Cement resistor Q

[dude]

A cathode biasing resistor calls for 10 watts (6L6s), would two 5 watts in series be equal to 10 watts?
Caps in series doubles wattage and halfs capactance, Resistors in series adds to resistance but what about the wattage?


al 

[92Volts]

Yes. Also, 2 parallel resistors (of double the intended resistance each) would work too.

In series each sees decreased voltage but the same current. In parallel each sees the same voltage but decreased current.

[sluckey]

Caps in series doubles wattage and halfs resistance...

No. EQUAL value caps with same voltage rating connected in series will double the voltage rating and half the capacitance. This is only true for equal value caps. If you connect two unequal value caps in series the total capacitance will be equal to the reciprocal of the sum of the reciprocals. The voltage felt across each cap will also be different.

[dude]

I miss-put resistance, instead I meant to say capacitance. I should have said same value caps too, I'm aware of unequal caps values in series and parallel not being doubled or halved, thanks.


So, I take it on the biasing resistors, two 5 watts of any value in series will be both wattages added together, 10 watts. One 160 and one 100 (5 watts each) = 260 ohms and 10 watts?


Thanks   

[Colas LeGrippa]

two 5w resistances in series equal 5W. even 100 in series equal 5 W.
two 2,5W resistances in parallel (of equal resistance) equals 5W
if you want to get 1K 10W, you can use 1k 10w OR  2x 2k 5w IN PARALLEL
DON'T FORGET THAT IN A GIVEN CIRCUIT, THE CURRENT STAYS THE SAME wherever you take the measurement.  The voltage drop changes with the resistances values for a total of drop that equals the total resistances, paralleled or in series. Uffff,,,clear I hope !!!!!!

[92Volts]

I miss-put resistance, instead I meant to say capacitance. I should have said same value caps too, I'm aware of unequal caps values in series and parallel not being doubled or halved, thanks.


So, I take it on the biasing resistors, two 5 watts of any value in series will be both wattages added together, 10 watts. One 160 and one 100 (5 watts each) = 260 ohms and 10 watts?


Thanks   

No, they need the same value to double the power limit. When they're dissimilar values, one is "working harder" than the other and we must respect that individual resistor's 5W limit. When we reach that limit, the other one dissipates less than 5W and the total power is less than doubled.

The current through each resistor is the same. By this or other rules, we know they form a voltage divider and the larger resistance takes more voltage across it.

If you have 26 volts across the pair (due to 100ma current), the 100 ohm sees 10v and the 160 ohm sees 16v.

The 100 ohm in this case dissipates 1W. The 160 ohm dissipates 1.6W.

This is still better than a single 260 ohm, 5W resistor! The 160 ohm reaches its 5W limit around 28 volts and 175ma. At that time the 100 ohm resistor sees an additional 17.5v and 3ish watts for a total capacity around 8 watts.

[92Volts]

two 5w resistances in series equal 5W. even 100 in series equal 5 W.
two 2,5W resistances in parallel (of equal resistance) equals 5W

This is wrong.

Yes, current is the same through each resistor in a series string.

But the resistance and voltage across each one will be lower, if you use a series string to replace a single, higher-value resistor.

If you were dealing with a constant current (somehow), each series resistor could handle the same current as it can on its own. But the total resistance and therefore voltage across and power dissipated in the entire string would increase.

[dude]

Thanks, gets kind if confusing but I think I get it. I assumed there's a difference in wattage ratings adding different value R's than when they're the same (vs different).


I get the cap thing, that's pretty clear if values and watts are the same but like Sluckey says when their different it a new ballgame. I assume Sluckey is telling me in the long run: Best to keep the capacitance and it's ratings the same when using them in parallel or series, not that it can't be done.

Probably a good rule of thumb with resistors too, unless you want the lowest rating of the lowest one to be safe. I just never thought about the "rating" of resistors in series or parallel, we usually just to what we have to get the correct value when we don't have the correct R, but we usually use the same ratings, like half watts.


Thanks everyone,
al   

[PRR]

Caps don't have Watts.

Otherwise you got it right the first time. All the rest was confusing.

[dude]

Caps don't have Watts.

Otherwise you got it right the first time. All the rest was confusing.

Sorry PRR, I meant to say capacitance, guess my age is catching up to me but thanks for pointing that out. Sometimes I scare myself walking to shop, 30 second trip only to forget what I going for...? Also, I'm saying capacitance and watts, really a cap has a wattage rating and not "watts". I'm out to lunch today. 
     

[sluckey]

I'm saying capacitance and watts, really a cap has a wattage rating and not "watts". I'm out to lunch today. 

I'll say! When ya comin back?

[shooter]

glad I had an ALt-beer before reading this one 

[dude]

What, what, you say?  What did I say...? I think it was a hamburger I was after

[jjasilli]

two 5w resistances in series equal 5W. even 100 in series equal 5 W.
two 2,5W resistances in parallel (of equal resistance) equals 5W

This is wrong.

Yes, current is the same through each resistor in a series string.

But the resistance and voltage across each one will be lower, if you use a series string to replace a single, higher-value resistor.

If you were dealing with a constant current (somehow), each series resistor could handle the same current as it can on its own. But the total resistance and therefore voltage across and power dissipated in the entire string would increase.

92volts is right. To simplify: 1X 10W resistor of R value can be replaced by 2X 5W resistors which ea having 1/2 R value, which total that R value.  The reason is that each 5W Resistor of, 1/2R value, will drop only 1/2 the voltage; hence dissipate only 1/2 the wattage.

[jjasilli]

Colas:  two 5w resistances in series equal 5W. even 100 in series equal 5 W.
92volts:  This is wrong.

92volts is right. To simplify: 1X 10W resistor of R value can be replaced by 2X 5W series resistors which ea having 1/2 R value, which total that R value.  The reason is that each 5W Resistor of, 1/2R value, will drop only 1/2 the voltage; hence dissipate only 1/2 the wattage.

Colas is right that the current is the same.  But that's only part of the story.  Per Ohm's Law that current has to be multiplied by the smaller resistance of ea series resistor.  Ea series resistor produces a smaller voltage drop than one bigger resistor.