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Simple voltage regualtor

Discussion in 'Electronic Basics' started by Captain Dondo, Oct 24, 2003.

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  1. I need to power some meters in my car. They require 5 VDC at < 50ma -
    some as low as 5 ma. I need less than 200 ma total.

    I've looked at the various 5vdc regulators, but all seem to say then need
    something like 7 VDC to produce 5vdc. Is it safe to run them with 13.8
    (or even higher) voltages like you might see in a car? Charging systems
    can produce - what - 14.5 VDC or something like that.

    Any basic circuits or guidance would be welcome.


  2. Almost all 5 volt regulators can handle 15 volts in, as long as you
    heat sink them well enough to carry away the waste heat. a 10 volt
    drop and a .2 amp current through them will produce 10*.2=2 watts of

    Since the battery voltage can have big, nasty spikes, occasionally, it
    is also a good idea to put a spike clamp (18 volt, 1 watt zener diode)
    across the input (after the fuse) and an inductor (a dozen turns
    through one of those big ferrite beads they put on monitor cables
    works well) or low value resistor (10 ohms 1 watt, say) in series with
    the positive supply line, followed by a big capacitor (220 uf, say) to
    ground. Add a .1 uf mylar cap across the 5 volt output right at the
    regulator, also.
  3. The common three terminal regulators like the 7805 need _at least_ 7
    volts to operate correctly, and will survive (and work correctly) with
    input voltages up to 35 volts or so.
  4. Baphomet

    Baphomet Guest


    An LM309 can handle up to +35 volts input at 1000 ma. output
  5. If you want to avoid some of the heat dissipated by a linear regulator, use
    a switching regulator, like the ones at

    They have ones that will take you from 12V to 5V, which would probably be
    what you want.

    There are lots of other manufacturers as well. You may be able to get these
    parts mail-order at or or

    You will probably need a few external components as well (unlike a 7805...)

    Agree with advice about using zener clamp for spike supression.

    Bob Monsen
  6. They work fine up to their maximum, which is greater than the car
    voltages. or and search for
    the data sheet for LM7805 etc.
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  7. Everything else I understand, but.... Connect the zener from hot to
    ground? Is that what you're saying? And if so, which end goes where?
    Shouldn't there be a resistor in line with the zener to limit current? I
    must not understand how zeners work (no surprise there, it's been 20+
    years since my high school electronics....)
  8. The striped end (cathode) goes to positive. Those transients can
    produce high voltages, but the total energy in them is not very
    large. The zener can survive them with only the resistance of a fuse
    in series. The zener will shunt all currents trying to raise the
    voltage above +18 or below -0.7
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