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Identifying the Wattage of resistors?

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bigal_scorpio

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Hi to all,

I recently bought some 3R9 and 4R7 resistors from a seller on ebay that were listed as 1/2w.

I only bought these because the only ones I had were 1/4w or 1/8w which is what the vast majority of my collection are (or were bought as) and I needed some 1/2w ones for a new project where the resistors are used in a LM317 regulator to keep current to a certain level and the calculations meant that the resistors would possibly carry 1/3w.

When the new resistors arrived I was a bit taken aback to see one set labelled as 1/2w carbon film 4R7 and the other as 1/8w carbon film 3R9!

Now that wasn't the only thing that struck me as strange, the resistors were both the same physical size and looked suspiciously like the thousands I already have that were bought as 1/4 and 1/8w.

Now I am wondering what they are and don't want to wait until I have built the circuits just to see if the resistors get too hot as at the moment I am waiting for some parts and if they do fail I would have to wait again for bigger resistors.

Is there any way I can tell what watts(pun intended) the resistors are? Is there a way to determine the wattage by the physical size? Are there differently made carbon film resistors that have higher or lower wattage capacity?

BTW I already contacted the seller only to be told that they are both 1/2w and mislabelled!

My problem is now that I don't trust what he says and would like to know for sure. Many years ago I worked with large resistors and they certainly did get bigger for more watts then. Has this changed? Are my thousands of resistors all mislabelled? Or are these two wrong?

Any info either way appreciated guys.

Thanks. Al

PS the resistors measured with my vernier calipers are (body)6.25mm x 2.2mm at thickest to 1.7mm in the center. Leads measure (3R9) 0.49mm (4r7) 0.51mm.
 
I suspect it's difficult to rate Resistors by their physical size, - although as you suspect I believe "you can't Transistorise the Watt" - if the Seller is playing up then return the stuff and write it off to experience. If they were cheap, then bin them!
+ If you have plenty of resistors, I'm surprised you are bulk-buying R's that are only slightly higher-rating....can't you parallel them? (or series, although sometime difficult to mount them).

If you have 1/4W then two in series, in parallel (ie 4x in all) will give you 4x the rating at the same (individual) Res . . you know that....so why spend money? If these are 1/4 W then you have made a 1W resistor..... and I think a multi-R may have a greater instantaneous rating, consult the Mfr data sheets.
- that may clarify what your own bulk-resistors are....and that still depends on the ambient temperature.
 
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Hi Harold,

Yes I had thought of the ser/par solution, its just the size in this case.

Also I am thinking about the larger implications too. How many other projects have people made in the past that contain resistors of the wrong wattage? Have these conditions ever caused harm or worse?

It just makes you think. Are we at the mercy of someone deciding that a part has a particular value and calling them what they want? If lots of different value components can look exactly the same then who is to say that even large suppliers are not at the mercy of the supplier or manufacturer further up the chain.

Sorry for the rant but I just think that values are slipping these days and we are all guilty of just looking the other way sometimes for the sake of an easier life.

Al
 
Obviously when you buy on ebay it's caveat emptor. If you want devices that are labeled properly and reliable then you need to buy from a reputable vendor.

Regarding resistors, yes the wattage is pretty well related to size (unless the resistor in mounted on a heat sink), since it's the area of the component that radiates the heat away and, for a given wattage, you need a given area to keep the temperature at a safe level. To determine the wattage just look in a catalog of standard resistors and see the size versus wattage. That should tell you the wattage of the devices you have.
 
Well, you can always try to stress them. Operate them at 1/2 W for a few days and see if they change value or take a look at the temperature of the resistors and a known 1/2 Watt resistor operating at 1/2 W.

You can also CYA by making sure the same package size is availabe in the wattage you need from a reputable vendor.
 
Post #3 - to an extent a "value" is depenmdent on what you want from it.... in general, components are "acceptable standards" which means reasonably reliable.
If you are building (old hobby-hores comes into view) a 400 year working electronic clock, then you derate components so they are "more reliable"
- but in practice this means "Less stressed" - since even wirewould resistors can fail - due to nicks in the wire when made, or an encapsulated part can fail due to stress when the encapsulant shrinks. Sometimes these effect can take years. I recall relays failiing on a project - relays that had be specially commissioned (in a hurry!) and digging further it seemed the coli winders were in the habit of eating oranges, then washing their hands before commencing work, but due to the "rush" this was not always the case and some "spray" managed to get into the windings - not something that would be noticed if it was at a much lower level - so your components may be "programmed to fail"
It is things like this that make a 400year electronic clock unlikely to survive.
The clock at Wells Cathederal is about this age and still going.
 
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