repurposing a power supply

Discussion in 'Homebrew and Kit Projects' started by KJ7VAB, Aug 23, 2021.

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  1. KJ7VAB

    KJ7VAB Ham Member QRZ Page

    First off, I'm looking to power my 818nd. When I plug in the supplied wall wart, I get a lot of noise on 80 meter.
    So I'm looking for a quiet alternative and thought I'd run through some of the useless junk I have laying around before buying anything.

    I have several old laptop power supplies. If I use some kind of voltage regulator to get the voltage to 12-13, will it still be noisy?

    I also have an old unused ATX power supply. Was going to connect everything up and see if it's noisy, but the 12 volt out was fluctuating between 9.8 and 10.5 (maybe it needs a dummy load on the 5v?), so I chickened out.

    Are either of these worth pursuing if the goal is quiet power?
     
  2. AA4PB

    AA4PB Ham Member QRZ Page

    Laptop power supplies are likely to be switching types which create RF noise. I'd recommend a linear type regulated supply if you want to avoid RF noise. Be sure to get one that has a continuous duty amp rating at least as high as you radio draws on transmit. Astron has a variety of linear supplies.
     
    KJ7VAB likes this.
  3. KX4OM

    KX4OM Ham Member QRZ Page

    I've repurposed an old ATX PC power supply. The load that I put on the +5 volt line is a cement 10 ohm 10 W resistor. I modified the supply for 13.6 output by changing the value of a couple of resistors in the 2.5v feedback circuit for the TL494 IC. Their are MANY articles on the web for doing that mod. Instructables.com used to have a list of articles on converting PC supplies kept by a user. I used mine on an 2m FM transceiver. To make a PC supply quieter, I suggest reading Manfred Mornhinweg's articles on his homebrew 40A switching supply that appeared in QST Dec 1998 and Jan 1999 and in ARRL Handbooks after that.

    I've also built a 13.8 VDC low dropout linear regulator board for several good and quiet laptop 16VDC and 19VDC 3.4 Amp switching supplies made by IBM years ago and Dell more recently. Adding the linear regulator also reduces the switching noise by over 10dB. The regulator is an LD1085 rated for 3.0 A output. It is in a small aluminum box with a barrel connector for the DC in and banana connectors out. I use the unit as one of my bench supplies. Using a switcher as a pre-regulator with a linear power supply is a common practice in industry.

    As a nod to Bob, AA4PB I built his automatic sealed lead acid battery charger several years ago. The article is from the May 2001 QST. One of my SLAs, which was a pull from a UPS, is still running after 15 years.

    Ted, KX4OM
     
    Last edited: Aug 23, 2021
    US7IGN and KJ7VAB like this.
  4. KJ7VAB

    KJ7VAB Ham Member QRZ Page

    Would adding a load stabilize the voltage fluctuation I'm seeing? Or is the power supply bad possibly?

    I'm really new at any kind of electronics at all.

    I have an older APC UPS with a dead battery, hmmm.....
     
  5. KX4OM

    KX4OM Ham Member QRZ Page

    PC supplies need a load to start. I have not even tried to turn on one without doing the modifications first.

    This is some old information, but then I had old power supplies to modify:

    https://www.informit.com/articles/article.aspx?p=30273&seqNum=6

    I use old discarded PC supply steel cases for a lot of my projects. A signal generator, a log power meter as part of my scalar network analyzer setup, and the aforementioned SLA lead acid battery charger to name a few.

    https://www.qsl.net/kx4om/

    Ted, KX4OM
     
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  6. VK2TIL

    VK2TIL Ham Member QRZ Page

    There are several options for this application.

    A linear supply is certain to provide "clean" power; a switching supply may or may not and this is usually discovered the hard way.

    A simple "CB" supply would do; there must be hundreds of these around. I'm in VK so can't help with places to look for these but people in the US may have suggestions; Craigslist, for instance.

    It was once said that every beginning amateur should build a power supply; there's not much in them, really.

    I built a "plug pack" to provide clean power to my N2PK VNA;

    Outside.jpg


    Inside 1.jpg

    You will require 3A capacity so an LM338 would be a good choice; Google will find the TI data sheet which contains the very simple schematic.

    Finding a suitable transformer may be difficult these days; that brings us to the laptop power supply.

    I built a little variable supply;

    2.JPG

    to use a Dell laptop supply; 17v @ 3.3A as I remember. The output of the supply looks like this;

    Output Noise Dell Power Supply.JPG

    but is considerably reduced after its passage through the LM338 regulator circuit;

    Output Noise 12v 0.5A.JPG

    Some things to think about.
     
