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NNTP-Posting-Date: Thu, 05 Dec 2002 10:16:58 -0600
From: email@example.com (John Fields)
Subject: Re: 24 VAC to 9VDC supply
Date: Thu, 05 Dec 2002 15:54:06 GMT
Organization: Austin Instruments, Inc.
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On Thu, 05 Dec 2002 14:31:31 GMT, email@example.com (John Fields)
>On 4 Dec 2002 22:27:09 -0600, Mark Lloyd wrote:
>>In that case "faith" is a synonym of "stupidity". It seems to be
>>easier for me to see that rather obvious fact, since I escaped a lot
>>of the early childhood brainwashing.
>Part of that [lack of?] brainwashing seems to have made you paricularly inept
>at determining the power handling capabilities of low-power
>regulators as well having left you with obvious problems with
>eliciting meaning from written text.
>Here is the OP's submission:
>"What is the quickest, dirtiest, cheapest way to get
>9 volts DC from a 24VAC source. Need about 150ma
>of current but the DC does not need to be regulated.
>Half-wave rectifying will give too much DC voltage,
>it would burn up a regulator since I don't have a
>place to heatsink one. Some solution which involves
>cutting up a wall-wart making use of the parts inside
>would be ideal."
>And here is the last part of your response:
>"I doubt it. The power dissipated in the regulator is:
>Where Vi is the input voltage, Vo is the output voltage, and I is
>current. In your case.
>(Vi-Vo)*I = (24V-5V)*.15A = 19V*.15A = 2.85W
>Less than 3 watts isn't that much. You could even use one of the
>low-power regulators (78L05). These handle up to 150mA, are in TO-92
>cases, and don't use heatsinks."
>You're a fucking idiot, and here's the proof:
>Stupid error #1.
>Notice that in (Vi-Vo)*I = (24V-5V)*.15A = 19V*.15A = 2.85W you've
>neglected to convert RMS to peak. 24VRMS is about 34Vpk which,
>allowing for a 10% variation in mains, could run as high as 37.3V.
>After subtracting 0.7V for a single rectifier we're still left with
>36.62V on the input of a regulator rated for 35V max. Even doing a
>full-wave rectification and subtracting 1.4V for both diodes leaves
>us with 35.93V, still too high.
>Stupid error #2.
>Notice that the OP asked for 9V, not 5V.
>Stupid errors #3&4.
>Assuming worst case of high line and a filter cap with a capacitance
>high enough to present little ripple to the input of the regulator,
>the power dissipation in the 78L09 would be about 36V-9V*150mA =
>4.05W. Expecting a 78L09 with a _peak_ output current capability of
>140mA to pass 150mA without going into limiting is stupid error #3,
>and expecting a device with a maximum junction temperature rating of
>125°C and an Rtheta(J-A)of 230°C/W to operate with a junction
>temperature of 931.5°C is stupid error #4.
>So, since you obviously _are_ stupid, your opinions as to what does
>and does not constitute stupidity are invalid and you can take your
>opinions about what constitutes faith along with the rest of your
>opinions and stick them right up your ass, where they belong.
Professional circuit designer
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