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/iframe></center><br><br> <br><br> </center> </td></tr> </table> <center> <table> <tr> <td width="600"> <center><hr width="400"><font size="2"><b>The <a href="http://www.cyber-spy.com/">Cyber-Spy.Com</a> Usenet Archive Feeds Directly<br>From The Open And Publicly Available Newsgroup<br>Sci.Electronics.Design<br><br>This Group And Thousands Of Others Are Available<br>On Most IS NNTP News Servers On Port 119.<br><br><a href="http://www.cyber-spy.com/">Cyber-Spy.Com</a> Is NOT Responsible For Any Topic,<br>Opinions Or Content Posted To This Or Any Other<br>Newsgroup. This Web Archive Of The Newsgroup And<br>Posts Are For Informational Purposes Only.<br><br></b></font><hr width="400"></center> <center><font size="2"><pre><b> Message-ID: <3DB4D802.519D9110@bellatlantic.net> From: ehsjr@bellatlantic.net Reply-To: ehsjr@bellatlantic.net X-Mailer: Mozilla 4.75 [en] (Win98; U) X-Accept-Language: en Newsgroups: sci.electronics.components,sci.electronics.design,alt.engineering.electrical Subject: Re: V regulator input cap size? References: <0001HW.B9D991AD000C5917163F7590@news.covad.net> <3db451b5.4005327@news.texas.net> <0001HW.B9D9CD54001A5D15163F7590@news.covad.net> Date: Tue, 22 Oct 2002 04:45:24 GMT NNTP-Posting-Host: 162.84.134.180 NNTP-Posting-Date: Tue, 22 Oct 2002 00:45:24 EDT DaveC wrote: > > On Mon, 21 Oct 2002 12:42:42 -0700, John Fields wrote > (in message <3db451b5.4005327@news.texas.net>): > > > THis is what you have: > > > > > > 35RMS>----------------+ > > | > > 35RMS>--+-[CR]-+-[CR]-+ > > | | | 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. > > What to do? I'm using 35vac from an HVAC control-voltage transformer. The > transformer is used for standard control circuits in the HVAC unit on the > roof, so I can't change the transformer. > > Short of getting a 35v/20v transformer (or such unavailable combination), any > suggestions how I can make this voltage useable for my 12vdc circuit? > Two-diode rectification? Single diode? You could build a pre-regulator using a power transistor with a zener to set the voltage at the base to 15 volts. That will give you about 15 volts on the emitter to the input of the 7812. It avoids the overvoltage problem and assists with the heat problem. The power transistor (assuming ~34 volts across it) will dissipate about 10.2 watts (assuming total current draw of 300 ma), and the 7812 will have to dissipate about a watt. That 300 ma is probably high - but your minimum is 200 ma, which still leaves a total of about 6.2 watts to be dissipated. Finding a lower current relay would be a big help. Another idea: Use 2 120 to 24 volt transformers. Connect the 120 primary of each transformer to the 35 volts. Connect the 24 volt secondaries (which will provide 7 volts each) in series aiding and you'll have 14 volts AC. That will give you roughly 19 volts DC for the 7812. It will need to dissipate about 2.1 watts at 300 ma. You can reduce that further by putting 8 1N4001 diodes in series after the bridge - that would reduce it to roughly .66 watts at 300 ma > -- > Note that my return address is corrupted in an attempt to reduce spam. If you > choose to e-mail me, please correct my address as described below. > > Thanks, > Dave > --- > Dave Carpenter > Sound Logic > voicebox@d8n8a8i.com > > Remove the numbers to reply via e-mail </b></pre></font><hr width="650"><a href="http://www.cyber-spy.com/electo-index.html"><font size="3">Go Back To The <a href="http://www.cyber-spy.com/">Cyber-Spy.Com</a><br>Usenet Web Archive Index Of<br>The sci.electronics.design Newsgroup</font></a><br><br></center></td><img src="http://cyber-spy.com/cgi-bin/livecntr.pl?0+1+unetsci" width="1" height="1" alt="The Cyber-Spy Toolkit - powerful internet spy software - live police scanners - usenet fbi secrets archive - newsgroup spy electronics archive - search engine spy tools - download TCST today!"> </tr> </table> </center> <center> <br><center><hr size="3"></center> <table width="100%" cellpadding="0" border="0" cellspacing="0"> <tr> <td align="left" valign="top"><small><b><i><a href="http://www.cyber-spy.com/index.html">Cyber-Spy.Com Live Resources</a></i></b></small></td> <td align="right" valign="top"><small> Copyright Alpha Omega Delta,<br> All rights reserved. </small></td> </tr> </table> <center><hr size="3"></center> <br><br> <br><br></center> </body></html>