Basic 7805 help - incorrect output voltage

Discussion in 'General Electronics Chat' started by waiexbldr, Nov 3, 2010.

  1. waiexbldr

    Thread Starter New Member

    Nov 3, 2010
    I'm working on my first PICaxe project and am having a little trouble with the very first step: the power supply! I'm pretty much brand new to hands-on electronics so bear with me.

    I purchased a brand new LM7805C voltage regulator from Radio Shack last night, set it up with a a 100uF cap & .1uF cap on both the input and output sides (as recommended by the book I'm working out of). When I set it up with a 9v 1200mA power supply (AC/DC adapter), I read 9.15v on the input side and 8.35v on the output side. I was assuming I'm supposed to get 5v out but the documentation that comes with the component is pretty scarce. Since the book indicates the PICaxe is very sensitive I can't move forward until I'm getting 5v out of this thing.

    So...I went out an bought a second one tonight, applied the 9v to it directly (no capacitors involved) and measured the voltage from both sides and received the same readings. Seems unlikely it's a problem with the component, but I'm not sure what I'm doing wrong. I'm glad to read about it in one of the ebooks if there is a section you can point me to. Thanks for any help you can provide.
  2. blueroomelectronics

    AAC Fanatic!

    Jul 22, 2007
    Post a photo of it. Do you have it the right way around?
  3. hgmjr


    Jan 28, 2005
    Try putting a load resistor on the output of the voltage regulator. Even 1K would be worth a try.

  4. waiexbldr

    Thread Starter New Member

    Nov 3, 2010
    OK, apparently I don't have a single camera in the house I can make work in macro mode right now. Maybe you have to be able to operate electronics before you can build them :).

    For setup: if you are looking straight at the 7805 you see the black box with the heat sink sticking out the top and the terminals/pins sticking out the bottom. I have the power going in the left terminal sticking out the bottom, I have the middle terminal hooked into ground on my breadboard, and the right terminal I use my volt meter to connect it to ground on the breadboard. That's where I'm getting the 8.35v.

    I added a 2.7k resistor (red, violet, orange) to the third pin and used my volt meter to connect that to ground and voltage has dropped to 8.14v.

    I'll try one more time to get a picture.
  5. blueroomelectronics

    AAC Fanatic!

    Jul 22, 2007
    Sounds right, might be a duff 7805.
  6. waiexbldr

    Thread Starter New Member

    Nov 3, 2010
    OK. I'm just going to be a bit surprised if I got two duds that are bad in the exact same way - I bought them at different Radio Shacks. The ground pin in the middle is reading 8.39v. Is it supposed to do that? I'm thinking maybe there is something wrong in my connections to the breadboard? I've had poor connections and such on previous prototyping I've done, but this is such a simple circuit and it's giving *some* result so I'm finding it hard to justify how it could result in a higher than expected output voltage based on poor connections. Not a good start to my microcontroller hobby...
  7. debe

    Senior Member

    Sep 21, 2010
    This is how it should be hooked up roughly & what should come out.( Sounds like faulty ground on cent pin)
    waiexbldr likes this.
  8. waiexbldr

    Thread Starter New Member

    Nov 3, 2010
    Faulty ground on the center pin FTW! Thanks very much for the picture and the hint. I rewired all the ground portion of the breadboard to refresh the connections and now I'm getting a rock solid 5.0v out of my capacitor supported 7805.

    Thanks to all for helping me work through this. I know a tiny bit more than when I started tonight.
  9. tracecom

    AAC Fanatic!

    Apr 16, 2010
    Just out of curiosity, if you're going to power the PICAXE with a battery, why not use three single cell batteries (AA, AAA, C, or D) in series and forego the voltage regulator? The batteries will last much longer than the 9 volt and the power supply is much simpler. Is it a physical space constraint?

    ETA: Never mind, I reread your first post and see that you are using a 9v wall wart. :)
  10. Jaguarjoe

    Active Member

    Apr 7, 2010
    3 AA's have the advantage of not letting too much smoke out of the wires if there's a wiring error or some other problem. That's the defacto standard on the Picaxe forum.
  11. waiexbldr

    Thread Starter New Member

    Nov 3, 2010
    Using the AC/DC converter just for prototyping and learning on the breadboard. I'm assuming my project will need some independent power so it can fly, run, roll or just be away from a socket when it actually turns into something useful. Good point on reducing the total size of the fire by using batteries, though. Also, didn't realize there was a picaxe forum. I'll have to check that out. Thanks!
  12. Audioguru


    Dec 20, 2007
    3 AA carbon-zinc battery cells made in China a few years ago have probably spent a year on a boat plus another year in a store so they are almost dead and might supply a few mA.
    But 3 AA new locally made alkaline cells can supply up to a few Amps.
    A thousand times the difference in current (and the amount of smoke).
  13. Dantali0n

    New Member

    Aug 23, 2013
    Just want to point out that even though this thread is 3 years old, it just made me fix my problem in my circuit big thanks! never would have quessed my l7805cv would have been death on delivery.
  14. joeyd999

    AAC Fanatic!

    Jun 6, 2011
    Ha! You'd be surprised how much smoke (and fire!) you can make with 3 'AA's. Been there, done that!
  15. sheldons

    Well-Known Member

    Oct 26, 2011
    if you have got the reg incorrectly wired or as one i had a while back the reg was correctly wired and fitted to the board but there was little or no output unless the reg was moved about due to the centre pin breaking away(the gnd pin) close to the regulator package and was found when the reg was desoldered from the board-the 78 series of fixed regs are very easy to work with