Calculating resistor power rating - basic electronics

[letterbeacon]

Hi there,

I'm planning building the RS124 soon, and I'm attempting to work out what power rating I should rate the resistors. I know I can do this via Ohm's law, but to someone like me who can put together a project by following a schematic, but my theory is a little rusty, I'm struggling to find the values on the schematic I need for me to implement Ohm's law.

For example in the attached schematic R26 is 100k and links the plate of the 6B6C and the 6CG7 together. I can see that there's a voltage of 175v at 20db compression on the plate of the 6B6C, but how can I calculate the voltage drop across R26?

I know this is super-basic, but in all the examples I find when I Google, they give you all the figure you need to work it out.

Thanks.

 

[CJ]

step 1: resize schematic to 600 x 800

never mind, still can't read it,

you need current, usually this is done by subtracting plate V from B+, then divide that V by 100 K,

or you can read bias voltage across cathode resistor and using Ohms law,

then multiply current by itself, then multiply that by 00 K  P=I^2 R

 

[ruffrecords]

In your specific example, the 6CG7 plate is at 142 volts. The other plate can be no lower than75V the the maximum volts drop across the 100K is 142 - 75 = 67V. Power is V squared over R. If R is in K ohms the answer will be milliwatts so we have:

67 x 67 / 100 = 45mW

Cheers

Ian

 

[PRR]

I would not build something I can't read.

> in all the examples ..., they give you all the figure you need

The figures are *right there*.

75V on one plate. 142V on the other plate.

If they were 75 foot and 142 foot buildings, you would need a 67 foot ladder to climb one to the other.

Power of 67V in 100K "should" be simple to figure.

If not, go back to Boot Camp. 200K+33K=?? 10K||10K=?? 10K||5K=?? 120V across 12r =?? 12V in 1meg=??

 

[letterbeacon]

Great, thanks everyone -that’s much clearer now.

 

[PRR]

And the rules of thumb:

#1 -- All resistors 1/2W unless otherwise noted! This because the resistor makers saw that "most" electronic resistors ran under 1/2W, so that was the smallest/cheapest they would make. Yes, since the 1930s we have been going "smaller" and currently a 1/8W part may be cheaper than a 1/2W part. How much cheaper? How many do you have to buy? See rule of Brain Pain.

In 300V guitar preamps: Plate Resistors should be 2W. Normally they idle at 0.1W. But when a tube shorts-out we have 300V on a 100K resistor. With your Watts training you can see 0.9W here. Maybe a 1W part will survive an all-night short. And maybe it degrades to crap-out a week after you remove your short. Also the "300V" may be 350V even 400V. And the inside of a guitar amp is hot. Also bigger parts at lower relative stress give less trouble. If you make a million, you can ill-afford the extra pennies. For a one-off and doing your own warranty service, put in the bigger part dangit!

Here we see only 145V on tap. Logically the idle and worst-case dissipations will be lots lower (define "lots" as homework).

Brain Pain has a value. From 1W to 2W may be just 10 cents. Paying full-retail fast-service, 1/2W to 3W is another $0.60. THINKING is billable at $10 to $1,000/hour. Even if you are only worth the 10 bucks, if you can't make a 0.5W versus 3W decision in under 4 minutes, the bigger resistor comes out cheaper (at one-off quantity). If you buy cheaper, it is even less worth the brain pain to figure it out.