How to Determine Resistor Watt Rating...Does SIZE Matter ?

[PCollen]

I received a large number/variety of 1/2 watt metal film resistor for an amp I'm building, and ALL are the same physical size EXCEPT the 100K's, which are about half the size. I think these 100K's may be 1/4 watt, given the size difference, but how does one know ? I don't know if they all come from the same manufacturer or not.

See attached picture. The upper resistor is the 100K, and the lower an 82K from the same order/shipment.

[ChrisM]

I have 1/2W MF that size if it makes you feel any better.

Maybe for very common values like 100K they shrink the size down?

[PCollen]

I have 1/2W MF that size if it makes you feel any better.

Maybe for very common values like 100K they shrink the size down?

Like maybe to get more in the baggie...I don't think so, since the 470K, 1.5K, 2.7K, 10K 47K, 56K 68K, 82K 150K, 220K, 330K , and 1M resistors were all the larger size.

[ChrisM]

Was just throwing the idea out there and sharing my experience.

Do a test then. Put 224V DC across the resistor, if it burns up or gets very close to burning up then it's 1/2W.

[Structo]

You know that in most instances on a amp circuit board, 1/2 watt resistors are fine.
But, I generally use all 1 watt resistors, especially on the carbon film resistors.
A little bigger size, easier to handle, thicker leads.

The mil spec Dales are only rated at .6 watt I believe.
But we know mil spec is way over rated, those are 1 watt resistors in civilian life.

[20to20]

I received a large number/variety of 1/2 watt metal film resistor for an amp I'm building, and ALL are the same physical size EXCEPT the 100K's, which are about half the size. I think these 100K's may be 1/4 watt, given the size difference, but how does one know ? I don't know if they all come from the same manufacturer or not.

See attached picture. The upper resistor is the 100K, and the lower an 82K from the same order/shipment.

I'd say they slipped you some 1/4's by mistake. They look like the same material for sure. Since you can bracket the odd one with larger sized R's. there's not much doubt.

[PCollen]

Was just throwing the idea out there and sharing my experience.

Do a test then. Put 224V DC across the resistor, if it burns up or gets very close to burning up then it's 1/2W.

I was just making a joke re: that baggie comment, and I appreciate your comments.

Only have to put about 160-175 Vdc across it to determine quickly if it's a 1/4 watt, or less. 1/2 W would be OK... what I ordered and expected to receive.

[PCollen]

I received a large number/variety of 1/2 watt metal film resistor for an amp I'm building, and ALL are the same physical size EXCEPT the 100K's, which are about half the size. I think these 100K's may be 1/4 watt, given the size difference, but how does one know ? I don't know if they all come from the same manufacturer or not.

See attached picture. The upper resistor is the 100K, and the lower an 82K from the same order/shipment.

I'd say they slipped you some 1/4's by mistake. They look like the same material for sure. Since you can bracket the odd one with larger sized R's. there's not much doubt.

And I went to Mouser and looked at the spec sheets for several brands of MF resistors, and it seems that 1/4 W 'bodies' are typically significantly smaller than 1/2 W. L (1/4) = 6.8mm, vs. L (1/2) = 10mm for example, which is what I see here.

[PCollen]

You know that in most instances on a amp circuit board, 1/2 watt resistors are fine.
But, I generally use all 1 watt resistors, especially on the carbon film resistors.
A little bigger size, easier to handle, thicker leads.

The mil spec Dales are only rated at .6 watt I believe.
But we know mil spec is way over rated, those are 1 watt resistors in civilian life.

I've had good luck in the past with 1/2W MF's from this vendor..this situation is an aberation. It will be interesting to read their response to my 'memo of concern' I sent them last night.

Update: Vendor informed me that they ran out of stock on the 'usual' 100K/0.5W, and bought these smaller 100K/0.5W to fill orders until the larger ones are re-stocked..surprise, Surprise, SURPRISE ! Wish I'd know that BEFORE the order was filled and shipped.

[guitarmike2107]

If they are the same brand of resistor then the 100k are definitely a lower wattage resistor.

Power dissipation is related to material type and heat dissipation (surface area) so a smaller resistor will not be able to get rid of the heat as much as a larger one.

There are some very small 1w metal film resistors available now , but they brown and are obviously a new type of material?

[David Root]

There are some very small allegedly 1/2W resistors around, mostly made in India, according to the Mouser bag anyway. They are actually slightly smaller than conventional 1/4W types.

I say allegedly because if you ask Andy Fuchs about them they cost him a grundle of refitting because they FAIL IN PLATE LOAD SERVICE.

They are apparently OK for lower voltage use, just about anything except plate loads, but I would personally not use them as screen resistors either.

[guitarmike2107]

thats the ones, they are Vishay/BC Components metal film Resistors from India... brown coloured ....avoid these.