Shop OBEX P1 Docs P2 Docs Learn Events
Parallel diodes — Parallax Forums

Parallel diodes

bambinobambino Posts: 789
edited 2008-09-06 01:03 in General Discussion
I have a PCB made that runs 3 batteries through 3 diodes into a regulator like so:
bat
diode
···························· \
bat
diode
regulator
·····························/
bat
diode

Finding a 3 pole single throw switch that will fit into my app to turn on power is now a bit of a problem!
What effect would I have if I ran all the power through one single pole single throw switch into the three diodes? ie:

bat----·····················
diode
········· \·················· /························ \
bat
switch
diode---
regulator
··········/·················· \························ /
bat----·····················
diode

I know it would somehow effect the voltage drop, I'm not to worried about that, their mainly there just for reverse polarity protection.
The boards are already made or I would just use one diode!
Any words of wisedom on this would be much apprieciated!

Comments

  • steve_bsteve_b Posts: 1,563
    edited 2008-09-04 14:46
    What value are the batteries?
    What leg of the regulator are then going on on the regulator....what's the regulator!?

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    <FONT>Steve



    What's the best thing to do in a lightning storm? "take a one iron out the bag and hold it straight up above your head, even God cant hit a one iron!"
    Lee Travino after the second time being hit by lightning!
  • Mike GreenMike Green Posts: 23,101
    edited 2008-09-04 15:00
    Do not use the diagram you showed with the single pole single throw switch!

    What will happen is that the higher voltage battery of the three will discharge into the lower voltage batteries until one of the batteries explodes or until the higher voltage battery drops in voltage, then the then higher voltage battery takes over. Eventually, all the batteries will either be dead or damaged.

    The diodes prevent this from happening. The only place the batteries can discharge into is the regulator. You need to put the single pole single throw switch between the common connection of the diodes and the regulator. If that's not possible because the diodes and regulator are already wired, you could put the switch in the ground lead of the batteries. The three batteries all have ground leads that are connected together, then that common lead is connected to the ground of the regulator or the main circuit board. That's where you put your power switch.

    Normally, you'd like to have your power switch so that it can completely disconnect the batteries from the circuit. In your case, you have three batteries and only a single pole switch. The diodes will allow only a few microAmps of leakage at most between the batteries which is tolerable.
  • davejamesdavejames Posts: 4,047
    edited 2008-09-04 15:12
    Bambino,

    I'm guessing from the first diagram that you're attempting to protect the regulator from a "reverse connection" on any battery. Is that correct?

    If you wanted to switch power to the regulator with the first diagram, then just insert an SPDT (single pole, single throw) switch in between where the diodes connect together and the regulator input.

    The second diagram will put all batteries in parallel, route that through the SPDT switch and then through 3 diodes in parallel, then to the regulator. Unless there is some concern for current capacity and the related voltage drop, 3 parallel diodes may not be required. To figure the voltage drop across the 3 parallel diodes, just divide the typical diode voltage drop (0.7 for silicon) by 3 (approx 0.233V in this case). To be really accurate, you'd need to consult the particular diode's data sheet for information concerning its specific voltage/current characteristics.

    DJ
  • davejamesdavejames Posts: 4,047
    edited 2008-09-04 15:17
    ...just read Mr. Green's response and I heartily agree. Paralleling the batteries would not be a good thing. Switching the ground-side of the batteries is a great idea.

    DJ
  • Beau SchwabeBeau Schwabe Posts: 6,568
    edited 2008-09-05 01:15
    davejames,

    "To figure the voltage drop across the 3 parallel diodes, just divide the typical diode voltage drop (0.7 for silicon) by 3 (approx 0.233V in this case)." - Paralleling diodes doesn't quite work that way... There will always be about a 0.6V drop across the silicon junction under nominal current demands, and upwards of 0.8V under high current demands.· What paralleling diodes will do is lessen the voltage drop over higher current demands so that the voltage drop is closer to 0.6V because the current is distributed among the parallel paths, but·it will not go below the band-gap or junction voltage of 0.6V.




    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    Beau Schwabe

    IC Layout Engineer
    Parallax, Inc.

