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Old 1st March 2010, 12:25 AM   #1
kroto is offline kroto  Indonesia
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Default Resistor Voltage Rating

Is resistor realy have voltage rating?

Based on power rating, a 2M2 1/8W resistor working on 300Vdc isn't exceeding their power rating, its only 41mW dissipation, so, why a resistor has its own voltage rating whether the power rating not exceeded?

Any corellation with noise performance?

Different type and/or manufacturer has different nominal voltage rating?
or I can use whatever type and/or manufacture with the same power rating?

I'll be back later, working time...
Thx folks...
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Old 1st March 2010, 12:40 AM   #2
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It has to do with the size and internal construction. You don't want it leaking across the surface or flashing over. Just because the power is reasonable doesn't mean the volts/mil is reasonable.
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Old 1st March 2010, 01:16 AM   #3
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Yes, certain resistors "resistivity" changes with voltage. Check the manufacturer's data sheet. There are a couple of paragraphs describing the effect in "The Art of Electronics".
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Old 1st March 2010, 01:49 AM   #4
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also many resistors can tolerate considerable "surge" - dynamic peak power over their continuous power rating but as pointed out could still fail due to over Voltage

power related failures likely will result in an open, over voltage failures permit huge currents during arcing, and may result in permanently lower resistance due to carbon tracking after the overvoltage is gone

the difference can be critical in a safety analysis

Last edited by jcx; 1st March 2010 at 01:55 AM.
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Old 1st March 2010, 04:20 AM   #5
kroto is offline kroto  Indonesia
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I see,, need to learn about resistor construction.

Anyone have that article on the web?
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Old 1st March 2010, 10:44 AM   #6
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Quote:
Originally Posted by kroto View Post
I see,, need to learn about resistor construction.

Anyone have that article on the web?
Just think about a resistor where R= infinity.

The package, construction, and surface (paint?) finish will sensibly have a maximum working voltage before breakdown, arcing etc, as already mentioned. This obviously has nothing to do with power dissipation as there isn't any!

So that rating is for the package / construction and becomes moot as the resistance value falls to where Pd becomes real.

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Old 1st March 2010, 11:32 PM   #7
Enzo is offline Enzo  United States
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COnsider that film resistors are made as a helical stripe around a ceramic form. (or other insulating material) Like the stripes around a baber pole.

The current flows through that stripe. Each turn of the stripe finds itself next to the previous turn and the following turn. As voltage drops along the length of the resistive stripe, a potential difference exists between the adjacent turns. The space between them is tiny. If the voltages across the resistor are high enough, there can be arcs between the turns.
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Old 2nd March 2010, 10:24 AM   #8
kroto is offline kroto  Indonesia
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Click the image to open in full size.
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Old 2nd March 2010, 10:31 AM   #9
kroto is offline kroto  Indonesia
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Click the image to open in full size.
you mean like that helix line?

well, any suggestion for the voltage rating per watt package?
I mean in general, I don't know who is the manufacturer though...
I've been read in ESP website that for safety, max operating voltage of 75V for 1/4W Resistor and 150V for 1/2W.
Am I can safely use that terms for any manufacturer?
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Old 2nd March 2010, 11:53 AM   #10
Mooly is online now Mooly  United Kingdom
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Quote:
Originally Posted by kroto View Post
Is resistor realy have voltage rating?

Based on power rating, a 2M2 1/8W resistor working on 300Vdc isn't exceeding their power rating, its only 41mW dissipation, so, why a resistor has its own voltage rating whether the power rating not exceeded?

Any corellation with noise performance?

Different type and/or manufacturer has different nominal voltage rating?
or I can use whatever type and/or manufacture with the same power rating?

I'll be back later, working time...
Thx folks...
Resistors do have a voltage rating... I have seen countless failures due to this in commercial equipment. Switch mode PSU's often have high value start up resistors, and these are a classic failure item.
On the basis of power dissipated only, they were well within their limits.
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