Windfreak Technologies
Berkeley Nucleonics Academy RF Boot Camp - RF Cafe
 

About RF Cafe

Kirt Blattenberger - RF Cafe Webmaster

Copyright: 1996 - 2024
Webmaster:
    Kirt Blattenberger,

    BSEE - KB3UON

RF Cafe began life in 1996 as "RF Tools" in an AOL screen name web space totaling 2 MB. Its primary purpose was to provide me with ready access to commonly needed formulas and reference material while performing my work as an RF system and circuit design engineer. The World Wide Web (Internet) was largely an unknown entity at the time and bandwidth was a scarce commodity. Dial-up modems blazed along at 14.4 kbps while tying up your telephone line, and a nice lady's voice announced "You've Got Mail" when a new message arrived...

All trademarks, copyrights, patents, and other rights of ownership to images and text used on the RF Cafe website are hereby acknowledged.

My Hobby Website:

AirplanesAndRockets.com

Electronics World articles Popular Electronics articles QST articles Radio & TV News articles Radio-Craft articles Radio-Electronics articles Short Wave Craft articles Wireless World articles Google Search of RF Cafe website Sitemap Electronics Equations Mathematics Equations Equations physics Manufacturers & distributors Engineer Jobs LinkedIn Crosswords Engineering Humor Kirt's Cogitations RF Engineering Quizzes Notable Quotes App Notes Calculators Education Engineering magazine articles Engineering software Engineering smorgasbord RF Cafe Archives RF Cascade Workbook 2018 RF Symbols for Visio - Word Advertising RF Cafe Homepage Thank you for visiting RF Cafe!
Anatech Electronics (RF Filters) - RF Cafe

Power Supply Question - RF Cafe Forums

RF Cafe Forums closed its virtual doors in 2012 mainly due to other social media platforms dominating public commenting venues. RF Cafe Forums began sometime around August of 2003 and was quite well-attended for many years. By 2010, Facebook and Twitter were overwhelmingly dominating online personal interaction, and RF Cafe Forums activity dropped off precipitously. If the folks at phpBB would release a version with integrated sign-in from the major social media platforms, I would resurrect the RF Cafe Forums, but until then it is probably not worth the effort. Regardless, there are still lots of great posts in the archive that ware worth looking at.

Below are the old forum threads, including responses to the original posts.

-- Amateur Radio
-- Anecdotes, Gripes & Humor
-- Antennas
-- CAE, CAD, & Software
-- Circuits & Components
-- Employment & Interviews
-- Miscellany
-- Swap Shop
-- Systems
-- Test & Measurement
-- Webmaster

ice745
Post subject: Power Supply Question
Unread postPosted: Sun Jul 17, 2005 4:22 pm
Offline
Lieutenant

Joined: Fri Apr 22, 2005 12:22 am
Posts: 3
I am trying to build a power supply for a talking clock my father made in the 80's (unfortunately he passed so I can't ask him). On the clock itself is a rectifier followed by two 7805CT 5 volt regulators then large capacitors. I just need to know what rating transformer both Volts and Amps I should use to power the clock. 12 volts 9/10 amp works but excessively heats the regulators; and 6.3 volts 2 amps works but only for a few hours before the clock rapidly turns on and off and has excessive static from the speaker. I hope someone can tell from just the rectifier and regulators what transformer would be best. And should I put a fuse on the 115 volt side or the lower voltage side of the transformer, and what rating?

Thanks for any help. If you need more information, please ask and I will try my best.


Top
Profile

IR
Post subject:
Unread postPosted: Sun Jul 17, 2005 11:40 pm
Offline
Site Admin
User avatar

Joined: Mon Jun 27, 2005 2:02 pm
Posts: 373
Location: Germany
Hi ice745,

You should add a high value capacitor between the rectifier and the voltage regulators. The value of capacitor should be of few hundreds uF (For example 220uF, 470uF, you can connect in parallel couple of these values) and they should have a working voltage higher than (At least 1.5 times) the voltage obtained at the output of the rectifier (Vmax=1.414*output voltage from rectifier). The capacitor keeps the voltage to the regulators' input with low ripple, and doesn't allow it to swing from 0 to Vmax.

I hope that you use full-wave rectifier. The rectifier itself should be able to stand the total current that your clock consumes.

If your regulators are 7805CT, then the voltage at their input should be at least 3V higher than the output voltage i.e. 8V. A 9V transformer is the choice (of standard voltages available). The current at the secondary of the transformer should be higher (1.5 times at least) than the current your clock consumes. You should check the current consumption of your clock, you mentioned that the regulators are heating. Maybe you can add another regulator to ease, or attach a heat sink to each regulator (If they are TO-220 package, then they have a tab with a hole exactly for that). If not, then attach them to a metal plate and glue them with a
heat-conductive paste to the plate.

_________________
Best regards,

- IR


Top
Profile

ice745
Post subject:
Unread postPosted: Mon Jul 18, 2005 2:01 pm
Offline
Lieutenant

Joined: Fri Apr 22, 2005 12:22 am
Posts: 3
Thanks. I am not sure how to check the current consumption of the clock. Would I just try with my current power supply and use an ammeter on the secondary side of the transformer?

It is a fullwave rectifier.

The volt. regs. only heat up if I use about 12 volts, when I tried 6.3 they would stay really cool. And both of them already have heat sinks. I may try a 9 volt transformer and see how that works.

Thanks for the help :)


Top
Profile

IR
Post subject:
Unread postPosted: Mon Jul 18, 2005 2:05 pm
Offline
Site Admin
User avatar

Joined: Mon Jun 27, 2005 2:02 pm
Posts: 373
Location: Germany
Hello,

You should connect an Amper-meter in series at the output of the regulators and measure the current consumption in DC mode.

Good luck, please keep me posted!! :smt023

_________________
Best regards,

- IR



Posted  11/12/2012
TotalTemp Technologies (Thermal Platforms) - RF Cafe
PCB Directory (Assembly)
LOTUS Communications Systems Modular RF Component Building Blocks - RF Cafe
 

Please Support RF Cafe by purchasing my  ridiculously low−priced products, all of which I created.

These Are Available for Free