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Series regulators in parallel

Discussion in 'General Electronics Discussion' started by Rory Starkweather, Apr 21, 2015.

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  1. Rory Starkweather

    Rory Starkweather

    Nov 13, 2014
    I was looking at an Application Note from Linear Tech. It suggested that connecting series regulators in parallel was more difficult than it looks. I'm wondering why this would be true.

    I received 10 LM7809 in the TO-220 case today. I had planned to use two of them, in parallel, to power a 30 meter HAM radio receiver with several add-ons. The power supply is a 12 vdc, 1 Amp wall wart, and after I build this, I just want to forget it. I don't want any overheating problems.

    I realize that the 7809 is rated for one Amp, but I'm paranoid.

    The App Note mentioned an LT series regulator designed to be operated in parallel. In the schematic each had a 10mOhm resistor between the Ref pin and the output.Suppose they used 1 x 10mOhm resistor with both the references connected to it? The current wouldn't be a really big deal. Manufacturing differencs in 2 resistors might be , though.

    Any thoughts on this?
  2. davenn

    davenn Moderator

    Sep 5, 2009
    just use a LM 338K its good for 5A :)
  3. BGB


    Nov 30, 2014
    I have wired a pair if LM7805s in parallel before, and haven't noticed any big difference between one and two of them.

    with one of them, it gets hot, and may eventually go into thermal shutdown.

    with two of them, both get hot, and thermal shutdown seems to happen after nearly the same amount of time as with a single regulator.

    my guess is maybe it works if you need more maximum amperage or similar, but otherwise, using a bigger heatsink and a fan on a single regulator seems a lot more effective IME.

    no real idea why this seems to be the case... (it seems a bit weird).
  4. Arouse1973

    Arouse1973 Adam

    Dec 18, 2013
    Hi Rory
    What LT device were you looking at? I think the small value resistors you are referring to are called ballast resistors and allow for equal current sharing within say 90%. Because the output voltages will be very close unlike the older regulators, you can get away with smaller ballast resistors. For instance the LT3080 will have a output voltage of +/-2 mV if two are used running off the same voltage set resistor. This allows them to use a 20 milliohm resistor at 1 A and only drop 20 mV on the output. The problem with the older regulators is you would need quite a large resistor in series with the output to share the current fairly equally between them. This obviously reduces your maximum current output and wastes power due to the resistors.
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