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PC PS to Run Mobile Rig?

Discussion in 'Homebrew and Kit Projects' started by KF7QLP, Feb 21, 2012.

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  1. KF7QLP

    KF7QLP Ham Member QRZ Page

    Bought myself a mobile rig (12V power.) The info with it says it can draw up 13A, xmit'ing at high power.

    In the car I can just tap its 12V system and in the garage I have a couple batteries and charger to power it.

    I'd like to also use it in my (home) office but don't want to bring the loose batteries in and the Explorer won't fit through the door. It occurred to me that PC power supplies have a 12V tap and the first one I pulled up on newegg, a 500W unit at about $40, claimed it was good for 18A on each of two 12V taps.

    I seem to remember that switchers, at least in the 'olden days' didn't like to fire up w/o some load. Is that still true of generic PC supplies, is the load included inside the ps case or has the design matured so that that's not true anymore (if it ever was.)

    How 'bout ripple at high load. What's a number I should be shooting for? External filtering?

    Any thoughts on using a PC supply to power a 12V rig?

    Benton 21feb12
  2. KC9AXZ

    KC9AXZ Ham Member QRZ Page

    This MAY work. You can put a load on the 5 volt side of the power supply and get 12VDC out the other side. The problem is the computer power supply may only output near 12VDC. You may not get enough voltage to run the radio properly. You might be able to make a few adjustments to the supply to increase the supply output to 13.8VDC.

    In my opinion if you are doing this just to experiment have at it. I personally wouldn't buy a new computer supply to do this job. I'd try to find a scrapped one to experiment with. If your willing to double the cost for a new PC supply you could easily find a suitable dedicated switcher that will run your rig. If you are looking to run your rig on the cheap, you should be able to find at a hamfest a older linear supply in the same price range.

    Jon KC9AXZ
  3. W9GB

    W9GB Ham Member QRZ Page

    Previous QRZ Q&A question on this same topic

    Several GOOD Internet articles -- DIY How To Modify Switch-Mode Power Supplies (SMPS).
    GOOGLE St. Louis Switcher by Matt Kastigar, N0XEU

    NOISE from a Poorly Filtered SMPS (cheap units) is a known issue, expecially for the HF (160-10 meter) users.

    Plenty of quality USED or Require Repair -- 13.8 VDC 20-35 Amp Linear PS on the market (eBay, hamfests, land mobile dealers)
    FOR $20 you can buy the Power Supply Kit from SparkFun Electornics
    This was designed for breadboard experimenters .. so 12 V heavy current (> 8 amp) should be tapped from drive connectors (CD, hard).

    PLUG-In you 20 or 24-pin ATX Power Supply -- and have binding posts for each voltage (3.3, 5. 12. -12) from the power supply.
    Case or enclosure is optional.
    Last edited: Feb 22, 2012
  4. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    I wouldn't buy a new PC power supply for this, but if you already have one (people throw away "tower" PCs by the thousands, every day, and probably most of them have a good power supply) it's worth a try.

    They never have a load in them.

    Some will deliver 12V without loads on the other outputs, some won't. You can try it and see. They're very low in "ripple," and most of them don't generate much switching noise that you would hear on VHF (might hear it on HF, though). Some have an output voltage adjustment that might crank up to 12.6V or even a bit higher -- and some don't. It would be preferable to crank it up if possible.

    Although "12V mobile rigs" can function on 12V, many won't meet full specifications like TX output power at such a low voltage and have that specified for 13.8Vdc only. No PC power supply I've ever seen will crank up that high without circuit changes.

    I think driving the Explorer inside is a great idea. If you ram the house at the right angle, you can probably just make a new door without damaging the Explorer much at all.
  5. W9GB

    W9GB Ham Member QRZ Page

    IF you are a SMPS Bargain Hunter ....go to eBay and Search on: MeanWell
    You will have hundreds of different models shown, look for the 13.8 VDC models at leat 15 Amps.

    IF you get for less than $50 USD, then you bought a bargain !
    You can also search on Astron, pleanty of units (used and new) on eBay each week.

    Last edited: Feb 22, 2012
  6. KD0AUO

    KD0AUO Ham Member QRZ Page

    I recently converted a PC power supply (350W) to a bench supply. Had to put a small load on the 5v line. Added a bunch of binding posts for -5, -12, 3.3, 5 and 12v.

    One thing I've noticed, it's not all that good at regulation. It's starts out about 12.34 and just loading it lightly with a small PC fan it dropped to 12.27. I'd don't know if it will stop loading down there or not as I don't have a bigger load to test it with.

  7. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    A car headlamp is a pretty good and very cheap 12V 5A load. Two in parallel are 12V 10A. A fan might only be 100mA or so, not much of a load.
  8. KF7QLP

    KF7QLP Ham Member QRZ Page

    Thanks for that link. I did look for the forum search tools before I posted, but that button was just a little to subtle for my hurry.
    I also found after posting here.
    I'm running a 2M/440 radio. One of the articles I read on DxZone, I think, included strategies for taming the conducted noise, if it proves a problem.

    Sure, no, I wasn't going to go buy new -- just tossed that out as a price point, after going to Newegg to check specs on PC supplies.

    Thanks. Also saw varying opinions for how stiff a load they need.

    The St. Louis Switcher article suggested finding a supply with enough info to ID the regulator, digesting its voltage-divider resister equation, and changing to boost the 12V (and 5V) supply.

    To the extent it's meant to run in a car, I expect it doesn't need to be well regulated, just as long as it doesn't sag too much under high load.

    Thanks everyone for all the pointers. One of the articles on DxZone suggested pulling half the components off the circuit board, and rewinding a toroid to adjust the voltage. That sounded a little more involved than I was hoping for. Tweaking up a voltage divider, I can do.

    Benton 22feb12
  9. KD0AUO

    KD0AUO Ham Member QRZ Page

    Didn't think of that. I think I have an old one in the garage somewhere. Thanks!
  10. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    Bear in mind that's an old article and unless you find a 10 year-old PC power supply, almost everything contained there is now irrelevant because PC SMPS power supply standards and designs have changed -- a lot.

    Automotive systems are very well regulated, actually. Typically 13.8-14.0V running, and 12.0V not running. Only time the voltage would sag below 12V is when cranking, or if you're running accessories a long time without the engine and alternator. Performance of rigs at 12.0V varies a lot with the rig. I have some that are perfect at that voltage, and some that will barely transmit at that voltage.
  11. WA9SVD

    WA9SVD Ham Member QRZ Page

    Things haven't really changed. Typical computer switching supplies still need a load on the 5 V and often the 3.3 Volt bus to startup, and a connection to simulate the "power up" signal from a motherboard to start as well. Many of the newer supplies actually have lower 12 Volt capability than in the past.
    The problems are multiple:
    1. The "12 Volt" rail is usually +/- 10%, which is sufficient for hard drives and DC/DVD Drives in a computer, but many Amateur Radios will not work well, (some won't work at all) if the voltage falls below 12 Volts. They ARE actually designed for 13.8 Volts, +/- 10%, since 13.8V is the typical automobile system voltage when the engine is running.
    2. The (RF) noise generated by such supplies can be horrendous; most of the supplies do NOT have any RF suppression or power-line filtering. (Open one up. You will see where the filter components are SUPPOSED to be mounted, but those spots are only provided when the unit is first submitted for RFI testing; once in production, the filter components are "conveniently" eliminated.

    YMMV. You MAY be able to use such a supply, or you may be bitterly disappointed. And different brand supplies can vary widely; even different models from the same brand name may have widely different performance.
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