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From: Mark Lloyd
Subject: Re: 24 VAC to 9VDC supply
Date: 5 Dec 2002 15:56:08 -0600
References: <email@example.com> <firstname.lastname@example.org> <email@example.com>
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On Thu, 05 Dec 2002 14:31:31 GMT, firstname.lastname@example.org (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.
I do apolagize for actual errors I made (most notably 5V instead of
9V), but will not participate in the general bad attitude expressed in
the rest of your reply.
>Part of that 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
20 days until the winter solstice celebration
"There's no more proof for the existence of God than there is for the existence of the
Easter Bunny. That's right. The Easter Bunny."
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