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Voltage regulators

Discussion in 'General Electronics Discussion' started by davelectronic, Aug 23, 2011.

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  1. davelectronic


    Dec 13, 2010
    I have a problem.
    Any ideas as to why would be appreciated.
    Any way i have built this circuit twice now, first time i thought it was my error.
    The circuit consisted of two 78S12CV 2amp voltage regulators, whats happened i belive is they wont share the input supply.

    I have built the same circuit with each regulator having it own rectifier and filter capacitor, so the pair did not parallel until the final isolation diodes, this circuit works fine, line and load are the same between regulators.

    I set out to build a second circuit, but this time i thought the regulators would share the same source, so my first attempt burnt out one of the regulators, the second attempt did the same.

    Yes i know some say there not meant to share current very well, but my first circuit works a treat, this time round i wanted to use one rectifier one filter capacitor, i have left my ok working circuit that has all separate input, and a diagram schematic of the one ive been trying, the bottom line is why wont they share the same input ? i could use diodes on the input, but this will drop the input, ground or common has two diodes to increase the voltage to 13.20 volts, as in the working first circuits it works fine, so any one have any idea why they wont share the same rectified filtered input, mad, lost a fare few components today. Dave. :(


    This is the working ok circuit.


    I got this off the REUK site, and if followed wont work if the inputs are shared.

    This link is the origin of the schematic, any ideas as to why they wont share the input, i would be grateful to know. Dave :)
  2. Resqueline


    Jul 31, 2009
    About the only thing that could burn out a 7812 in this setting is oscillations, imho.
    I'll bet the un-decoupled common diode D (together with a common input) is making the regulators interact.
    Try putting a cap across this diode, and make sure the wiring between regulators and cap's are 2" or less.
    The 78S12's may have different behaviour/ requirements than the 78L12's in this respect.
  3. davenn

    davenn Moderator

    Sep 5, 2009
    The real problem is that you cant really parallel 2 regulators and get away with it
    Because of variations in the manufacture of the devices, one reg will end up passing all the current
    ( the other will do nothing) and it will soon expire ... as you discovered

    If you really need the extra current capabilitied there are 2 standard ways of doing it
    1) a single regulator like your 7812 and use current pass transistors... works well
    especially for really serious current requirements. I built one supply many yrs ago with a single 7812 and
    5 x power transistors, it could supply 35Amps

    2) if you only need 1 - 5 amps use a LM338, a hi current, adj regulator
    a very easy low component count circuit

  4. davelectronic


    Dec 13, 2010
    Yes thanks for your input, i had thought the 78S12CV might have different characteristics than it 1 amp cousin, as i say ive paralleled two but they had completely separate inputs from two rectifiers and two filter caps.

    Well i doubt i will try it from a single input again, components over here in the UK are expensive with VAT at % 20, ive used the LM338K and the LM338T yes both of them are fine, ive yet to try the pass transistor circuit, iam thinking of using the 2N3055 TO3 epitaxial,
    Or the TIP2955 TO-3P , the pass transistor is an appealing option, expect i will try that next, yes i know the two transistors are NPN and PNP configuration. Thanks for the reply's. Dave. :)
  5. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    The important difference is that with the first you lose current limiting and over temp shutdown, with the latter you don't (although your max current will be lower)
  6. davelectronic


    Dec 13, 2010
    Hi there Steve.
    The pass transistor i think you mean, i would not drive the circuit beyond its rated current, so current limiting not an issue, as with over temp shutdown i wont drive the circuit to the max, but i could knock up some thermal shut down control, current limiting not really powering RF linear amplifiers, but the circuit i built would meet the amps required power needs, plus a bit in reserve. Dave. :)
  7. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    Yeah, sure. I was mostly adding the comment for others who might look at this thread in the future.
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