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7805 output is ~4.25Vdc — Parallax Forums

7805 output is ~4.25Vdc

Possum79Possum79 Posts: 30
edited 2010-10-27 10:23 in General Discussion
Its been a while sense I have logged on here but I read the forums daily. I am tryin to make a simple 5Vdc power source. I have a 7805 TO-220 package with .22uf and .1uf caps being fed by a 12 volt power source. Cheapy battery charger I think. The charger is outputing about 12 volts and I am getting only 4-4.25 volts on my output of the 7805. Any suggestions as I am a little stumped. Currently I am just trying to power a small 555 timer blinking led circuit and I know the 555 requires at least 4.5V to power it. Right now I have just a solid light and no blinking. If I use my boe bot as a power source instead of the 7805 I do get a blinky light so I know that circuit works. Any ideas?

Comments

  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2010-10-24 15:16
    If your 12V source is, as you say, a "cheapie battery charger", it's probably unfiltered, which means the output ripple will extend all the way down to 0V. You will need to either add a large cap (1000uF, 35V) to the power source, ahead of the 7805, or (better yet) get a different power source.

    -Phil
  • LeonLeon Posts: 7,620
    edited 2010-10-24 15:16
    The output capacitor needs to be close to the pins of the 7805 to prevent oscillation. The input capacitor should be 330nF, but that won't cause your problem. What is the output voltage with no load on it?
  • Possum79Possum79 Posts: 30
    edited 2010-10-24 15:30
    At Phil I also switched over to a wal wart 9.6V supply that was outputing a around 10.5V and it still is getting me only around 4-4.25V. I have to go through my stash of caps to see if I have a higher rated cap. At Leon if I just put a volt meter on the 7805 thats when Im getting about 4.25V. if I have the 555 ciruit and i probe with a voltmeter I get about 4.15V All the caps are real close to the input and output of the 7805 as I am building this on a breadboard right now. Once I get it working I plan on making my own pcb.


    Edit. I uploaded a screen shot of the schematic I tried making with Eagle. I'm new to Eagle so bear with me.
    Schematic.png
    1024 x 728 - 88K
  • LeonLeon Posts: 7,620
    edited 2010-10-24 15:43
    The capacitors should be the other way round, the 100n goes on the output. Check the 7805 connections, I've never had any problems with them.
  • Possum79Possum79 Posts: 30
    edited 2010-10-24 16:04
    So Phil I went through my caps and added a 1000uf to the input and wham it works now. Output is right at 4.95V. You guys are really awesome with the quick replies.

    Also good catch on the caps being in the wrong spots. I changed them around and it still didnt output enough. Then once I added the larger cap to the input it worked.


    Thanks again guys! I really need an O-scope.
  • Bobb FwedBobb Fwed Posts: 1,119
    edited 2010-10-25 08:35
    Leon wrote: »
    Sorry about the duplicate, it often seems to happen with this forum software.
    I haven't seen it at all...but whatever.

    You can delete the post by selecting edit, and clicking the delete button.

    As for the topic. Can you hook up to a scope? See if you are getting a constant voltage, or some type of bounce caused by input voltage fluctuations (or something else). I know you solved the problem, but, if you know the cause of the problem, you can prevent it in the future.

    1000µF sounds big, but if the cap is holding the input voltage high enough between input cycles, when you get much of a load on the regulator, it won't be able to do that any more.
  • Dave HeinDave Hein Posts: 6,347
    edited 2010-10-25 09:43
    Actually 1000 uf is not that large. The voltage ripple from a full-wave rectified source is Vpp = I/(2fC). Based on his schematic he should be drawing about 50 ma. Assuming his AC power is 60 Hz the voltage ripple will be 50ma/(2*60*1000uf) = 0.4 volts. In theory, he could get by with an input ripple of 5 or 6 volts, which means he could get by with a 100 uf capactior, but less input ripple is better.

    You can get an idea of how much ripple there is by measuring the voltage using the AC mode on a meter.

    Dave
  • Possum79Possum79 Posts: 30
    edited 2010-10-27 10:23
    I do not have scope to hook up. Im thinking about getting the usb scope from parallax soon just to play with. I have only dabbled with them in college a little. I find Im learning more from these forums then i did in school even though i dont do much electronics for a living right now and i have been out of school for 6 years.
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