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ATX Power Supply for Ham Radio?

Discussion in 'Homebrew and Kit Projects' started by NC4JB, Apr 30, 2010.

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  1. NC4JB

    NC4JB Ham Member QRZ Page

    I'm familiar with pages like this one that describe how to take an ATX power supply and turn it into an experimenter's bench supply with several voltages and other nice features. I'm curious though to see if it would be feasible to not modify the power supply in any way, and simply wire a molex connector to the power cables on my mobile rig and plug it right into one of the 12V connectors. Would this provide clean enough power, with enough voltage and amperage to run the rig? The power supply I had in mind is one I pulled form a P4 Dell Dimension 4400, and is rated for 14A @12VDC, and the radio I need to power is a 10W/45W 2-meter mobile rig. The specs for the radio say it needs 13.8VDC (+/-15%) and 10amps continuous, so I think this PSU will work, but I'm definitely interested in second opinions before I do something stupid and fry an expensive radio.
  2. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    The DC power from an ATX power supply is clean and regulated.

    The only problem you might have is if the power supply requires a minimum load on the 5v bus in order to assure the 12v bus regulates. If you can, check the complete specification for the power supply. If you can't, you might "bench test" this first to make sure it works.

    Another small drawback is usually the +12v really is +12v and may nor may not adjust to anything above that. Most of our rigs will "work" at 12v (since they can be battery powered) but many won't run full power with such a low supply voltage and are really optimized for about 13.8Vdc, which is why when you buy a "factory adjusted" 12v power supply for ham radio work, it usually comes "out of the box" adjusted for 13.8Vdc (and not 12v).

    One great advantage to the modern ATX and ATX12V etc. PC power supplies is they are almost all "universal input" and work fine with any AC line voltage from 90 to 250V, at 50 or 60 Hz (and often well beyond) so you can travel around the world and only change the wall plug without having to change the power supply.
  3. KI6ZIF

    KI6ZIF Ham Member QRZ Page

    I will also add to this. Depending upon the ATX power supply itself as well, some of them actually need a load (I use a 10w 10ohm resistor) on the 5v and 12v circuits. That is, so the power supply see's a load, and retains "latch".

    On SOME of the power supplies, they may even need one on each "rail". As some power supplies actually have 2 12v circuits. Even more power supplies have 2 5v circuits now, and some are 2+ 12v circuits. This drasticly complicates repurposing the power supply for other then "PC" use.

    The good news is, most 400w power supplies can output 10+ amps @ 12v, and maintain excellent voltage regulation.
  4. 2E0GOM

    2E0GOM Ham Member QRZ Page

    I actually use this configuration....

    although as the 12v rail is slightly above 12v and the radio likes 13.8, I actually use a 12 deep cycle battery as a float load so that the radio gets a nice steady flow...

    i'm using a 350w power supply with a total of 17 amps for the 12v rails, now bear in mind that you may need to tie some wires together to get the full current output and that you need to b careful which wires you tie togther if it has more than one physical 12v rail.

    the way mine worked out is that all the hdd wires are on one rail and th motherboard connector has the other rail, so I just tied all the 12v leads on the atx connector (along with their associated grounds) and the ones for the hdd i tied together too with their associated grounds.

    as far as the floppy drive 12v wires go, they are used for low current applications like my tnc and so forth.

    at the moment the system which comprises of the following (to clarify)

    350W psu -> 12v deep cycle battery -> 1 IC-V8000 50W, 1 IC-2200H 50W, and an IC745 running at 100W

    now i'v not tried all of them txing at the same time for obvious reasons.... but the 2200H and HF rig will happily sit there and tx at full power at the same time w/o any problems....

    there is only one caveat you MUST take care of.... you need an automotive power diode in the positive lead if you do it the same way I have, that's because if the power goes to the psu, the battery will backfeed 12v into the power supply.

    oh... and dont forget to ground the green wire :)

  5. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    Floppy drives use +5V only, no +12V.

    Normally this shouldn't be necessary; I'd look at the circuit, but usually there's nothing in the output stage of a PC power supply that could conduct reverse current. The drawback to using a diode here is its 0.6-0.8V drop, which will reduce the output voltage even lower than it normally is, which is routinely "too low" for a lot of ham gear to begin with.

    It takes about 13V to charge a 12V battery typically, and if you reduce the voltage to anything as low as 12V or lower, the charging current can be very small (possibly zero) so you're only charging with a "trickle" and not a rapid charge. I'd monitor the charge current and assure it's always a positive number that is equal to the actual load current to prevent depleting the battery.
  6. BG5JQ

    BG5JQ Ham Member QRZ Page

    I have been to do it.It's a good idea.
  7. 2E0GOM

    2E0GOM Ham Member QRZ Page

    yes but there's still a 12v line on the connector - at least on th one I have

    I agree, although YMMV somewhat, I dont use one myself and when the commercial power goes, the battery powers the fan lol. why i havent worked out yet, but it's very rare for our power to go out anyways (gen backup).

    as far as "charging" yes it keeps it at a nice float charge. it's been sitting like that for 6+ months now and works nicely..

    like i say... YMMV

  8. NC4JB

    NC4JB Ham Member QRZ Page

    Well in case anyone else wants to give this a try, I got it working just fine with no modification at all. I'm using a 250W PSU from a Dell Dimension 4400. I shorted the green wire on the ATX connector to a ground wire on same connector, spliced in a spare 2xMolex->PCIE connector I butchered for the purpose, and wired that to the DC cord that came in the mail today along with the mag mount 144Mhz antenna. I have not (a cannot currently) transmitted with it, so I'm not sure if it would handle that load, but just as a receiver everything works fine.
  9. KI6ZIF

    KI6ZIF Ham Member QRZ Page

    I can guarantee you it will not handle the load. The math IIRC, says that peak the power supply should deliver 20.3 amp's. However you need to calculate efficiency into that as well.

