replacing resistors: 0.6W film with 0.25W carbon

Frequently asked question regarding resistors, potentiometers, types, requirements, ratings etc.
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Post by Rumbler »

Hi there,

I am trying to assemble the Fuzz unit in Penfold's "Music Project" book.
It requires metal film resistors, all 0.6W, but I only have carbon 0.25W.
After reading here and there in the forum, I understand that the material of the resistors will affect the signal introducing noises.

But what about the power rating? Am I going to fry the carbon resistors?

If I understand correctly, power rating is the maximal limit for heath dissipation, that is how much power the resistor can convert into heat.
Now, if the pedal is powered with a 9V battery, 9V is also the maximal voltage applied to each component in the circuit. Then, according to Ohm's law, a 100 Ohm resistor with a power rating of 0.25 W will melt with a 9V voltage ( 9V x 9V / 100 Ohm = 0.81W). On the other hand, a 100 KOhm resistor in the same connditions will have no problem (9V x 9V / 100000 Ohm = 0.00081 W).

Is that correct?


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Post by galmar »

Carbon film resistors have more inherent hiss noise. A smaller part (usually of lower wattage) will also have more noise too.

For the above statement regarding power, your assumptions are correct. Of course, they refer to the worst case scenario - that is, a resistor sees the whole supply across it. This saves you the trouble of burning a resistor if another fault occurs and it has to bear the whole 9V.

What is the schematic, actually? Good designs ensure that no resistor is normally operated at more than its half power capability. Sometimes I even use a rule of not exceeding the 1/10 of the nominal power.

So, if you could provide us with schematics we could give you more accurate information. :)

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Post by Rumbler »

Hi there,

Here is the link for the schematic (it's a picture taken with my phone, no time to redraw it)

The part list requires all the resistors to be 0.6W 1% metal film.
Except for the hiss, I think there should be no issue using resistors with 0.25W carbon.
All the values are pretty high (2.2 to 100 KOhm), I think 0.25W resistors should be ok.
I am only worried about R9 and R10, two resistors of 470 Ohm. I am not sure about the voltage at that point.

In the worst scenario the maximum power for R9 and R10 is around 0.17W, which is below the 0.25W limit of my carbon resistor, but is it safe to be so close to the limit?

Maybe I am worrying too much about the details, but this helps me understanding while I learn to work with schematics.
And sorry for the convoluted English, it is not my mother language.


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Post by DrNomis »

You shouldn't have any problems using .25 watt resistors in the circuit you're building, other than maybe a bit of noise since carbon composition resistors do tend to be inherently noisy, .25 watt 5% carbon film resistors will work fine..... :thumbsup
Genius is not all about 99% perspiration, and 1% inspiration - sometimes the solution is staring you right in the face.-Frequencycentral.

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