Metal Film Resistors. Power Ratings and Physical Sizes.

Thread Starter

RAMBO999

Joined Feb 26, 2018
256
I have had a good rummage about and can't find anything definitive.

Ther are two key parameters to every resistor. Power and resistance.

From those you can derive maximum voltage that can be applied to the resistor should
so that it doesn't burn out.

I am given to understand that there is some relationship between power rating and
physical dimensions. It's to do with heat dissipation.

Does anyone know of a chart or a list that details the standard dimensions of metal film resistors
as they relate to power ratings.

I would expect a 2W 50 ohm resistor to be bigger than a 0.25W one for example.

I have hundreds of mf resistors. Just need a way to categorize them power wise.

Thanks.
 

crutschow

Joined Mar 14, 2008
27,466
Look up the typical size for resistors vs power rating.
That should enable you to guess what the ratings are for the mf resistors you have.
 

WBahn

Joined Mar 31, 2012
26,398
I have had a good rummage about and can't find anything definitive.

Ther are two key parameters to every resistor. Power and resistance.

From those you can derive maximum voltage that can be applied to the resistor should
so that it doesn't burn out.

I am given to understand that there is some relationship between power rating and
physical dimensions. It's to do with heat dissipation.

Does anyone know of a chart or a list that details the standard dimensions of metal film resistors
as they relate to power ratings.

I would expect a 2W 50 ohm resistor to be bigger than a 0.25W one for example.

I have hundreds of mf resistors. Just need a way to categorize them power wise.

Thanks.
I'm not aware of any "standard dimensions" for these types of components.

For small power ratings (fractional watt) most resistors from most manufactures using similar form factors will probably be pretty close.

But once you start talking about larger resistors, there are a whole lot of additional factors at play. The resistor may have been built with features that enhance heat dissipation (such as fins or provisions for mounting on a heat sink). Also, other considerations, such as tolerance over temperature, may result in the exact same physical resistor being sold as two different part numbers with two different power ratings, the smaller one being a power level that keeps the temperature variations within a smaller range.
 

OBW0549

Joined Mar 2, 2015
3,566
Ther are two key parameters to every resistor. Power and resistance. From those you can derive maximum voltage that can be applied to the resistor should so that it doesn't burn out.
Beware: resistors usually have a maximum voltage rating that may be significantly lower than the maximum tolerable voltage you would calculate by this method, often as low as 125 volts.

For example, if you applied the P = V^2 / R formula to a 1/4W, 10 MΩ resistor, you would conclude its maximum applied voltage is 1580 volts. Don't try it.
 

dl324

Joined Mar 30, 2015
12,929
Does anyone know of a chart or a list that details the standard dimensions of metal film resistors
as they relate to power ratings.
Things were simpler before manufacturers started making 1/4W resistors that were the same dimensions as 1/8W.

This is for Yaego metal film:
upload_2019-7-29_13-41-2.png

This is for SEI resistors:
upload_2019-7-29_13-35-40.png

If you don't have the full part number (which can't be printed on the resistor), the reasonable thing to do would be to use the smaller power rating.

They also make unusual power capacities (e.g. 1/3W axial through hole). I'll measure the one example of 1/3W I have someday...
 

sagor

Joined Mar 10, 2019
469
OBW is correct, some MF resistors with higher voltage ratings will be longer but not as "fat" as lower voltage rated resistors, for the same wattage rating. I'm talking about 2W, 3W or 5W rated resistors. I have some higher voltage rated ones that are longer than the "standard" ones....
It varies by manufacturer and specified usage....
 

Thread Starter

RAMBO999

Joined Feb 26, 2018
256
Beware: resistors usually have a maximum voltage rating that may be significantly lower than the maximum tolerable voltage you would calculate by this method, often as low as 125 volts.

For example, if you applied the P = V^2 / R formula to a 1/4W, 10 MΩ resistor, you would conclude its maximum applied voltage is 1580 volts. Don't try it.
I would normally do the V = SQRT(P x R) thing which would give me V = 1580 volts.

Is there a practical rule of thumb for safety purposes? 75% or something?
 

Thread Starter

RAMBO999

Joined Feb 26, 2018
256
OBW is correct, some MF resistors with higher voltage ratings will be longer but not as "fat" as lower voltage rated resistors, for the same wattage rating. I'm talking about 2W, 3W or 5W rated resistors. I have some higher voltage rated ones that are longer than the "standard" ones....
It varies by manufacturer and specified usage....
All sounds like it's a bit of a mess standards wise. Right? Maybe stick with one supplier.

But that doesn't help me out with my stock.
 

dl324

Joined Mar 30, 2015
12,929
I would normally do the V = SQRT(P x R) thing which would give me V = 1580 volts.

Is there a practical rule of thumb for safety purposes? 75% or something?
Consult a datasheet. If you don't have the complete part number, be conservative and use the lowest figure.

