1/4 watt resistors... no go?

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sluckey
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Re: 1/4 watt resistors... no go?

Post by sluckey »

The rule of thumb I learned back in the '60s was calculate the real wattage (in your case .37W). Double that number for safety factor (in your case .74W). Now pick the next highest wattage resistor you have on hand (probably 1W).

Save those 1/4 watters for a nice pedal project. :wink:
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pompeiisneaks
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Re: 1/4 watt resistors... no go?

Post by pompeiisneaks »

Yeah makes sense. I should probably just go for 1 watters on the norm to remove the potential for noise if it starts dissipating. I think that's part of the reason the tweedle Dee deluxe has 3 10 watters instead of the common 5 watter. (Or was it one 10 watt).

~Phil
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sluckey
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Re: 1/4 watt resistors... no go?

Post by sluckey »

pompeiisneaks wrote: Tue Dec 19, 2017 12:05 am I think that's part of the reason the tweedle Dee deluxe has 3 10 watters instead of the common 5 watter. (Or was it one 10 watt).
I doubt there was any high level thinking going on with that decision. More likely, I need a 250Ω@5W. Damn! Ain't got one. I'll just use a couple 100Ω and one 50Ω. And watch the internet go crazy trying to make some special mojo out of it. :mrgreen:
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martin manning
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Re: 1/4 watt resistors... no go?

Post by martin manning »

Thanks, RG. Using E24 series values makes the table size quite manageable. Here's another version of the log-log plots above. The scales are the same so both sets of curves can go on the same page. I'm finding this pretty quick to use. For a given resistor you might know either the current or the voltage, so it's handy to have both options. Go in with resistance and either voltage or current, and choose the wattage rating just above, which ia already derated 50%. Basically a graphical equivalent to the process described by Steve Luckey above. If you use the current, you can also read the corresponding voltage using the dissipation curves as a guide, to see if it is over the resistor's spec. Note this will be actual voltage even though the dissipation curves are derated.
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R.G.
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Re: 1/4 watt resistors... no go?

Post by R.G. »

That's another good way to present the info.

I chewed for a while on how to stick the derating info in. The datasheets showed a number of max surface temps in the usual difficult to use manner. My rule of thumb that a resistor at max dissipation is 200C on its surface turned out to be wrong, of course. That number is actually more like 155C for most of the resistor families on today's data sheets, and 125C for one of them.

The thermal derating I got from my datasheet review of the brands that Mouser sells came down to this:
> CF and CC resistors can dissipate their rated power up to 70C ambient temperature
> Above that, derated linearly with ambient temperature to a max ambient temperature, where the internal dissipation must be derated to zero.

Hard to put that in a simple, eye-friendly way.
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pompeiisneaks
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Re: 1/4 watt resistors... no go?

Post by pompeiisneaks »

Thanks Martin and R.G. Great info again :).

Such a depth of information here, love it!

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martin manning
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Re: 1/4 watt resistors... no go?

Post by martin manning »

I fooled around with this spreadsheet idea a bit more. Three numbers taken from the resistor data sheet (the red symbols) will duplicate the derate curves (for one rating anyway), then with a little more Excel-smithng you can have a calculator where you input an ambient temp and a max surface temp, and get a derate factor out for each. In the screen shot below, I determined both an ambient temp and surface temp (using derate factors for 1/2W KOA Speer MF resistors) producing the rule-of-thumb 50% derate. The result seems comfortably conservative given that at an ambient temp of 235F (!) the surface temp is still right around the max working level for G10. For a 100k plate resistor, the 50% derate max current will be 1.58 mA, and the max voltage will be 158. Fine for a 12AX7 preamp stage. A smaller, cheaper 1/4W part could be tried (the derate parameters would need updating), but for a DIY project the cost savings really doesn't matter.
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