Disparity between resistor power rating and rated voltage?

[darkfeffy]

Hello there,

I am wondering why the resistor maximum voltage is not simply the square root of the product of its wattage and its resistance. According to the attached datasheet (pdf) for example, the maximum voltage of a 2.2 mega ohm, 0.25W resistor, is 250V (instead of 742V as calculated). Other datasheets show similar disparity. Could anyone explain what's going on?

Thanks
ed.

 

[darkfeffy]

I suspect this limit represents the strength of the dielectric. If so, then how can the resistor's rated power ever be attained/achieved? Of what use is that figure then?

 

[dlgoff]

A resistors power rating in Watts = Volts x Amps. There is a point where increasing Volts can cause this:

http://en.wikipedia.org/wiki/Electrical_arc

 

[darkfeffy]

Thanks for the reply, Don. I guess I should re-phrase my question by asking: Assuming the datasheet resistance value stays roughly constant, then of what importance is the wattage rating if one cannot even attain that without risk of arcing? Shouldn't the datasheet wattage be taken from V^2/R (since this other quantity is clearly less than the datasheet wattage)?
Thanks.
ed.

 

[nsaspook]

Traditionally 'wattage' in small carbon resistors is set by standard sizes (the greater its surface area the more power) instead of computed dissipation.

Originally the JAN-R-11 spec defined standard sizes for mil-spec resistor parts.

 

[willem2]

Thanks for the reply, Don. I guess I should re-phrase my question by asking: Assuming the datasheet resistance value stays roughly constant, then of what importance is the wattage rating if one cannot even attain that without risk of arcing? Shouldn't the datasheet wattage be taken from V^2/R (since this other quantity is clearly less than the datasheet wattage)?
Thanks.
ed.

You could exceed the voltage rating, and not the power rating with a signal consisting of short pulses.

 

[darkfeffy]

Hi Willem2

Thanks for your reply but I have to disagree with you because, as you can see in the datasheet that I originally attached, it is possible to have a pulse power in the tens of kilo watts (yes, it says 10,000W) as long as the pulse duration is shorter than 1 micro second.
Apart from that remark, were you trying to say that we should not interpret the 'wattage' advertised on the datasheets as a continuous rating?

Thanks
ed.

 

[Averagesupernova]

We SHOULD interpret the wattage rating as continuous. Think about this: On a sine wave, the RMS volts is .707 * peak voltage. Average power is derived from RMS volts * RMS amps. So if you apply an AC voltage (sine wave) to a resistor that almost exceeds it's power rating, it may be possible you ARE exceeding its voltage rating. Not likely, but not impossible. A more likely scenario is what was described in post #6.

 

[jim hardy]

Posts 5 & 6 both hit the answer.

Place yourself for a moment in the shoes of manufacturer.
You have a line of resistor bodies in various body sizes, each capable of dissipating X_size watts (or milliwatts)
Each size offers resistance values ranging from probably one ohm (or 0.1 ohm) to megohms. And by size is how they're sorted in your catalog.

What is the limiting factor for each body size - watts or volts per inch of the internals ? observe dlgoff's arcing, that's a 1000 word picture if ever there was one. But only in high ohm values will you hit the volts-per-inch limit before the watts limit.

It's unusual in garden-variety circuit design to bump against the volts-per-inch limit because most consumer electronics doesn't have high enough voltage.
So it's not emphasized much in our education .

But the prudent designer is aware of it as just another limit to be observed.

By the way - it's good practice to use resistors at about half their continuous rated wattage.

 

[sophiecentaur]

Thanks for the reply, Don. I guess I should re-phrase my question by asking: Assuming the datasheet resistance value stays roughly constant, then of what importance is the wattage rating if one cannot even attain that without risk of arcing? Shouldn't the datasheet wattage be taken from V^2/R (since this other quantity is clearly less than the datasheet wattage)?
Thanks.
ed.

I'd say you would always need to read the sheet carefully and aim at whatever happens to be the lower limit. Manufacturers (and Engineers who use real components) are not interested in the sort of 'anomaly' that you feel you may have found. If you exceed either of the ratings then you have nothing to complain about. That's life. They have covered themselves for any liability by publishing their two figures.

 

[AlephZero]

What jom hardy said. The data sheet in the OP says, in big letters, on a red background,

SERIES SPECIFICATIONS

Those three limits apply to all the resistance values in the series. In different situations, for different resistance values, you hit one limit before you hit the other two, but you don't always hit the same limit first for every resistance value.

That's just the way specifications are written, in the real world. Printing a big table of the "exact" limits for all 150 resistance values in the range wouldn't add any value.

 

[Mike_In_Plano]

Voltage stresses are a very real problem to reliability of resistors. Even if a resistor can "absorb" a large impulse of energy, damage occurs between the grains and especially between the grains and metallization. Thus manufacturers make surge rated devices for applications with short, high transients.
Ignoring this phenomena invites warranty issues. Personally, I don't push resistors beyond 80% of Vrating.

- Mike