Question on power resistors

[rhdwave]

Could someone please look over this schematic and tell me if the power rating is correct for use with an 8ohm amp? I'm not quite understanding how this works, it seems to me that for the 8 ohm switch it is two 8ohm 50 watt resistors in parallel joined together to an additional two 8 ohm 50 watt resistors in parallel.  So, to me it makes sense that the ohmage stays at 8 as the two parallel combinations will become both 4ohms but then since they're in series you add them together.  Now, i'm not even sure this is correct from what i'm looking at.  Anyone know for sure?

And secondly, i couldn't find any information on what happens to the wattage of resistors when combined in parallel and series.  Thanks for any responses!!!

http://www.aikenamps.com/DummyLoad.pdf

It's the top left handed schematic i'm looking at.

[R.G.]

The power rating of a resistor remains whatever it is however it's combined with other things.

What that means is that there is a maximum current that it can withstand (as P=I^2*R) and a maximum voltage it can withstand (as P = V^2/R). Those are unchanged by connection.

In a series connection, the current flowing through two resistors is identical - it's the same current - so the amount of power that can be dissipated by the series connection without burning something up is just the power which happens when the current or voltage would burn up the lesser-power-rated resistor.  And the total power which is dissipated is the power in each one, or

Ptotal = R1-power + R2-power = (I^2) * (R1+R2)

So the power is the sum of the resistances times the square of the current which is the max current that would burn up the smallest-power rating resistor. If the two resistors happen to have equal resistances and equal power ratings, the power rating of the two is double the power of one.

In parallel, the voltage across the resistors is the same, so the power is
Ptotal = V^2/R1 + V^2/R2

The voltage which can be used without burning out the parallel combination is the smallest voltage which would burn out the lowest rated resistor again. The bigger one would still be fine. If the two resistors happen to be the same resistance, then the power dissipated in each one is identical, and the max power for the pair is the power which would burn out the lowest-rated one. If the resistors happen to be the same power rating, this is just twice the rating of either one.

So for identical resistors, you get the power rating of a single resistor times the number of resistors whether it's series or parallel. But you have to calculate if they're not identical.

[rhdwave]

Thanks RG !!
So for the dummy load, i could use 4 50 watt resistors and it would cover up to 200watts? Is that correct, as shown in the diagram on the link i provided?

[R.G.]

I don't know which diagram you're looking at, but yes, four 8 ohm/50W resistors can be hooked up in series/parallel to get 8 ohms, 200W.

[rhdwave]

Thanks again RG!!! Here's the diagram i was looking at: http://www.aikenamps.com/DummyLoad.pdf