Resistor wattage question (Schematic example posted)

JoeyPee wrote in news:e6c03f8c-8e38-49fa-b294- snipped-for-privacy@f38g2000pra.googlegroups.com:

Either it's impossible, or it depends on other parts in unseen context, so what's the context?

The only way I can make sense of the claim without extra info is that the switch appears to a single ganged pair, so in any position there is a minimum of ten watts safe dissipation per circuit, total 20, but if that's what they mean it sounds like bad specsmanship.

There is no other context. It is a component substitution box, and that is the full schematic for this section. What you see is the entire section.

My only issue is that the R25 and R38 positions cannot dissipate 20 watts because each is only a 10-watt resistor and no other resistors are in series with them. Hence, the "20-watt" rating claim is not accurate for those positions.

I was just asking for another opinion in case I was missing something obvious. As I see it, the 20-w claim is only accurate for the 2nd position on each top and bottom section. Higher positions would actually be rated above 20w, while the lowest positions are clearly

Disagree?

The single resistor position certainly can't be better than 10W, unless there are some heroic means to cool the resistors. There may be case cooling issues that would limit the box to 20W total, no matter how many resistors are switched in.

I think you're correct.

I'm not sure where the original verbiage came from but it sounds like, and the schematic looks like, something from a Popular Electronics article 30 or 40 years ago. Not exactly peer-reviewed or anything :-).

It's interesting how slowly the standard E12/E24 values have been accepted in the power resistor world. Many of the values on that schematic are still commercially produced today.

Tim.