I'd like to build my own DC power filter - help, please?

Discussion in 'Homebrew and Kit Projects' started by KV4JW, Oct 11, 2018.

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  1. KV4JW

    KV4JW Ham Member QRZ Page

    I have a keen interest in keeping clean DC power flowing to my equipment at all times. As obvious, we can't always trust power supplies to be clean, and even in a direct to battery mobile situation, noise could possibly still be injected into power leads.

    I see a lot of commercialized "DC power conditioners" and "DC filters" that I can buy - but I'm cheap, handy with a soldering pencil, and enjoy making simple devices that benefit me in some form. I'd like something simple. Perhaps a chain of capacitors contained within a small project box that could be inserted in line with power leads? Could it be that simple? Or will I need to wind some chokes and install as well?

    If anyone can point me to some online plans, that'd be awesome too. Thanks!
  2. KV4JW

    KV4JW Ham Member QRZ Page

    Something along the lines of this, perhaps?


    Attached Files:

  3. W9GB

    W9GB Ham Member QRZ Page

    DC Power Conditioning can be “A Cat Chasing it’s Tail”, especially for Radio Amateurs and Audiophiles. Both of these hobbyist groups spend thousands of $$ on latest technology (radio, amplifiers) ... THEN become stupid with their DC Power Source choices. FAILED LOGIC.

    For CLEAN DC Power — Address EMI, RFI, or Bad Filtering Problems at the Power Source (Cure the Problem, or Sell/Dump the voltage source).
    DC Power Problems, that I have observed over past 40 years, were Operator Inflicted (Pilot Error).
    The two (2) most common:

    Many Radios DO NOT have Reverse Voltage Protection
    Loose DC wiring (bare wire hookups) in the Radio Shack is “Bush League” ...
    and based on Q&A Tech Forums (damaged radios), it continues with many radio amateurs.

    Standardize 12/13.8 VDC wiring in Radio Shack and Mobile Ops on a Plug-N-Play Standard.
    ARES/RACES chose Anderson Powerpoles (APP is now an Ideal Company, Sycamore, IL)
    http://www.arrl.org/files/file/Public Service/TrainingModules/Technical/Anderson powerpole.pdf

    Almost twenty years ago Mike Bryce, WB8VGE noticed that many 13.8 VDC power supplies
    (Heathkit HP-1144, Kenwood/Yaesu/Icom DC power supplies until recently) had NO Over Voltage Protection (OVP). DC power supply failure — damages radio ($$).

    The Protector (June 2003, QST magazine) was designed to address those DC power supplies,
    with DIY retro-fits to resolve the omission.

    West Mountain Radio developed an “off-shelf” commercial solution.
    Last edited: Oct 11, 2018
    KB0MNM likes this.
  4. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    What kind of equipment?

    If it's intended for mobile operation and noise conducted by the DC power leads impacts performance or operation in any way, that's not a good mobile equipment design.

    Linear regulated power supplies have no "noise" on the DC output bus, except a miniscule amount of 120 Hz ripple. And that's a really small amount, usually in the millivolt range under full load. Switchmode power supplies can conduct higher frequency noise by its output leads and can be more effectively (further) filtered with outboard systems but for ham gear I've never seen a case where that would improve anything. With sensitive audio gear (mike preamps, mixers, phono preamps, etc) I could see where it might.
    WA7PRC and KV4JW like this.
  5. VK4HAT

    VK4HAT XML Subscriber QRZ Page

    They all have ripple and ripple is noise. Its just the amount of ripple on the DC that varies. How much is acceptable depends on the application circuit that is attached to it.

    What you do depends on the nature of the power noise. Just because there might be noise does not mean that there is noise or that it is even going to be a problem.

    Start by measuring the noise/ripple on the DC and work from there. Its all frequency dependent, just tossing a filter at it is pointless if the cutoff of the filter does not adequately attenuate the ripple on the DC.
    Last edited: Oct 12, 2018
  6. WA7ARK

    WA7ARK Ham Member QRZ Page

    A lot of modern equipment is very tolerant of power supply variations. Can you identify a "real" problem that you are actually having, or is this just a "feel good" exercise?
  7. KA9JLM

    KA9JLM Ham Member QRZ Page

    Don't forget the fuse(s).

  8. KE8FWJ

    KE8FWJ Ham Member QRZ Page

    If you configure your linear regulation circuit like a gain amplifier configuration you can self determine the ripple and noise to what you want amplified. That way you can rid your circuit of those ungainly expensive capacitors used to smooth out the ripple on the supply line.

    Once you dabble with this euphoria you will never go back :) You may even start introducing regulated dual rail linear regulated PSUs to your audio amplifiers for enhanced listening heaven.

    Have fun, have fun, have fun.
  9. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    They all have "some" ripple at some load current, but it's generally all 120 Hz and you'd have to look really hard to find "noise" (I define "noise" as something not line-power related and generally not at one distinct frequency), and is so attenuated that you'd need to look hard to find it.

    The switchmode designs are more likely to have real "noise" content at the switch frequency and its harmonics; the power lines can act as antennas to help radiate this where you'd rather not have it radiated, so it's best to filter it right at the power supply (source) and not somewhere down the line.
  10. KV6O

    KV6O Ham Member QRZ Page

    Without knowing what you're trying to do, and with what power source, it's hard to address.

    If you're running on batteries, for example, basic power protection is probably what you need - fusing, undervoltage cutout, reverse polarity protection.

    If you're using a linear supply, the caps provide the conditioning, you still need fusing, reverse polarity protection, and now you might want to thru in over-voltage protection (like a crowbar circuit) as linear's work by bringing down the voltage from some higher voltage.

    If you're using a switcher, then you might want some actual filtering to supplement the filtering in the supply if it's not adequate.

    There really isn't a one size fits all answer...

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