    Post Edited (Beau Schwabe (Parallax)) : 9/5/2008 6:20:48 PM GMT
  • bambinobambino Posts: 789
    edited 2008-09-05 01:36
    The boards are allready made so placeing the switch between diode and regulator isn't possible without tedious trace cutting.

    The batteries where originally going to be 9v rechargeables. The people engineering the container for my app have reduced the amount of room I have not only for the switch, but the battiers as well.

    I am now looking at a pack of six AAA cells for a total of 7.2 volts(rechargeable), which I like better. Less heat to dissipate.

    I like the ground switching idea, it would be simple enough!
    pdf
    pdf
    39730397242004ADP3335_a.pdf
    443K
    pdf
    pdf
    20cjq030[1].pdf
    138K
  • bambinobambino Posts: 789
    edited 2008-09-05 13:22
    @Beau
    Now that I am going with a series of batteries(theoritically one cell of 7.2 volts), using the parallel idea would help keep me above the drop out voltage for the regulators longer, correct? I mean, their allready soldiered to the board, i might as well use them, right? Or should I just use one and leave the others open?
  • Beau SchwabeBeau Schwabe Posts: 6,568
    edited 2008-09-05 15:14
    bambino,
    ·
    I'm not sure that I follow you...
    ·
    Depends on the 9V·rechargeable used...
    Some use 7 cells in series that give about 8.4V @ 100mAh
    Some use 6 cells in series that give about 7.2V @ 150mAh
    ·
    Placing 3 in parallel would give you a capacity ranging from 300mAh to 450mAh
    ·
    ·
    ·
    Using just one·six pack of·"AAA" in series you get about 7.2V @·900mAH ... twice as much as the three 9V·battery configuration.
    ·


    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    Beau Schwabe

    IC Layout Engineer
    Parallax, Inc.
  • davejamesdavejames Posts: 4,047
    edited 2008-09-05 16:37
    Mr. Schwabe,

    Jeesh! What was I thinking?! Of course you are exactly right.

    I think I'll head back to the cave and just observe.

    (shuffle, shuffle, shuffle)

    DJ
  • Beau SchwabeBeau Schwabe Posts: 6,568
    edited 2008-09-05 18:25
    davejames,

    Please, get out of the cave and keep posting! smilewinkgrin.gif

    This forum is a living learning entity and everyone contributes wonderful ideas. One of the things that I find so interesting about the forum is that there are always serendipitous ideas that are created as people go from point A to point B. There are always many ways to get the job done... some may be better than others, but that's part of the learning process.

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    Beau Schwabe

    IC Layout Engineer
    Parallax, Inc.
  • davejamesdavejames Posts: 4,047
    edited 2008-09-05 18:42
    (shuffling up to the cave entrance, sticking nose outside)

    Mr. Schwabe,

    OK - I will. Though I'll spend more time thinking about the post before pushing the submit button!

    DJ
  • bambinobambino Posts: 789
    edited 2008-09-05 19:24
    Series AAA configuration was the intended method.
    The 9v was canned when they realized the size of the package and the limited milliamp hours.
    You answered my question , thanks Beau!
  • Beau SchwabeBeau Schwabe Posts: 6,568
    edited 2008-09-05 20:50
    bambino,
    ·
    Just be sure and double check the mAh rating of your rechargeable "AAA"'s ... I have seen them range all over the board as little as 550mAH to as much as 950mAH.· The stores really have fun with this too when it comes to determining their selling price.
    ·
    Read the fine print, read the mAh rating, and select your batteries based on $$/mAh
    ·


    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    Beau Schwabe

    IC Layout Engineer
    Parallax, Inc.

    Post Edited (Beau Schwabe (Parallax)) : 9/6/2008 3:30:31 AM GMT
  • bambinobambino Posts: 789
    edited 2008-09-06 01:03
    Indeed, Thanks for the reminder! I'm close to the end on this one and the urge to get the first thing I see and run with it is great!
    The ones I saw today where 17USD for a pack of two. But they were rated at 800mAh.
    I bought some non-rechargeables for testing this weekend. Going to find out how fast this thing can suck em dry.
    Only problem was when I got home I noticed The Store-Who-Must-Not-Be-Named doesn't include an amp rating on there batteies.
    O well, I guess it's back to town for me!
Sign In or Register to comment.