.

I've never been good at math, but it makes sense that if I'm using 1/3
of the resistor, I am getting 1/3 of the wattage (about 8W).

I don't need to do this, because I have a pair of 8 ohm 100W fixed
resistors, but I was just curious.

This is why the old wire wound rheostats were marked with values
like 0.05 Amps Max. Even though the pot was rated at 50 watts.
(That would be a 20K ohm total resistance for this example.)

Another thing to keep in mind, wire wound resistors have inductance.
Although probably not enough to become a substantial issue at audio
frequencies.

Good ain't cheap and cheap ain't good, but Caddock makes non-
inductive thick film resistors in 20 and 50 watt sizes.

--
Jeff-1.0
wa6fwi
http://www.foxsmercantile.com

 

[mike]

On 2/26/2017 11:51 AM, oldschool@tubes.com wrote:

While looking at resistors on ebay, to use as an 8 ohm speaker load, I
developed a question.

There is a 25 ohm 25 watt wirewound resistor with adjustible slider. In
other words, the two ends of the resistor are 25 ohms, but there slider
can be adjusted to get any resitance between 1 and 25.

So, if I had one of these, and put the slider at 8 ohms, to use for a
speaker load, would it still be rated at 25 watts? -OR- does the allowed
wattage drop when only part of the resistor is being used?

I'm not sure how this works????? (In this case, I'd be using about 1/3
of the entire resistor).

If you tie the ends together, you get a 12.5 ohm resistor.
I'm too lazy to do the math, but as you dial it off center,
the resistance goes down until you get to 8-ohms. You have two
parallel resistors that total 8 ohms.
Limit the current in the smaller part to 1 amp.
Calculate the additional current in the bigger part.
Sum 'em up, calculate the max power.
Sure, you can write a formula for that, but I'm way too lazy.

 

[Phil Allison]

Foxs Mercantile wrote:

Another thing to keep in mind, wire wound resistors have inductance.
Although probably not enough to become a substantial issue at audio
frequencies.

** Typical hollow, ceramic WW resistors rated at 100W show no change in value until the frequency is well over 100kHz.

The metal encased sort that need a heatsink show much less.

Inductance in WW resistors sometimes becomes an issue with very low values - like 0.1 ohms.



..... Phil

 

[Phil Allison]

Phil Allison wrote:

** Typical hollow, ceramic WW resistors rated at 100W show no change
in value until the frequency is well over 100kHz.

** The above applies to resistors of 4 ohms resistance, or greater.

The trick is that for a given physical size, inductance values do not vary that much while resistance values vary over a range of 1,000,000:1.


...... Phil

 

[mike]

On 2/26/2017 4:49 PM, Phil Allison wrote:

mike wrote:



If you tie the ends together, you get a 12.5 ohm resistor.



** Wrong - you get a 6.25ohm resistor IF the slider in centralised.

There are then two, 12.5 ohm sections in parallel.



.... Phil

Yep, brain fart on my part.