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Electronics Repair (sci.electronics.repair) Discussion of repairing electronic equipment. Topics include requests for assistance, where to obtain servicing information and parts, techniques for diagnosis and repair, and annecdotes about success, failures and problems. |
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#1
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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). |
#3
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Is the wattage linear on a wirewound adjustible resistor?
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#4
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Is the wattage linear on a wirewound adjustible resistor?
On Sun, 26 Feb 2017 14:08:57 -0600, Foxs Mercantile
wrote: On 2/26/2017 1:51 PM, 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. |
#5
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Is the wattage linear on a wirewound adjustible resistor?
On Sun, 26 Feb 2017 14:08:57 -0600, Foxs Mercantile
wrote: On 2/26/2017 1:51 PM, 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. |
#6
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Is the wattage linear on a wirewound adjustible resistor?
On 2/26/2017 2:42 PM, 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 |
#7
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Is the wattage linear on a wirewound adjustible resistor?
On 2/26/2017 2:42 PM, 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 |
#8
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Is the wattage linear on a wirewound adjustible resistor?
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#9
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Is the wattage linear on a wirewound adjustible resistor?
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#10
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Is the wattage linear on a wirewound adjustible resistor?
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 |
#11
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Is the wattage linear on a wirewound adjustible resistor?
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 |
#12
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Is the wattage linear on a wirewound adjustible resistor?
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 |
#13
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Is the wattage linear on a wirewound adjustible resistor?
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 |
#14
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Is the wattage linear on a wirewound adjustible resistor?
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. |
#15
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Is the wattage linear on a wirewound adjustible resistor?
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. |
#16
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Is the wattage linear on a wirewound adjustible resistor?
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 |
#17
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Is the wattage linear on a wirewound adjustible resistor?
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 |
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