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  • 7805 Question for the Guru's

    In an effort to eliminate my inverter completely (laptop setup) I need to power my 2 usb hubs.

    One is a Belkin 7 port calling for ~3.5 amps
    The other is a no name 4 port calling for ~2.5 amps

    I have looked into the 5v POL mp3car sells, but they are WEAK.

    I have looked at these two as well:
    http://jopeldesigns.com/acppsv1.html
    http://www.trcelectronics.com/Meanwell/sd-50a-5.shtml

    But, 7805's are so cheap... however they are only rated at 1 amp...

    so, thoughts?

    Anyone using them to power a hub or just individual usb devices?

  • #2
    Heatsink em, Run parallel em

    This will increase your amps
    2004 Holden WL Caprice Auto GENIII
    Base System = Raspberry Pi
    Everything else is pending for now as switched from a Mini-ITX setup

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    • #3
      I originally used a couple 7805 to power my usb hub and accessories, but it ended up pumping out a LOT of heat. 7805's are simple, but VERY inefficient. I replaced with the Jopel 5V power supply, and love it. It also doubles as a decent accessory control relay, with the 12V unregulated output. It supplies a MUCH cleaner 5V, provides more than enough current, and even has the nice and convenient USB control signal option. By the time you build a linear regulator using 7805's, board, terminals, proper heat sinks, a decent case, switching relay, fuse, the $80 for the Jopel starts to not look so bad.
      2000 Ford Expedition Eddie Bauer - Bi-Fuel Gasoline/CNG
      Intel D945GCLF2 w/512MB RAM, CL Audigy w/KxProject, M2-ATX, Lilliput EBY701

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      • #4
        as beautiful as the Jopel is, I already have some spare parts lying around...

        2 - 7805s, plenty of components (resistors/relays/capacitors/wires), breadboard, heatsinks(an old Vantec set for motherboard w/fan)

        So not to be completely cheap, would running lets say 4 - 7805s in parallel achieve my goal? And how much heat would that produce?

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        • #5
          I'm hitting the same problem as you. So far I've decided against paralleling 7805, way too much heat.

          This is what I've found so far: power supply 5V 5A by 7805+Mj2955 for digital circuit

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          • #6
            To quote something from another website, not mine,....

            And finally, you can run several 7805s in parallel. The easiest way to do it is to make the inputs a common connection and the grounds a common connection. But don't connect the outputs directly together. The 7805 don't all regulate at exactly the same voltage and the one that regulates at the highest voltage will do all the work while the others loaf along and you'll don't have solved anything. Instead, put a small-value (e.g., 0.47 ohm, 1-watt) resistor in series with each output and THEN connect the other ends of those resistors in common as your final output. That small value of resistance will allow each regulator to work independently of the others and the current will be shared by all of the regulators fairly equally. The down side of this fix is that it makes the regulation a little bit "softer" (i.e., a higher internal resistance for the supply), but in most cases, won't be much of a problem.
            This Source

            Another page from same site

            One more as well

            If you do it as dean suggests, then you should be all set.
            Also i would get a small metal case and enclose them in it and use it as the heat sink... Think of how the normal car inverter cases are....
            2004 Holden WL Caprice Auto GENIII
            Base System = Raspberry Pi
            Everything else is pending for now as switched from a Mini-ITX setup

            Comment


            • #7
              I realize this is not an option for most people and is a wasteful method, But if done right can be a cheap and quick alternative in the mean time until a more "Complete" power supply system can be built...
              2004 Holden WL Caprice Auto GENIII
              Base System = Raspberry Pi
              Everything else is pending for now as switched from a Mini-ITX setup

              Comment


              • #8
                Being realistic in this approach, heat in a vehicle is a factor. But on the other hand, my laptop gets quite warm, amps, laptop power brick, ect....

                The 2995 circuit looks nice as its rated 5 amps, HOWEVER...
                7805's are readily available at Radioshack, if one was to order something wouldn't it be wiser to order a Jopel? Yes

                I am looking for a permanent solution. I'm confident in my handy work building a 7805 system properly sink'ed. How many watts of heat are we talking about for 4-7805s? Pushing a ~6amp load? And would this be a wise/viable permanent solution?

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                • #9
                  nope.... My setup is with a Toshiba satellite. Ive got my 5v tapped from the laptop, but its how a high output. I just use it to connect to the main hub, and drive 2 5v relays.

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                  • #10
                    Originally posted by Hellrazor978 View Post
                    nope.... My setup is with a Toshiba satellite. Ive got my 5v tapped from the laptop, but its how a high output. I just use it to connect to the main hub, and drive 2 5v relays.
                    I've realized that, so I took the comment out.

                    Originally posted by Hellrazor978 View Post
                    How many watts of heat are we talking about for 4-7805s? Pushing a ~6amp load? And would this be a wise/viable permanent solution?
                    14.4V - 5V = 9.4V

                    P = U * I = 9.4V * 6A = 56.4W

                    This is a lot of heat to dissipate. At 6A you need a DC-DC converter, which are > 80% efficient.

                    Are you really going to pull 0.5A from each USB port? Calculate the real power draw, and then aim for supply for 125% of that value.

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                    • #11
                      Main Hub(powered), Touchpad, Touchscreen, OBD, GPS, Bluetooth, RF for Keyboard, Xbox360 Joypad, USB Capture for Camera, 4 Port hub(bus), 4 Port Hub(powered/accessible)

                      11 Devices, so lets say 15 devices so I can utilize my accessible 4 port hub for anything on the fly.

                      15 * .5A(max) = 7.5amps(max)...

                      So yeah, DC-DC is going to be the way to go.... that being said.

                      Which of these two units seems like the better buy?
                      http://jopeldesigns.com/acppsv1.html
                      http://www.trcelectronics.com/Meanwell/sd-50a-5.shtml

                      If I'm reading the Data sheet correctly, the Mean-well runs at 70% efficiency?

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                      • #12
                        If you already have some 7805s you could throw the circuit together and start measuring draw. I did that, and found that all the components I am powering off the hub only draw ~400mA, so I didn't even need the second 7805 I was planning on using. Plus at the ~13V to 5V drop they are good for more like 1.5A apiece. Used thermal epoxy to attach 1 7805 to a heatsink, then thermal epoxied that to a beam in my car.

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                        • #13
                          so back to the 7805 topic...

                          My original intention was to use 4-7805s in parallel and attach an old 12v northbridge fansink. Do you think that'll do the trick, or will it need a bigger sink?

                          Also, how do you measure amperage draw?

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                          • #14
                            Im not one for the maths, So i hope someone else can give you a better answer for the heatsink size.

                            As for the amp measuring, That can be done with either a multimeter inline with the circuit or there are some little automotive devices that basically have a LCD display and a cable that plugs into a blade fuse holder.
                            2004 Holden WL Caprice Auto GENIII
                            Base System = Raspberry Pi
                            Everything else is pending for now as switched from a Mini-ITX setup

                            Comment


                            • #15
                              Originally posted by Hellrazor978 View Post
                              Do you think that'll do the trick, or will it need a bigger sink?
                              If you going to max out each 7805, then you need a fan/heatsink like that per each chip.

                              Look at it this way, how big are the AMD Socket A heat sinks? Those CPUs dissipate ~60W when loaded, just like your 7805s when providing 6 Amps...

                              Go with a DC-DC converter.

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