From: Lizard Blizzard
Subject: Re: V regulator input cap size?
Date: Tue, 22 Oct 2002 07:50:55 -0700
References: <0001HW.B9D991AD000C5917163F7590@news.covad.net> <firstname.lastname@example.org> <email@example.com>
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Frank Bemelman wrote:
> "John Fields" schreef in bericht
>>On Mon, 21 Oct 2002 11:12:45 -0700, DaveC wrote:
>>>Using existing design of 12v regulated ps. Input voltage is 35vac rms,
>>>rect, 470uF input cap, KA7812 regulator, 22uF output cap.
>>>The data sheet for this regulator specifies much smaller caps: 0.33uF
>>>Are the larger caps way too big? Is there a down side to having such
>>>caps? This circuit will power 3 ICs (CD4020 and two 555's), two LEDs, and
>>>200mA-coil relay, plus misc support passives.
>>THis is what you have:
>> | | | 7812
>> | +-----------+---IN OUT--+--->>--+
>> | | | GND | |
>> +-[CR]-+-[CR]-+ [470µF] | [22µF] [RL]
>> | | | | |
>>Your 35VRMS is ~ 49.5V peak, and after two diode drops through the
>>bridge will wind up as about 48V across the input of the 7812.
>>Unless that KA7812 is rated for substantially more input voltage
>>than a standard 7812 (35V) you've been shot down before you've even
>>gotten off the ground.
> It's amazing how components behave beyond their specs, see my experiment
> with the 4060. My benchsupply does not go much beyond 30V, so I can't
> try frying a 7812.
> Another point is that relay drawing 200mA. Dropping 36V at 200mA, things
> will run more than a bit hot. If the PCB is upside-down, the 7812 will
> probably desolder itself, before dying ;)
> If I were on a bounty island and these were all the available
> parts, I would spend the rest of the day on the beach.
Where's the suntan oil - I'll come join you.
I goofed, I said LM317 in the other followup, substitute LM340 or
whatever in its place. For 12V output, the unreg'd supply shouldn't
have to be more than 18 to 20V, nor less than 15V.
Speaking of the bare basics, quite literally and a lesson in
improvisation, here's an interesting URL:
Use an old aluminum drink can and a piece of wire you find down at the
beach, and improvise a battery out of some dissimilar metals and salt
water. Make yourself a milliwatt beacon transmitter. If you're lucky,
you might get it to work on 121.5 MHz and summon a search and rescue
team. Tell them, in Morse Code, to bring you some more suntan oil. :-))
> Frank Bemelman
> (remove 'x' & .invalid when sending email)
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