Shop OBEX P1 Docs P2 Docs Learn Events
Making a basic 5v Power supply — Parallax Forums

Making a basic 5v Power supply

deanwesterburgdeanwesterburg Posts: 25
edited 2013-04-07 11:00 in Learn with BlocklyProp
I am trying to venture away from the educational boards into making my own projects from the chips themselves. I am concerned about using a 9v battery to make consistant 5v power.

I have seen several diagrams (such as this one) for basic 5v power supplies that have a few capacitors, a few resistors and a regulator.

I have connected a 9v battery to a 7805 regulator and I get a very consistant 4.98v output. Are these other components really needed and if so, can someone explain what they do? I'm sure there are things I am not aware of that are problems and I really want to understand!

Thanks,

Dean

Comments

  • LeonLeon Posts: 7,620
    edited 2013-04-06 15:48
    The 7805 data sheet will have all the information you need.
  • RickInTexasRickInTexas Posts: 124
    edited 2013-04-06 16:37
    Leon wrote: »
    The 7805 data sheet will have all the information you need.
    The capacitors are there for stability. The 78xx series is pretty bulletproof. I've used dozens over the past 2-3 decades with no problems under less than ideal conditions. I'd say your fine as long as you're seeing no appreciable AC on the 5 volt output.

    Best to use some caps even if you don't have exact values.

    Note: while easy to work with, this is not a very efficient setup. It will burn through 9 volts quickly. I'd find an old DC walwart with a 7 to 12 volt output.

    Supply more details if you need more help.
  • deanwesterburgdeanwesterburg Posts: 25
    edited 2013-04-06 17:20
    Thanks Rick! I don't have a scope yet so I have no way to see what the output look like but I am thinking of purchasing something like This one from Adafruit since Parallax dropped the prop scope.

    Is there a method or formula for choosing a capacitor for this situation?

    Thanks again!
  • wcumminswcummins Posts: 12
    edited 2013-04-06 17:44
    Most Goodwill stores and many second hand stores have a box somewhere in the back of the store that has dozens of old cell phone chargers. If you look at the voltage output of these chargers you will find some that charge 5 volts, some with more than five volts and some that charge 3.3 volts. They sell for only a couple of bucks. Pick up a couple of the five volt charges and test the voltage output. If the output is in fact about 4.8 to 5 volts you will not need to use the 7805 with this charger as your power supply. If you want to use your 7805 pick up any charger with an output of let’s say 8volts to 12 volts and you’re in business for just a few bucks, just cut off the plug used to plug into the cell phone and strip back the wire and test for positive and negative.
  • davejamesdavejames Posts: 4,047
    edited 2013-04-06 20:24
    deanwesterburg wrote: »
    Is there a method or formula for choosing a capacitor for this situation?

    Dean - if you really want to jump into the subject, I suggest the follow voltage regulator handbook from On Semi:
    http://www.onsemi.com/pub_link/Collateral/HB206-D.PDF
  • Dr_AculaDr_Acula Posts: 5,484
    edited 2013-04-06 22:08
    Are these other components really needed and if so, can someone explain what they do? I'm sure there are things I am not aware of that are problems and I really want to understand!

    The 100uF on the input smooths the input voltage. That is especially important if the original volts came from mains voltage and a transformer and recifier. For a 9V battery input, maybe you can leave it out. But it serves another function too. If you disconnect the input voltage, and the capacitor on the 5V side is bigger than the capacitor on the input side, current will flow backwards through the regulator. Some regulators may not like that. So in general terms, the input cap is bigger than the output cap (10x bigger in your example in post #1). Best to leave this in.

    The 10uF on the output keeps the output smooth if something happens to turn on and suddenly need more current. It stops integrated circuits resetting and other odd behaviour. Best to leave this in as well.

    The 0.1uF (or 100nF, same thing) also helps smooth the supply. In general terms, use one 0.1uF capacitor right next to every IC, and maybe every 10 chips or so, also a 10uF capacitor.

    You can get right into all the theory, or just follow the circuit diagram like a recipe book.
  • RickInTexasRickInTexas Posts: 124
    edited 2013-04-07 10:48
    deanwesterburg wrote: »
    Thanks Rick! I don't have a scope yet so I have no way to see what the output look like but I am thinking of purchasing something like This one from Adafruit since Parallax dropped the prop scope.
    Is there a method or formula for choosing a capacitor for this situation?
    Thanks again!

    You probably don't need a scope yet. I have an old 100Mhz analog scope, but a decent VOM is all I use >99% of the time.

    For your power supply question, DrAcula summed it it very nicely.

    BTW, exact values aren't critical here, e.g. a 0.33uf could be subbed for a 0.1uf etc.
  • PublisonPublison Posts: 12,366
    edited 2013-04-07 11:00
    deanwesterburg wrote: »
    Thanks Rick! I don't have a scope yet so I have no way to see what the output look like but I am thinking of purchasing something like This one from Adafruit since Parallax dropped the prop scope.

    Is there a method or formula for choosing a capacitor for this situation?

    Thanks again!

    Be advised the Prop Scope is available. The Parallax USB Oscilloscope was retired.
Sign In or Register to comment.