Page 1 of 2 12 LastLast
Results 1 to 10 of 16

Thread: 7805 Question for the Guru's

  1. #1
    Newbie
    Join Date
    Jun 2008
    Posts
    7

    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. #2
    Variable Bitrate mayhembdm666's Avatar
    Join Date
    Mar 2008
    Location
    Adelaide, Australia
    Posts
    234
    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

  3. #3
    Variable Bitrate
    Join Date
    Nov 2006
    Location
    Rubidoux, CA
    Posts
    275
    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

  4. #4
    Newbie
    Join Date
    Jun 2008
    Posts
    7
    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?

  5. #5
    Low Bitrate
    Join Date
    May 2009
    Posts
    64
    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

  6. #6
    Variable Bitrate mayhembdm666's Avatar
    Join Date
    Mar 2008
    Location
    Adelaide, Australia
    Posts
    234
    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

  7. #7
    Variable Bitrate mayhembdm666's Avatar
    Join Date
    Mar 2008
    Location
    Adelaide, Australia
    Posts
    234
    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

  8. #8
    Newbie
    Join Date
    Jun 2008
    Posts
    7
    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?

  9. #9
    Newbie
    Join Date
    Jun 2008
    Posts
    7
    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.

  10. #10
    Low Bitrate
    Join Date
    May 2009
    Posts
    64
    Quote 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.

    Quote 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.

Page 1 of 2 12 LastLast

Similar Threads

  1. A quick question for the gurus...
    By poppasmerph in forum Power Supplies
    Replies: 1
    Last Post: 05-13-2005, 02:33 AM
  2. Quick question for the Gurus
    By poppasmerph in forum Newbie
    Replies: 3
    Last Post: 05-12-2005, 06:58 PM
  3. lcd power question
    By jrbless in forum LCD/Display
    Replies: 1
    Last Post: 04-07-2002, 12:27 PM
  4. Question For Electric Gurus
    By brentp4 in forum General Hardware Discussion
    Replies: 4
    Last Post: 02-01-2002, 07:31 PM
  5. Another inverter question...for electronics gurus
    By moahdib in forum Power Supplies
    Replies: 10
    Last Post: 06-07-2001, 08:30 PM

Bookmarks

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •