LM317 vs. 7803

Discussion in 'General Electronics Chat' started by TexAvery, Apr 19, 2010.

  1. TexAvery

    Thread Starter Member

    Oct 7, 2009
    When building a 3v power supply, why not use a 7803 voltage regulator in place of a LM317?
    Using a LM317 requires 2 to 3 resistors, 7803 does not.
    Am I missing something?
  2. kingdano

    Senior Member

    Apr 14, 2010
    The LM317 is a linear regulator - so depending on your input voltage, you could dissipate a lot of heat through it, and waste power.

    The 7803 is a switching regulator - which is an entirely different topology for voltage regulators. Switchers often inject high frequency noise into surrounding circuitry.

    Are you doing this for work? Project? Fun?

    @Sgt Wookie


    Its the first and second devices listed - one for +3.3V the other for -3.3V
    Last edited: Apr 19, 2010
  3. SgtWookie


    Jul 17, 2007
    I think you meant 7805? I haven't seen a 7803.

    The last two digits indicate the voltage.

    The 78xx series regulators are fixed, the LM317 is variable.

    You CAN increase the output voltage of the 78xx series by adding resistors.

    The LM317 has somewhat better regulation and higher current rating than the 78xx series. If a 78xx series regulator's output happens to be higher than what you want (due to factory tolerances) you are stuck. With a LM317, you can adjust the output right where you want it; anywhere from about 1.2v up to around 37v, depending on input voltage, current and thermal limitations.

    Note that the 78xx series regulators require a 0.33uF cap on the input to ground, and an 0.1uF cap on the output to ground, as close to the regulator as you can get them. If you omit these caps, you may have instability problems (high frequency oscillations and reduced output voltage). You can add more capacitors if you like, but you must have those two present.

    Note that the 78xx series regulators must have at least 5mA load on the output before guaranteed line/load regulation is achieved. With the LM317, you can build in the minimum 10mA load by using a 120 Ohm resistor for R1.
  4. TexAvery

    Thread Starter Member

    Oct 7, 2009
    7803 is what I asking about, but now I see a BIG price difference compared to LM317.
    Work? = No, I am not getting paid
    Fun? = No, when finished there will be no amusement.
    Project?=Yes, you have to learn to stay young.
    I am driving a PIR sensor for a scopeclock.
  5. retched

    AAC Fanatic!

    Dec 5, 2009
    Last edited: Apr 19, 2010