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5v Supply Circuit — Parallax Forums

5v Supply Circuit

idleupidleup Posts: 46
edited 2005-10-17 22:43 in General Discussion
I am looking to make a 5v supply for my LCD. I have found two ways of doing it.

1. Use a 7805 with CAPS to stabilize current surges

2. Use a LM317T Variable Voltage Regulator IC

I am new to this, I am sure both would work fine but which is the way an experienced electronics guru would use? or is there even a better way?

- Matt

Comments

  • Jon WilliamsJon Williams Posts: 6,491
    edited 2005-10-17 21:36
    You might consider an LDO regulator like the LM2940-5 -- it's what we use on our boards. In the downloads section of our web site you'll find schematics for various boards and you can lift a supply design from one of them.

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    Jon Williams
    Applications Engineer, Parallax
  • idleupidleup Posts: 46
    edited 2005-10-17 21:54
    That looks simple enough, couple questions though:

    1. Are the capacitors between VIN and GND and then again between VOUT and GND to keep a constant/steady voltage and current as the circuit fluctuates a load?

    2. When and LED is shown in a schematic like that and just says "Resister LED" what kind of LED should it be? I know they come in different voltages, etc. Would I assume a 5v LED?

    3. Is the output current the same as whatever the input current is?

    Is there a resource for looking up schematic symbols? I am new to this and I notice just on the BS2 Demo board schematic there are two different symbols for Caps. One is parallel lines and one is a line with a smile face next to it.



    - Matt
  • Jon WilliamsJon Williams Posts: 6,491
    edited 2005-10-17 22:23
    1. We use big caps in the supply because a lot of those boards are connected to robots that can have spikey loads (starting a servo, for example). This keeps the 5v side from sagging too much and prevents the BASIC Stamp from inadvertently resetting.

    2. That's a component designed for 5 volts -- you could safely use a standard green LED and a 470 ohm resistor (will consume about 7 mA -- use a bigger resistor to reduce the current [noparse][[/noparse]LED will be less bright]).

    3. The output current will be whatever your circuit consumes, up to what the regulator and input supply are capable of providing.

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    Jon Williams
    Applications Engineer, Parallax
  • ElectronegativityElectronegativity Posts: 311
    edited 2005-10-17 22:43
    Hi Idleup, I had similar questions not long ago.

    This thread has an image attachment at the bottom of one of Guenther's posts that shows a circuit to use with a 7805:

    http://forums.parallax.com/showthread.php?p=546903

    Here's a thread where questions about resistors and LED's are adressed in detail:

    http://forums.parallax.com/showthread.php?p=548249

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    I wonder if this wire is hot...
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