Wattage for resistors

  • Thread starter Beanyboy
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  • #1
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Am trying to build an Astable 555 timer 8 bit computer clock. Ben Eater's project.


Resistors are specified, but with no reference to wattage. Does anyone know the appropriate wattage for the carbon film resistors please?
 

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  • #2
gleem
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Power dissipation is equal to V2/R or RI2 where the V is the voltage drop across the resistor and I is the current through the resistor. Since you seem to be limited to 5 Volts anywhere in the circuit and the lowest resistor is 220 ohms I would say the 1/4 watt resistors would be fine.
 
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  • #3
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Very much appreciated. Those were the ones I had ordered - phew!!!
 
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  • #4
sophiecentaur
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carbon film resistors
Not metal film? I don't think carbon resistors have been used for years.
 
  • #5
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If you order a pack of budget resistors from eg A*z*n, and they come from Far East, unless specified as metal film, they may well be carbon.
Buyer Beware.
 
  • #7
CWatters
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Not metal film? I don't think carbon resistors have been used for years.
Places like Mouser here in the UK still stock quite a lot (4000+ matches) but I think 1/4W is the smallest. They have Metal film in 1/8W, 1/10W even some 1/20W ranges.
 
  • #8
davenn
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Not metal film? I don't think carbon resistors have been used for years.

carbon film are still very much a standard. I use 1000's of them in my commercial constructions
on a day to day basis
You are probably remembering the much older solid carbon resistors


Dave
 
  • #9
sophiecentaur
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carbon film are still very much a standard. I use 1000's of them in my commercial constructions
on a day to day basis
You are probably remembering the much older solid carbon resistors


Dave
Yes. That's probably it. It's a while since I constructed circuits and, where I worked, the occasional carbon resistor could be found in the bottom of a component drawer or on an old circuit board.
Metal film resistors are so good nowadays, for accuracy and stability, what are the advantages of modern carbon film resistors? I could imagine that they could be made low inductance and possibly cheaper??
 
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  • #10
davenn
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Metal film resistors are so good nowadays, for accuracy and stability, what are the advantages of modern carbon film resistors? I could imagine that they could be made low inductance and possibly cheaper??

I use metal film where low noise is important eg audio circuits

where noise isn't an issue, I use carbon film, yes they are cheaper and when using them by the 1000's
the savings in cost start to add up :smile:

Dave
 
  • #11
sophiecentaur
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I use metal film where low noise is important eg audio circuits

where noise isn't an issue, I use carbon film, yes they are cheaper and when using them by the 1000's
the savings in cost start to add up :smile:

Dave
Ha! I was never involved in procurement. We just had 'expensive' stuff or else 'very expensive' stuff to play with. No wonder they cut the money to R&D whenever there were cuts in general.
I guess that was 'bad engineering', really.
 
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  • #12
Most 555 type circuits use the 1/4 w resistors as noted. In some cases the 1/8 w are fine given the power dissipcaton. When looking at the power dissipation of resistors be careful to look at the temperature rise. The really old carbon composition resistors were fairly large and the with that large surface area did not as hot as the newer, smaller, resistors with the same power rating. Thus a 2 watt carbon comp. with 1 watt of power would have a temperature rise much less than a modern 2 watt metal oxide resistor that has much less surface area. The metal oxide is designed to be reliable at those higher temperatures. However in some circuits you could have a plastic capacitor next to that resistor and would not want it subjected to the higher temperature. So, in effect, not all resistors of the same rated wattage are equal. I remember cases where I had to choose one companies 3 watt resistor to be an equivalent to another's 2 watt resistor. Lots of decisions when choosing components. We found that in most cases it was better to buy the much more precise metal film resistors than the less expensive carbon film resistors.
 

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