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linear voltage regulator

Discussion in 'General Electronics Discussion' started by milen, Dec 20, 2009.

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  1. milen

    milen

    17
    0
    Dec 20, 2009
    I am building simple PIC based PCB and I need proper power supply for it.
    Can I use L78M05CV 500mA, 5V votage regulator?
    Can I connect this IC like below schematics from datasheet shows?
    Do I need any additional capacitors...?
    [​IMG]
     
  2. Resqueline

    Resqueline

    2,848
    2
    Jul 31, 2009
    Q1: yes.
    Q2: where? Vi or Vo?
    Vi: what do you intend to run it off? A transformer & rectifier requires a 470uF cap.
    Vo: the answer is in the PIC datasheet.
     
  3. milen

    milen

    17
    0
    Dec 20, 2009
    input will be 12V car's battery. I intend to power up PIC16Fxxx and few ICs (1 darlington array and 2 multichanel optocoupler) plus three relays (each 40mA @ 5V)
     
  4. Resqueline

    Resqueline

    2,848
    2
    Jul 31, 2009
    Ok, you won't need much more then. The PIC seems to want a 0.1uF decoupling cap also, and this can be shared with the 0.1uF that the regulator wants - if in close proximity to each other. But it never hurts to add a couple of extra caps, like ten times bigger.
     
  5. milen

    milen

    17
    0
    Dec 20, 2009
    100nF capacior is on the MCU socket allready.
    Can you tell if this configuration will work on a car electrcal system.
    What I am worry about is spikes at engine startup and when turning the xenons on.
     
  6. Resqueline

    Resqueline

    2,848
    2
    Jul 31, 2009
    Ok. If you add an 1N4000 diode in series with the power input and a big cap in parallel with the 0.33uF you'll be protected against most dropouts, and any positive spikes won't ever reach the 35V limit of the 78M05. Car power is rarely that bad.
    Beware however of possible noise inducing ground loops, where you take power ground from one place and have a sensor (for example) ground at another place.
     
  7. milen

    milen

    17
    0
    Dec 20, 2009
    will this 1N5347BG PCB zener also be ok?
     
    Last edited: Dec 24, 2009
  8. Resqueline

    Resqueline

    2,848
    2
    Jul 31, 2009
    Zener? What zener? What for?
    I can't imagine you'll ever need one for this power.
    If you are dead set on transient protection you'll have to use transient protection diodes, not zeners.
     
  9. neon

    neon

    1,325
    0
    Oct 21, 2006
    Why a diode in series what for? A zener in series make more sense to me limit the positive source a .7v does nothing
     
  10. Resqueline

    Resqueline

    2,848
    2
    Jul 31, 2009
    I never claimed the need to limit the positive source.
    The diode I suggested is beneficial to prevent this supply's capacitor from being quickly drained by actions like starting the engine.
    This way the cap is kept charged to the positive spikes instead of being drained by negative-going spikes.
     
  11. alex88

    alex88

    8
    0
    Jan 20, 2010
    if this is used in the car, I would suggest to use fuse rather than diode for protection.
     
  12. Resqueline

    Resqueline

    2,848
    2
    Jul 31, 2009
    What's the fuse supposed to protect? And why not a diode?
     
  13. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,477
    2,820
    Jan 21, 2010
    My recommendation would be a diode in series with the +12V from the battery and a series resistor calculated to drop about 4 volts at the max current expected from the circuit. Using a 330uF filter capacitor (which I think was recommended earlier, the time constant should be long enough that the capacitor will not charge to above 35 volts during the duration of a short voltage spike. The capacitor should be rated for 35V (approx).

    For additional protection, you could connect a 30V zener across the filter capacitor. The power rating of this will and the series resistor will depend on the magnitude of spikes you're expecting and their duration. As someone suggested, purpose designed transient supressors may be a better option, but they may be harder to source than a zener.

    A disadvantage of this circuit may be the relatively slow rise time of the 5V rail. The solution to this depends on what you're circuit is doing and how it reacts to "brownout" conditions. Another disadvantage is the ability to work with a highly discharged battery (i.e. do you want it to continue to operate if the battery voltage falls to 10V?).

    If those are significant issues, then I would replace the resistor with an NPN transistor having a Vce > 100V used as a pre-regulator. connect the collector to the cathode of the diode, the emitter to the 330uF capacitor. Connect a "suitable" resistor from collector to base and a zener diode (say, 8V) from base to ground. This transistor will regulate the voltage to the main regulator to a max of just under the zener voltage. The resistor from base to collector needs to supply sufficient base current to the transistor, but not so much that you waste significant power in the zener during large spikes.

    Depending how well regulated you need the output voltage to be, you can use a 5.6V zener and dispense with the three terminal regulator. The component count will be similar, but the line and load regulation will be relatively poor.
     
  14. Resqueline

    Resqueline

    2,848
    2
    Jul 31, 2009
    Those are very good recommendations. So good that they'll practically allow you to power a circuit safely straight from the coil.
     
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