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+ and - 5v power supply — Parallax Forums

+ and - 5v power supply

kelun1kelun1 Posts: 10
edited 2012-03-25 13:27 in BASIC Stamp
Hi guys!

I need help on the following issues.

I'm trying to build a dual 5v power supply for one of my application in my hardware. and below are the list of the item i have in my hand:

LM7805
LM7905
9v DC battery as input power for my regulator

Im able to get the 5v regulated power out from 7805 with the negative end of the power supply as ground. but i cant seems to get 7905 to work properly. and i know that both output should have a common ground. can anyone teach me how should i build this?

Thanks a million for reading this guys! and the help too! =)

Kelun

Comments

  • LeonLeon Posts: 7,620
    edited 2012-03-25 07:20
    There should be a schematic in the 7905 data sheet.
  • PJAllenPJAllen Banned Posts: 5,065
    edited 2012-03-25 07:49
    Are you using a center-tap transformer? Two separate transformers? A transformer with two separate windings?

    N.B.The pin out of the 7805 (I-G-O) isn't the same as a 7905 (G-I-O).
  • PJAllenPJAllen Banned Posts: 5,065
    edited 2012-03-25 07:54
    I hasten to add that the 7805's tab is Ground, but the 7905's tab is Input.
  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2012-03-25 08:32
    If you're going the battery-power route, you will need two 9V batteries: one for the 7805, one for the 7905. You can't use one battery and expect have a common ground.

    -Phil
  • kelun1kelun1 Posts: 10
    edited 2012-03-25 12:43
    @Leon : hi leon thank you for the reply! i have notice the diagram of the circuit. and i follow the diagram but cant get it to work.

    @PJ : hi PJ thank you for the reply! i am not using a center tap transformer as my design only require me to use 9v battery. and yeah i notice the pin are different too. but just cant get it to work. =(
  • kelun1kelun1 Posts: 10
    edited 2012-03-25 12:51
    Hi Phil thank you for the reply!

    if i were to use different battery for both regulator, do i just connect 7805 GND pin with 7905 GND pin together to form the common ground?

    Below are the connection set up from what i know, please let me know if i am wrong.

    >> for 7805, the INPUT pin is connected to the positive end of the battery and the GROUND pin is connected to the negative end of the battery.

    >> for 7905, the INPUT pin is connected to the negative end of the battery and the GROUND pin is connected to the positive end of the battery.

    this is the part which make me cant get the "common" ground.

    or if it is possible, will you mind showing me the overall diagram as to help me to visualize it? really appreciate it!
  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2012-03-25 12:56
    kelun1 wrote:
    if i were to use different battery for both regulator, do i just connect 7805 GND pin with 7905 GND pin together to form the common ground?
    Yes. And you must use two batteries to obtain a common ground. Also, don't forget the caps that are required by these regulators for stability. Be sure to read their respective datasheets.

    -Phil
  • Mike GreenMike Green Posts: 23,101
    edited 2012-03-25 13:00
    You need two 9V batteries. One battery is for the 7805 and its positive end is connected to the 7805 input pin with the negative end connected to ground (for everything). The 2nd battery is for the 7905 and has its negative end connected to the 7905 input pin and its positive end connected to ground (for everything). The 7805 and 7905 ground pins are connected to ground (for everything). There is only one ground (for everything).

    I do suggest that you have a DPST (Double Pole Single Throw) power switch connected between the batteries and the regulators. That would allow you to turn things on and off without removing the batteries. It's optional. One side of the switch would go between the positive end of one battery and the 7805 input and the other side of the switch would go between the negative end of the other battery and the 7905 input.
  • kelun1kelun1 Posts: 10
    edited 2012-03-25 13:01
    Hi Phil and MIke, thanks alot for the clue! i will research on that! you guys just light up my life! Thanks a million guys!

    Kelun =)
  • PJAllenPJAllen Banned Posts: 5,065
    edited 2012-03-25 13:27
    Batteries ! ?
    Well... OK, but this won't last long.

    "...will you mind showing me the overall diagram as to help me to visualize it?"
    5Vp&m.JPG
    5Vp&m.JPG
    297 x 276 - 9K
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