    Honestly, I would say that "safely" and build wise, I doubt it would sustain 10amps very well at all.
  10. AB8RO

    AB8RO Ham Member QRZ Page

    A 250 watt computer power supply is fairly small particularly if it is of the older variety that doesn't expect the CPU to be running from 12v.

    This replacement claims 9 - 13 Amps max

    One reasonable way to use it, of course, is just to run the radio on its lowest power setting.
  11. 2E0GOM

    2E0GOM Ham Member QRZ Page

    that's the whole reason why i have a battery sitting there as a floater...

    i can run my hf rig at full power (100W) - ssb or CW and also have the packet node up running and transmitting at the same time as the hf rig....

    works fine no dimming of lights....

    i'm sure that the battery takes most of the grunt, but the 17 amps that the psu can supply will probably be doing most of the work.

    I know that the psu on it's own can handle a 50w 2m rig without the battery, but i like it there as it's automatic battery backup heh

  12. N0WUE

    N0WUE Ham Member QRZ Page

    how do you keep from overcharging the battery when it idles for too long?

    Do you shut off the power supply when radios aren't in use, or do you pick a power supply capable of just enough current to drive the radios in receive with some extra to spare?

    I tend to be overparanoid of exploding batteries... long story
  13. K7JEM

    K7JEM Ham Member QRZ Page

    Lead acid batteries can be floated from a supply for years or decades, as long as the voltage is correct. Typical float voltage can be anything from 13.2v to 14+ volts, and still be OK. Typically, the voltage will be optimized for the particular battery being used, and the ambient temperature, but that is not usually an issue.

  14. 2E0GOM

    2E0GOM Ham Member QRZ Page

    as I always have the radio on (it runs the local wl2k node) and also I have a 400W inverter running the 1u server via 12 v (until i can get a 12vdc psu for that)

    there's always a load on it and it seems to be running happy enuff....

    as far as charging, i just keeps floating, and as they're a gel cell it's not too bad and tends to seem to handle itself.

  15. KI6ZIF

    KI6ZIF Ham Member QRZ Page

    You know, since most 12v computer power supplies are VERY close to 12v (not 13.8v). And the voltage of a "low" battery is near 12.1v I wonder if a 12v PC power supply is capable of overcharging a Lead Acid 12v battery anyways.
  16. WA7PRC

    WA7PRC Ham Member QRZ Page

    My PS is three Group-27 batteries in parallel, float-charged by my homebrew 30A linear supply (from this Handbook article). The PS is set to 13.8V. It's been working fine for three years. That equipment is in the garage, with 1/0 welding cable (my auto jumper cables) feeding the radio gear. I have a 50A Maxi-Fuse (datasheet) at the battery connection to "prevent my day from being ruined". ;)

    I'm ready for Field Day (Class E), by just turning the float charger off! :D

    Bryan WA7PRC
  17. K7JEM

    K7JEM Ham Member QRZ Page

    No. And it's not capable of charging one, either. At 12v, a lead acid battery is near discharged. It is not a good idea to float a battery so low.

  18. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    You can't charge a 12V battery with a 12V source: No charge current can flow.
  19. KI6ZIF

    KI6ZIF Ham Member QRZ Page

    Precisely as I thought.
  20. ZL3GSL

    ZL3GSL Ham Member QRZ Page

    The best approach is to change the supply to produce 13.8V. :D

    All the supplies I've looked at get the 12V from a few extra turns on the 5V windings, so since you won't be using lots of amps at 5V, a 230W supply can provide lots of amps at "12V". I've converted a 230W (AT) supply which gives 20A at 13.8V quite happily. Its nameplate rating is 12V 12A. The main limit might be the rating of the diodes in the 12V circuit. The board layout will let you replace the 12V rectifier with the larger 5V one.

    Often an ATX supply has a preset pot in the voltage divider which feeds the regulator chip. That may not have enough range, but it's worth a try first.

    If it doesn't give enough, have a look at the PC board and identify the regulator chip. Often it's a TL494 or equivalent. If so, check the reference voltage on pin 2 (it will usually be 2.5 or 5V) . That's the voltage required to appear on pin 1 when the 5V (and often the 12V, too) are correct. I disconnect any feed from 5V and feed a voltage divider from the (now 13.8) "12V" output. If the reference voltage is 5V, pin 2 will normally have a high value resistor to ground, rather than a 2k-5k resistor as the low side of a voltage divider. If you put a preset in the divider, put it in the low side so that if it fails open circuit it will not cause the output toi rise and test the over-voltage protection in the supply. (The overvoltage should operate at 14V).

    Because the comparator takes some current, a calculated divider won't give the exact voltage. A quick way to adjust the divider is to put 13.8V from an external supply into the output of the (non-powered) supply being modified.

    Other regulator chips have different pinouts, so what I've said here will need interpretation to suit. A circuit diagram is a great help, but you can get the datasheet for the chip. ;)
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