1/2 watt metal film looks like 1/4 watt

Passives Resistors
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1

40+ year user of carbon film 1/4W, 1/2W 5% resistors and a few 1/4W metal film 1%.

My recent order contained some cut tape 4.3 Ohm 1/2W metal film. I’m scratching my head since it looks like a 1/4W. DigiKey specs show 0.098" Dia and 0.295" Length. TE Connectivity specs only show D(max) 3.5mm and L(max) 10mm. I’m measuring 0.10" dia and .26" length which might match DigiKey’s specs, but when converting to MM, I get 2.5mm dia and 6.6mm lenght which is the size of their 1/4W … (IE ROX025 D(max) 2.5mm and L(max 7.5mm).) Since they don’t spec a min and typical I have no way of knowing if the ROX05 1/2W can actually be smaller than their RO025 1/4W max, or if DigiKey packaged the wrong wattage…

I get that metal film can be smaller due to makup materials being thinner and better heat stability, but my concern is heat dissipation over the same size surface area in a closed environment with no air circulation. I only chose metal film since carbon film wasn’t availalble and choose wattage based on Power = Isquared *R plus wiggle room so if I’m anywhere close to 1/4W, I always go with 1/2 watt.

2

Do you have the part number you ordered/received. The measurements look like they line up with an ROX05S . S=Small size.

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3

Greetings,

The datasheet appears to list both “normal” and “small” size variants for the product series, with the xxxx05S indeed showing a half-watt rating.

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Those ratings are typically reckoned as the power the causes the resistor to reach maximum rated temperature in some specified environment, e.g. free-air at 70°C. De-rating from nominal figures in practice is a good idea, because not all installations are in free air, and a crazy-hot resistor can start melting other things that come into contact with it…

A resource discussing this and related ideas that may be of interest can be found here.

4

Thanks, now that makes sense finally. Guess I didn’t pay attention to “small size”, since I let DigiKey sort my specs and ended with no cut tape but this one. ROX05SJ4R3 and misread small size line as having the same list and sizes when questioning. I’ll try one and do a temp check and go from there. Thanks all.