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Help with a circuit using an AC adapter. — Parallax Forums

Help with a circuit using an AC adapter.

Private19872Private19872 Posts: 61
edited 2015-03-09 15:51 in General Discussion
I'm designing a board for my propeller. At the moment, it's running fine off of D batteries but I also want to have the option of plugging into the wall using this. ( http://www.parallax.com/product/750-00009 ) My one question is, how do I wire the board so that if I use the wall-wart it doesn't drain the batteries? I also don't want the wall-wart to try and run current into the batteries because I'm pretty sure they don't like that... My theory was just to put a diode on both sources' anodes but I figured I'd ask here first. Thanks!

Comments

  • Mike GreenMike Green Posts: 23,101
    edited 2015-03-07 10:57
    The simplest way is to put a diode in the positive lead from the batteries (anode towards battery, cathode towards Propeller board). Any diode with an adequate current rating for your needs would do. There'll be about a 0.7V voltage drop across the diode, so you'll need to account for that in your battery voltage. If that's a problem, use a Shottky diode which has a smaller voltage drop. A wall-wart will have a voltage regulator of some sort which should prevent the batteries from discharging through it when it's not plugged in, but you could play it safe by putting a diode in that lead as well. Again, there'll be a voltage drop of about 0.7V across the diode that you'll have to account for.
  • Private19872Private19872 Posts: 61
    edited 2015-03-08 11:43
    I tried as you said. Although everything worked this ( https://www.sparkfun.com/products/8884 ) screen was so dim, I almost couldn't read it. I measured (without a load) that one diode I tried had a drop of about 0.36v and the other I tried had a drop of 0.04v. Do the diodes have some kind of current limit I haven't accounted for? What would you recommend I look for if I need to buy a new diode? Thanks.
  • PliersPliers Posts: 280
    edited 2015-03-08 11:59
    Diodes are not, in general, current limiting.
    Your screen is dim, how many volts are going to it?
    Sparks Fun documents say it needs 6 to seven volts.
  • Private19872Private19872 Posts: 61
    edited 2015-03-08 12:04
    I have the screen currently hooked up directly to the diode, although before it was hooked up to the 5v regulator and working fine. The screen seems to be getting about 5.4v or so, but it should be working fine considering there were no problems with it running off of 5 volts without the diode.
  • PliersPliers Posts: 280
    edited 2015-03-08 14:53
    I miss read your data. 5.4 volts sounds good. Have you done a current check? Docs say 220ma



    Features:
    Voltage: 6V – 7V DC
    Current: 220mA (backlight at 100%)
    Input: 0-5V, 115,200bps (adjustable), 8 data bits, 1 stop bit, no parity
  • PliersPliers Posts: 280
    edited 2015-03-08 15:04
    What size (wattage) diode are you using?
    5.4 volts * 220 ma = 1.1 watts
    Can your diode handle 1.1 watts?
  • PliersPliers Posts: 280
    edited 2015-03-08 15:50
    It would be an interesting experiment if you used a variable power supply and lowered the voltage to the display and noted the results.
  • Private19872Private19872 Posts: 61
    edited 2015-03-09 12:10
    After some more testing, I think my previous numbers are lies. The voltage drop I measured was without a load on the diode. With only the screen connected to the batteries through the diode, the voltage drops from 5.84v to about 3.5v. Without the diode it only drops to 5.4v. As for what diode I'm using, it was just some switching diode I picked up in a ten pack at RadioShack, probably not very capable...
  • PliersPliers Posts: 280
    edited 2015-03-09 15:51
    Good luck with your project.
    I'm starting a hexpod project, 20 or more servos! I should start posting it's progression the project forum.


    You could put 5 diodes in parallel to take all that current.
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