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From: "Ken McDonald"
References: <7FYW9.1208$PR2.email@example.com> <firstname.lastname@example.org>
Subject: Re: Don't do this! Just don't!
X-Newsreader: Microsoft Outlook Express 6.00.2800.1106
Date: Mon, 20 Jan 2003 20:46:57 GMT
NNTP-Posting-Date: Mon, 20 Jan 2003 15:46:57 EST
"Spehro Pefhany" wrote in message
> On Mon, 20 Jan 2003 20:20:44 GMT, the renowned "Ken McDonald"
> >"Spehro Pefhany" wrote in message
> >> On Mon, 20 Jan 2003 20:06:27 GMT, the renowned "Ken McDonald"
> >> wrote:
> >> >>
> >> >>
> >> >> +15
> >> >> |
> >> >> p
> >> >> 10K o <--------------10K -----x--- small offset
> >> >> t |
> >> >> | 49R9
> >> >> | |
> >> >> | 0V
> >> >> -15.
> >> >>
> >> >>
> >> >> It's a bit nonlinear and the output impedance varies a teeny bit,
> >> >> (from 12.5K||49R9 to 10.0K||49.9) but not bad.
> >> >
> >> >If the pot is adjusted to the rail, won't the 49.9 ohm resistor smoke?
> >> Perhaps I should have mentioned that the 49R9 resistor should be rated
> >> for a *minimum* of 1/8 mW.
> >How much power will a 49.9 ohm resistor connected to 15 volts dissipate?
> >keep coming up with 4.5 watts.
> You're somehow missing the 10K series resistor.
> If you crank the pot (say) to 15V, the pot element sees 30V and
> dissipates 90mW. The 10K resistor dissipates (15 * 10/10.0499)^2/10K =
> 22.3mW. The 49.9 ohm resistor has only 15 * 49.9/(10049.9) = 74.47mV
> across it, and dissipates only 0.111mW.
Okay. I'm having trouble seeing which side of the 10K resistor the 49.9 ohm
resistor is on. The schematic would have made more sense to me if "10K" was
on the left side of the 49.9 ohm resistor. I didn't miss the 10K resistor,
it just looks like both resistors are connected to the wiper of the pot.
> Best regards,
> Spehro Pefhany
> "it's the network..." "The Journey is the reward"
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