    KX4OM, US7IGN, G4COE and 1 other person like this.
  7. WA9SVD

    WA9SVD Ham Member QRZ Page

    The problem with many switching supplies (especially those not specifically for amateur use) is they usually are NOT "filtered," and can cause tremendous interference:(, especially on HF radios. (This also applies to all but the best "computer" supplies.) Of course, some are better (or far worse:mad:) than others. If you can filter a laptop supply, perhaps with ferrites, you will probably still need to regulate the voltage to around 13.8 Volts +/-, since many laptop supplies put out more than 14 Volts; they let the computer "finish" the filtering and final regulation/battery charging. An LM338(K) WILL regulate up to 5 Amperes, but WILL need a good sized heatsink. Adding additional (large) capacitors on the input to a regulator MAY help in reducing the final voltage stability of a supply.
    If you choose to try a "computer" supply such as an ATX design, then you DO normally need a load on the 5 Volt section (usually @ 1 Ampere or so) to get proper regulation. One problem, though, is the output (unmodified) will only be 12 Volts (+/-) NOT the "normal" 13.8 Volts the radio really wants, and if it dips below 12 Volts under load, you may experience increased transmit audio distortion, or the radio may simply shut down. (Some radios ARE quite "finnicky" about power, and do NOT like anything at all below 12.00 Volts.) Your best choice might be to find a linear supply with 3-5 Amperes capability; if it has at least 16 Volts, you can easily add a linear regulator (such as an LM338K) for 13.8 Volts if it isn't regulated. Good luck, whatever you choose.
     
  8. SM0GLD

    SM0GLD Ham Member QRZ Page

    Newer ATX12V standard do not need any load on the +5V
     
  9. W9GB

    W9GB Ham Member QRZ Page

    Tim,
    That is the problem for Many, that enter Amateur Radio (A Technical Hobby).
    A Terrible Disease, promoted in 1980s, Leading to “Leave No Child Behind” …
    and Off shoring of those employment opportunities.

    This hobby is Expensive (Appliance Ops), AND the Best Method to Reduce $$ Spent …
    is to LEARN “MAKER SKILLS” to DIY Build (Vocational Arts at my High School in 1970s)

    BTW, Virtually Every Test Instrument on my Electronics Bench was Broken (and I fixed them),
    SAVING me THOUSANDS of DOLLARS !!
    FIRST, You need a WORKING ATX Power Supply.

    SECOND, Did you Know their are Published Standards and Design Guides?

    Desktop Platform Form Factors Power Supply
    Design Guide (ATX type SMPS), 67 pages
    Intel Corporation
    June 2018, Revision 2
    https://www.intel.com/content/dam/w...nts/guides/power-supply-design-guide-june.pdf

    The St. Louis Switcher
    by Matt Kastigar, N0XEU
    ARRL QST magazine, May 2002
    https://www.arrl.org/files/file/Technology/tis/info/pdf/0205035.pdf
    835F54D8-D0BB-456B-86B7-F80B059EF40B.jpeg
    SparkFun Benchtop Power Board Kit $22.95
    https://www.sparkfun.com/products/12867
    304C572A-6287-4593-B7B9-A3954AD48D29.jpeg
    SparkFun ATX Power Breakout Boards
    https://www.sparkfun.com/products/15035
    2DB38C5B-D5A6-4DA6-861B-978D37D62DE4.jpeg
    Breakout Hookup Guide
    https://learn.sparkfun.com/tutorials/atx-power-connector-4-pin-breakout-hookup-guide

    Switchmode Power Supply Handbook (343 pages), circa 1989.
    by Keith Billings
    https://xdevs.com/doc/_Books/Switchmode Power Supply Handbook.pdf
     
    Last edited: Aug 24, 2021
  10. G4COE

    G4COE Ham Member QRZ Page

    If you want a noise free power supply use a linear one, that is one that usews a mains transformer, stay clear of 'switchers'.

    Lol, I have a BT DECT cordless phone's and one of the wallwart chargers kept crashing my computer and sending the microwave oven clock 'bananas' - I discovered why when I opened it up, I blamed anything but the darn charger-:

    Mains ac in, to a fuse then to a bridge rectifier, out it came to a 4.7uF 500V electrolytic - yes 500V I had to look twice, then to a very small 220uH axial choke I think the value was, it then went to a 2.2uF 500V electrolytic..... one leg of the 4.7uF was badly soldered that had cracked all round the leg, the base unit wallwart used a transformer which I found odd!

    I've seen many a wallwart switchers lacking any 'proper' RFI input protection, the switxching spikes must have been getting back down the mains.

    If using a transformer power supply if there's no AC rated capacitor across either it's primary or secondary fit one.... around 220 - 470 nF will cut all the trash out that comes down the power lines, doesn't matter which side but do make sure its AC rated with a safety class emblem.

    Dave
     

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