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5A 7805 power "supply"?

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  • 5A 7805 power "supply"?

    I have a new PCISA-C800EVR-1G single board computer. Ive been debating myself as to how to power this fellow. It takes approx 4A at 5V to run. No 3.3v or -12v required.

    I have noticed that you all do not like running regulators for power (Ive searched here till my eyes go fuzzy, and i havent nearly seen half of this wealth of knowledge!). I have found regulators capable of 5A and was going to use those.

    Besides the obvious heat reasons, what are the downsides to this? I mean it seems almost too good to be true, i have a cheap regulator, a big heatsink (off a Athlon XP 2100+) and it seems to add up to easy power supply.

    I have another question about powering my SBC too (using the 5VSB and the PS-ON connector without a backplane) i think ill put in another post later.

    Thanks! and great place you have here!


    EDIT: heres the link to the datasheet of what i ordered:
    its the LM1084-5.0

  • #2
    If your comp. requires naught but 5V, and you're proficient enough to build a circuit to supply that, go for it.

    Most computers (that I'm aware of) require (clean) 12V, 5V, 3.3V, etc, and its easier for most of us to purchase a pre-built supply that'll give us all that.

    On a somewhat-related note, I'll shortly be building a 10A 12V regulator (using 7812s in parallel), to power a couple of external HDDs and other stuff. When/if you build this, post info - we always appreciate the sharing of knowledge!


    • #3
      I would be happy to post if it works! (Paitiently waits for the regulators to arrive in the mail)

      I was just hoping that there was someone with vaster (is that a word?) knowledge than I to confirm that all i need is that regulator and some capacitors as the Application notes show.

      PS- On a side note... does anyone know about the PS-ON line and 5VSB? My SBC uses the standard 4pin molex conector for power, but it also has a three pin header labled "PS-ON" that has GND, PS-ON, 5VSB pins. I tested that the 5VSB pin on my board OUTPUTS 5v and so i didnt want to INPUT 5v for standby voltage for fear of hurting something. I wasnt sure if that only worked if i attached my board to a backplane, or if im terribly missing something. I need a way to shutoff power since i wont be building a fancy ATX type supply. just 5/12v ON and 5/12v off. The manual doesnt go into enough detail for "fab your own software controlled powersupply"


      • #4
        Don't you mean that people here have a big aversion to "inverters", not regulators? Inverters convert DC power to AC power, that AC power is then run into an AC converter (the ubiquitous "power bricks" that come with nearly every electronic device you have) that converts it to the proper voltage DC power for the computer.

        As you can see, this can seem like an unnecessary step since your computer needs DC power. However, some rigs require more power than the DC power supplies/regulators like Opus, Carnetix, CarPC, M1A1 can provide. The inverter is necessary for them. Also, many people use them because they are cheap and easy to plug an AC power cord into.

        Please don't ask "which is better" unless you wish to start a holy war. Some people are very opinionated about using inverters vs. DC power supplies.
        Originally posted by ghettocruzer
        I was gung ho on building a PC [until] just recently. However, between my new phone having internet and GPS and all...and this kit...Im starting to have trouble justfiying it haha.
        Want to:
        -Find out about the new iBug iPad install?
        -Find out about carPC's in just 5 minutes? View the Car PC 101 video


        • #5
          Originally posted by Bugbyte
          However, some rigs require more power than the DC power supplies/regulators like Opus, Carnetix, CarPC, M1A1 can provide. The inverter is necessary for them. Also, many people use them because they are cheap and easy to plug an AC power cord into.
          Necessary? No - those folks are just unwilling to shell out the extra $$$ or unable to construct their own PSU that'll supply the wattage they need.

          Cheap? Well, perhaps. And inefficient - certainly good for testing/PoC purposes, but not for the long-term (IMHO).

          PS- I hadn't realized that DC-AC inverters vs DC-DC converters/regulators were the topic of any (potential) holy wars. Its usually just a noob - as we all once were - cutting an admittedly inconsequential corner to get a carpc up and running.

          PPS- its 'M1-ATX' not 'M1-M1' (ok, so I'm anal)


          • #6
            Sorry, but I don't know much about SBCs; however, if any part of your (potential) rig requres +12v, you'll need a regulator to clean up what your car's electrical system can provide. Voltage droop during cranking, the fact that your alternator puts out more than 12v when your engine's running (~14v, I think), etc. - all are things that you'll have to take into consideration.

            Check the output specs for any of the commercial power supplies listed at the store to see what an average system needs, and compare that to your SBC.

            As for parts needed for your circuit - I'd just follow disgram 5 on p. 8 of the PDF you linked to, maybe adding a filter capacitor to Vin. Oh, and you'll definitely need a heat sink of some sort, unless you want your project to go up in smoke down the road.


            • #7
              I appreciate the replys! Ill try it and see how it goes.

              Yea the inverter vs. direct power. I had that debate in my head once, but then i smoked my inverter (modding it to allow 550W through even tho it was rated for 400W, took about a day of running then it just died )


              • #8
                The problem with a Linear regulator (such as the 7805) is the power dissipation.
                e.g 14 in 5v out @ 5A = 45W to dissipate as heat.
                if you can tolerate a 20 degC rise about ambient (potentially 60 degC in some environ's) you still need a 0.4 DegC/W heatsink system, which will be quite large.
                SMPS is the way to go, but not as easy for the novice.

                My advice - Buy it. M1-ATX is only $70.
                If you really have no cash, but are able to make PCB's look for manufacturer data on SMPSU, but i bet you'll spend $70 making it.