June 12, 2012

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More Power! ... QRO Power Supplies

Armed with some basic principles and the right components, you can design just the right solution for your needs!


One of the things we usually take for granted is the good ol' DC power supply for the projects on our bench.  The near-ubiquitous "12-volt" supply will power most of our circuits and we usually don't even give this important function a second thought.  But what happens when you need a good deal of current, beyond what you can get from that handy-dandy AC adapter borrowed from laptop use?  Or suppose you are unwittingly powering a receiver with one of those new-fangled switching power adapters and you are getting some unusual hash in the spectrum that cannot be explained? 

So in this week's session we'll be delving into the wide-ranging world of electrical power conversion, and exploring the various types of supplies out there and what the preferred use is for them.  And in our usual tradition, we have a hands-on, DIY project that illustrates many of the principles of the discussion, and it's something that you can easily build with confidence of it doing the job for your specific needs.

73, George N2APB  & Joe N2CX

Audio Recording ... (Listen to the MP3 podcast)

Discussion Notes:

<20:24:35> "Ray K2ULR": Clack and Clack.... still on the air!!!
<20:24:55> "Ray K2ULR": Click & Clack... the Tappet brothers. :)
<20:26:44> "Armand WA1UQO": Sorry to say they retired this week. All shows from now on will be from their archives .
<20:28:09> "Pete - WB2QLL": New shows 'til they retire in September.
<20:28:52> "Armand WA1UQO": Great ! Thanks :)
<21:02:32> "Rick K3IND": So those lumps on the power supply cables are toroids to grab that noise?
<21:08:37> "George - N2APB": Cypress PSoC links ...
<21:08:40> "George - N2APB": • Training for PSoC and PSoC Creator (including an introduction to PSoC 3 and PSoC 5) can be found here: http://www.cypress.com/?rID=40547  and here http://www.cypress.com/?id=2232&rtID=134
• PSoC Insider - a blog with complete projects, videos, and more is here: http://www.cypress.com/?id=3503
• A description of the new features in Creator 2.0 can be found at this URL:http://www.cypress.com/?id=2232&rID=38406
• The book ""My First Five PSoC 3 Designs", available in printed form or PDF, is here:http://www.cypress.com/?rID=43123
• The Cypress Developer Community, featuring Forums, Videos, Blogs, and Training is here: http://www.cypress.com/?id=2203
• Need Creator on CD? See http://www.cypress.com/?rID=53608
<21:09:25> "Frank N3PUU": marlin p jones usually has some surplus xformers mpja.com
<21:17:22> "Frank N3PUU": its a shame you guys werent around when I was trying to build up a 100A supply.. made lots of smoke :) good presentation, thanks!
<21:17:56> "Mike - WA8BXN": forgive me, i have had to be in and out a bit tonight, has the "This weeks project = 13.6 v supply" been discussed yet?
<21:18:49> "Ray K2ULR": I think it's a switcher.
<21:18:59> "Pete - WB2QLL": super capacitor
<21:19:26> "Ray K2ULR": http://www.gammaresearch.net/hps-1a.html
<21:20:54> "Sid KC2EE": Gammaresearch.net HPS-1A supply
<21:22:21> "WA0ITP Terry": Thank You
<21:29:16> "Rick K3IND": 73

SESSION NOTES  ["Under Construction" up to showtime]

Wikipedia ...

A power supply is a device that supplies electric power to one or more electric loads. The term is most commonly applied to devices that convert one form of electrical energy to another, though it may also refer to devices that convert another form of energy (mechanical, chemical, solar) to electrical energy. A regulated power supply is one that controls the output voltage or current to a specific value; the controlled value is held nearly constant despite variations in either load current or the voltage supplied by the power supply's energy source.

Every power supply must obtain the energy it supplies to its load, as well as any energy it consumes while performing that task, from an energy source. Depending on its design, a power supply may obtain energy from:

A power supply may be implemented as a discrete, stand-alone device or as an integral device that is hardwired to its load. Examples of the latter case include the low voltage DC power supplies that are part of desktop computers and consumer electronics devices.

Commonly specified power supply attributes include:

  • The amount of voltage and current it can supply to its load.
  • How stable its output voltage or current is under varying line and load conditions.
  • How long it can supply energy without refueling or recharging (applies to power supplies that employ portable energy sources).


Linear, Programmable Power Supplies, from Hameg                Adjustable, Linear, 15A (MFJ)          Classic ham use 20a liner PS (Clegg)



 Open Chassis Linear (Sola)       13.6V fixed, 2A Switching PS (MFJ)


Linear versus switching supplies

You all know how a typical linear power supply operates: A heavy transformer takes the line voltage and converts it into something slightly above the desired final voltage. Some diodes rectify it, a big filter capacitor smoothes out the DC, and a series pass transistor burns up the excess voltage, so you get the desired output. A simple control circuit drives the pass transistor to hold the output voltage constant. The circuit is simple and uses few parts, but several of these parts are big, heavy, and expensive. And the efficiency is usually only around 50%, often even lower. That produces a lot of heat, which must be removed by big heat sinks and fans.

The switching approach is totally different: The line voltage is directly rectified and filtered, resulting in about 300 or 150V DC (300 is more commonly used). This feeds a power oscillator which produces output at about 20 to 500 kHz. This relatively high frequency allows the use of a small, lightweight and low cost transformer to reduce the voltage. The output is then rectified and filtered. And now comes the most interesting feature: Instead of just burning up the excess energy, in the switching power supply the control circuit steers the power oscillator in such a way that it delivers just the amount of energy needed. So, very little energy is wasted, resulting in high efficiency (75 to 90%), almost no heating, and a much reduced electricity bill!

