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Subject: Re: 24 VAC to 9VDC supply
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Date: Thu, 05 Dec 2002 19:24:45 GMT
NNTP-Posting-Date: Thu, 05 Dec 2002 19:24:45 GMT
Organization: AT&T Worldnet
What do you think about my answer? Please don't hold back, don't be a
wallflower or a shrinking violet. Tell me what you really think.
Standing by in a underground lead lined room. :-)
>You can use a series non polarized capacitor in the 24vac line.
>Calculating the value is a bit more than just XsubL=Vdrop/current,
>but this will get you close. I have used this when I didn't want the >heat
of a dropping resistor.Capacitors used in speaker >crossovers have worked
snip from previous response.
>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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