Yaego MFR-12 and MFR25S resistors are the same physical size and have the same maximum working voltage. But the MFR-25 and MFR50S have different working voltages even though they're the same size.
upload_2019-7-29_13-52-20.png
 

Thread Starter

RAMBO999

Joined Feb 26, 2018
256
Things were simpler before manufacturers started making 1/4W resistors that were the same dimensions as 1/8W.

This is for Yaego metal film:
View attachment 182776

This is for SEI resistors:
View attachment 182775

If you don't have the full part number (which can't be printed on the resistor), the reasonable thing to do would be to use the smaller power rating.

They also make unusual power capacities (e.g. 1/3W axial through hole). I'll measure the one example of 1/3W I have someday...
Now that looks good. Do you have a link to those charts that you could post? So I can bookmark them.
 

WBahn

Joined Mar 31, 2012
26,398
I would normally do the V = SQRT(P x R) thing which would give me V = 1580 volts.

Is there a practical rule of thumb for safety purposes? 75% or something?
The absolute maximum voltage that can be applied is a separate issue from the power rating of the resistor. Different failure mechanisms involved. The absolute maximum voltage is determined largely by the effect of the resulting electric fields in space around the resistor (think arcing and such) and is largely independent of the value of the resistor. The power rating of the resistor establishes a different maximum voltage based on thermal considerations. The actual maximum voltage is the lower of the two.
 

WBahn

Joined Mar 31, 2012
26,398
All sounds like it's a bit of a mess standards wise. Right? Maybe stick with one supplier.

But that doesn't help me out with my stock.
Most manufacturers try to take this approach. Find and stick with suppliers that you can trust and rely on and then use their products unless there is an overriding reason not to. While unit cost is certainly a factor, it is often not the deciding factor. Chasing the lowest unit costs often incurs other costs that end up totally swamping any savings.
 

Thread Starter

RAMBO999

Joined Feb 26, 2018
256
Ok guys. Thanks very much for your input. I was looking for an easy solution but doesn't look like there is one.

Maybe it's a case of burning a few and see what point they start to smoke and divide by three.
 

OBW0549

Joined Mar 2, 2015
3,566
Is there a practical rule of thumb for safety purposes? 75% or something?
No. You have to consult the data sheet for that family of resistors to find the maximum voltage rating. DO NOT exceed that rating.

And if you cannot find a data sheet for them, either throw them out or limit their usage to circuits which will not expose them to more than about a hundred volts.
 

WBahn

Joined Mar 31, 2012
26,398
Ok guys. Thanks very much for your input. I was looking for an easy solution but doesn't look like there is one.

Maybe it's a case of burning a few and see what point they start to smoke and divide by three.
You will probably want to divide by a bigger number than that. You might have to go well above the rated power before you start "burning" it noticeably, but you might be operating at much higher temperatures than the device is rated for and you might be damaging the device and changing its resistance permanently.

If you have an IR thermometer (they are getting pretty cheap) then you can slowly ramp the power up until it reaches something near the rated temperature. I don't know what the typical surface temperature of a resistor would be that is operating at it's maximum core temperature, but you could take resistors of known power rating and run them at max and measure it and use that as a guide.
 

Thread Starter

RAMBO999

Joined Feb 26, 2018
256
No. You have to consult the data sheet for that family of resistors to find the maximum voltage rating. DO NOT exceed that rating.

And if you cannot find a data sheet for them, either throw them out or limit their usage to circuits which will not expose them to more than about a hundred volts.
There's not much chance finding a datasheet if you can't deteremine their origin. That's the problem. Binning them is not a bad idea I suppose especially for the small low power ones. They are extremel cheap. But the bigger ones are a bit pricey by comparison.
 

Thread Starter

RAMBO999

Joined Feb 26, 2018
256
You will probably want to divide by a bigger number than that. You might have to go well above the rated power before you start "burning" it noticeably, but you might be operating at much higher temperatures than the device is rated for and you might be damaging the device and changing its resistance permanently.

If you have an IR thermometer (they are getting pretty cheap) then you can slowly ramp the power up until it reaches something near the rated temperature. I don't know what the typical surface temperature of a resistor would be that is operating at it's maximum core temperature, but you could take resistors of known power rating and run them at max and measure it and use that as a guide.
Actually I did just that with my IR thermo. 70C is the nominal temperature I believe and none of them reached anywhere near that. What I also did was assume the power rating and apply the max voltage according V = SQRT(W x R) eg: W = 0.25, R = 47 ohms, V = 3.4 volts.
Then repeat with W = 0,5. Tested a sample on that basis. The focus on the temperature.
 

dl324

Joined Mar 30, 2015
12,929
Thanks very much for your input. I was looking for an easy solution but doesn't look like there is one.
Just don't use your unknown resistors in circuits that will push them to any of their conservative limits.

In many applications, power dissipation and working voltage limits are don't cares. Qualification - they do matter, but not if you're using a 1/4W sized resistor and it's only dissipating a small fraction of that.

For instance.
upload_2019-7-30_7-48-6.png
R2 dissipates about 65mW and R1 dissipates 16mW. A 1/4W resistor would be far nowhere near it's maximum power dissipation or working voltage specs.
 
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