[For a more in-depth discussion on Linear-vs-Switching Power supplies, see ...  http://power-topics.blogspot.com/2007/08/linear-vs-switch-mode-power-supplies.html]



And now, our presentation! ...


Converting electrical power to a more convenient form


         AC line voltage to DC to power rigs

         AC line voltage to operate your computer

         Convert battery power to AC

         Battery power from solar cell to steady 12V DC

         Battery Power to 5V to charge cell phone or USB device


Types of supplies


         Step down supply

     Transformer to drop 120V AC to about 12V AC

     Rectifier and filter to get DC from AC

     Usually a regulator to get constant output voltage

     Linear regulator

     Switching regulator


         Step up supply

     Transformer to increase 120V AC to several hundred V DC

     Rectifier and filter to get DC from AC

     With tube rigs also step down winding for filaments

     Often no voltage regulator


         DC to DC converter

     Oscillator to generate sine wave AC or square wave

     Transformer to step up or down voltage

     Rectifier and filter to convert AC back to DC

     May or may not have regulator


         DC inverter

     Oscillator to generate sine wave AC or square wave

     Transformer to convert to 120V AC


Power supply circuitry



         Linear regulator power supplies

     Simple circuitry

     Need heavy iron transformer

     Large filter capacitor

     Output has low noise and ripple

     Loses power in pass transistor


         Switching power supplies

     Often rectify and filter at AC line voltage

     Generate square wave AC at KHz to MHz

         Transformer for line isolation

         Much smaller, lighter than 60 Hz

     Rectify and filter DC out of xfmr

         Filtering much easier at higher freq.

     Sense output level and send sample back to regulator

         Regulator works at rectified line voltage

         Regulates by PWM on HF square wave

     More complex circuitry

         But IC's can make simple

     On-line design tools

         Needs special transformers and chokes

     Smaller and less expensive than 60 Hz

         May generate kHz or MHz noise

     Both conducted on DC output and radiated


Power supply physical format

         Familiar large metal chassis

         Wall warts

         Small pc cards



Linear Power Supply vs. Switching Power Supply vs. Unregulated Power Supply

  Switching Regulated
Power Supplies
Linear Regulated
Power Supplies
Power Supplies
Circuit Design complex moderately complex simple
Part Count
high medium low
Load Regulation
0.05% to 0.5% .005% to 0.2% +10% (1/2 load to No Load (NL))
-10% (1/2 load to Full Load (FL))
Line Regulation 0.05% to 0.2% 0.005% to 0.05% directly proportional to
AC input change
Ripple (RMS) 10 mv to 25 mv 0.25 mv to 1.5 mv 0.5 v to 5 v
300 microseconds
(1/2L to FL)
50-100 microseconds
(NL to FL)
Efficiency 70-85% 40-60% 90-95%
Hold-up Time 15-30 milliseconds 1-2 milliseconds  
EMI high very low very low
Leakage high low very low
(power density)
small size
(high power density)
large size
(low power density)
medium size
(medium power density)
(Power to Weight Ratio)
Power Factor 0.6 - 0.7 without PFC
>0.95 with PFC
0.6 - 0.7 0.6 - 0.7
Cooling convection or fan convection or fan convection
Isolation yes yes yes
Input Voltage
90 - 132 VAC
(without PFC)
180 - 264 VAC
(without PFC)
90 - 264 VAC
(with PFC)
105 - 125 VAC
210 - 250 VAC
0 - 125 VAC
0 - 250 VAC
Output directly
proportional to input
Values in chart above are for comparison purposes only.


THIS WEEK'S PROJECT:   13.6V, 5A Linear Power Supply 

IN-PROGRESS PROJECT:   Remote Controlled Antenna Switch  ... (Continued from previous week's session)




1. SGS Thomson Power Supply Design Basics ... Do a Google search for "Power Supply Design Basics"


2. On-line linear reg design  ... <http://www.changpuak.ch/electronics/power_supply_design.php>


3. TI WEBENCH power supply designer ... <http://www.ti.com/ww/en/analog/webench/power.shtml>


4. Linear Technology LTPower CAD software ... <http://www.linear.com/designtools/software/#LTPower>


5. Comprehensive listing of switchmode power supply design ... <http://www.smps.us/smpsdesign.html>


6. Linear vs. Switching Power Supplies ... http://power-topics.blogspot.com/2007/08/linear-vs-switch-mode-power-supplies.html


7, 13.8V 20A linear power supply ... http://ludens.cl/Electron/Ps20/Ps20.html


8. 0-28V, 6A linear power supply using the LM317 ... http://electronics-diy.com/power-supply-lm317-2n3055-0-28v-6-8a.php


9. 10A power supply using the LM317 ... http://www.phoenixcomputerlabs.com/All-About-Mixers/LM317.html

10.  Reverse engineering iPhone chargers ... Here's a couple of really neat blog posts, tearing down an Apple iPhone charger as well as a non-Apple iPhone charger.  Both of them are tiny; the Apple charger is beautifully engineered, while the non-Apple one is slightly terrifying.   73, Josh AJ9BM

11.  Adafruit ... Josh AJ9BM says: "I learned about these from the Adafruit blog, which is a pretty good source of interesting electronics (and other DIY) articles: http://www.adafruit.com/blog/."






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