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5 Volt regulator — Parallax Forums

5 Volt regulator

lrohnlrohn Posts: 33
edited 2004-10-30 04:21 in BASIC Stamp
Hello everyone,

I'm building my own 5 volt regulated power supply for my basic stamp projects. I'll be using the 7805 voltage regulator and would like to use a capacitor across the input to ground to take out any ripple coming from the transformer so that the 7805 is receiving a smooth input voltage. I would also like to use a second capacitor across the output to ground to act as a load balancer to ensure consistent output from the 7805.

Can anyone help me determine what's the best size capacitors to use for this project? Are the values important? I want to use electrolytic capacitors also..

Thanks again for any help you can offer.

Luke

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Comments

  • NewzedNewzed Posts: 2,503
    edited 2004-10-24 20:14
    Luke, I use a .1uf across the input and a .1uf and a 47uf electroytic across the output.· Works well for me.

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    Sid Weaver
    Try the Stamp Tester

    http://hometown.aol.com/newzed/index.html
    ·
  • scribbscribb Posts: 8
    edited 2004-10-25 12:40
    Hi,

    You should have higher value capacitor at the input (to lower the ripple, especially at higher loads): I usually use 100 - 1000uF.· I use 10 - 22uF on the output.

    You can also calculate the ripple voltage for a known power supply type and a known load.· Example:
    For a full wave bridge rectified power supply, the ripple voltage is (peak-peak):
    = (load current * t)/C, where t = (1/2f) and C = capacitor.· f = frequency.

    Sheldon wink.gif
  • DonDon Posts: 34
    edited 2004-10-26 04:26
    The output capacitor can be larger than the input capacitor but in this case you should add a diode from output to input (anode on the output) to allow the output capacitor to discharge into the input.

    In order to calculate the size of the capacitor, you need to know how much ripple your circuit can tolerate.· This can be determined from knowing the regulator output voltage, the regulator's dropout voltage (the minimum input to output differential)·and the load current.· Another factor is whether your input is full-wave or half-wave rectified and what the line frequency is.

    Essentially, you calculate the capacitor to prevent the "droop" between half-wave cycles from allowing the input to the regulator to drop below the minimum voltage at the maximum current draw.· You can make some linear approximations that will get you pretty close and then add 20% or so to the capacitor size for a safety margin.


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    Don Kinzer

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  • lrohnlrohn Posts: 33
    edited 2004-10-27 04:42
    Thank you everyone for all the help, I appreciate it very much. Right now I am using a 470 uf 35V across the input to ground and the same across the output to ground because I had them redily available and they appear to be working just fine. I don't have a scope so I really don't know what the ripple is.

    Maybe someone can help me figure this out without a scope. I don't have all the values mentioned in the previous message but what I am using is a plug in class 2 transformer (simple wall wart). Input: AC 120V 60hz 7.0 w Output: DC 9V 300 mw. I cut the barrel plug of the end and outfitted it with banana plugs to easily plug into my breadboard terminals.

    Also can the regulator be demaged if I was to reverse polarity? I will be adding a diode in the future to prevent any harm by recersing the polarity but just want to know what would happen without actually trying.

    Thanks again guys,

    Luke
  • m_fabio2m_fabio2 Posts: 49
    edited 2004-10-27 10:57
    One of the problems I have noticed with the cheaper class two transformers is they burn out very easily. I would recommend using a computer power supply, because A) They are regulated, and B) If a short occurs, the voltage will collapse, instead of burning out the transformer as it will the cheaper ones.
  • Chris SavageChris Savage Parallax Engineering Posts: 14,406
    edited 2004-10-29 15:07
    m_fabio2,

    · I was without a scope for a few years until recently, so I will have to check again on this, but about 7 years ago I tried using some PC Power Supplies in some controller circuits, and found them to have much more ripple/noise than my regulated supplies, which were nothing more than a Transformer, Bridge Rectifier, Filter Caps and fuses.

    ·· I will see if I can get this done today, because PC supplies are readily available, and cheap, especially from HAM Fests and Computer Shows, but I am still uncertain about Noise & Ripple.· Noise Moreso...

    ·· As a side-note, the Power Supplies I had tested back then were AT Power Supplies, and were tested with a Home-Built Heathkit O-Scope.· Now they will be ATX, and tested with a Parallax USB O-Scope.



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    Chris Savage

    Knight Designs
    324 West Main Street
    P.O. Box 97
    Montour Falls, NY 14865
    (607) 535-6777

    Business Page:·· http://www.knightdesigns.com
    Personal Page:··· http://www.lightlink.com/dream/chris
    Designs Page:··· http://www.lightlink.com/dream/designs
    ·
  • LoopyBytelooseLoopyByteloose Posts: 12,537
    edited 2004-10-29 15:23
    I saw a great little printed circuit board that plugs into a breadboard and provides the regulated +5. It provided a barrel plug, a 7805 with filter caps, an on/off switch, and an LED indicator.

    Since the S&H to Taiwan is too much, I just made one for myself.

    This eliminated THREE problems.

    1. you do not have to solder those Banana Plugs onto a wall wart - just plug and go.
    2. Reverse polarity is impossible[noparse][[/noparse]unless you change wall warts]
    3. The short lengths of wire on the regulated supply are where they should be.
    ·[noparse][[/noparse]the input filter capacitor is at the end of the long wire and you do not have a long lead on the regulator side. ]

    You can buy one for about $10 [noparse][[/noparse]I think it is at the Athena PIC web site -- or just build your own.

    Personally, I think those banana plugs are very retro 1950s sci-fi stuff.

    I didn't buy the board that provides them either. It looks equally hokey with its lables and wire trays.

    I just bought the breadboard modules AND a one sided, unetched copper-clad printed circuit blank.
    You can just stick the modules on the copper with the provided double adhesive tape and have the noise sheilding you want.
    If you are really concerned about noise you can solder a jumper from the copper to the ground side of voltage supply for further shielding.

    There you go. Clean, cheap, and DIY.

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    G. Herzog in Taiwan

    Post Edited (Kramer) : 10/29/2004 3:26:28 PM GMT
  • Chris SavageChris Savage Parallax Engineering Posts: 14,406
    edited 2004-10-30 04:21
    Okay, I tested my old Regulated Power Supplies, which are using Transformers from Radio Shack, one 300mA, and one 2A.· The supply is +/- 5VDC & +/- 12VDC.· Negligable ripple...

    Following m_fabio2's suggestion on PC Power Supplies, I went ahead and tested the 5 volt line on one and found it to be around the same...Negligable in terms of ripple/noise...It was a MicroATX 180 Watt Supply.· These are alot smaller than standard PC Supplies.



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    Chris Savage

    Knight Designs
    324 West Main Street
    P.O. Box 97
    Montour Falls, NY 14865
    (607) 535-6777

    Business Page:·· http://www.knightdesigns.com
    Personal Page:··· http://www.lightlink.com/dream/chris
    Designs Page:··· http://www.lightlink.com/dream/designs
    ·
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