High Current Linear Regulators

Discussion in 'General Electronics Chat' started by Wendy, Feb 14, 2010.

  1. Wendy

    Thread Starter Moderator

    Mar 24, 2008
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    This is a hypothetical project, but we keep getting requests for voltage regulators in the 10 amp range. I'm not excepting myself, eventually I'll be building something along those lines.

    One of the first thoughts I had was something like this. It would not be totally stable, since as the transistor heats up (at 10A with any voltage drop it will need lots of heatsinking) it will drop more voltage across the BE junction. The current limiting will disappear, and a lot of safety circuits protecting the load besides.

    It is simple however, and should work.

    [​IMG]

    A better approach may be to put the transistor right after a standard LM317 regulator, it occurs to me the variation in BE voltage will be multiplied, whereas a simple voltage follower will only drift as much as the transistor.

    [​IMG]

    Ideas from other folks?
     
    Last edited: Feb 14, 2010
  2. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    Here are some circuits as used in the 78XX series datasheet:

    [​IMG]

    Greetings,
    Bertus
     
  3. t06afre

    AAC Fanatic!

    May 11, 2009
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  4. Wendy

    Thread Starter Moderator

    Mar 24, 2008
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    I remember that chippie, even used it. Didn't like it much though, too many parts overall, 3 terminal regulators have spoiled me forever. :D

    It is definitely in the running however. You see them used a lot in elpacs.
     
  5. SgtWookie

    Expert

    Jul 17, 2007
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    One of the big problems with these types of projects is making the novice appreciate the power dissipation issue.

    Building a 0V-30V 10A supply doesn't sound terribly hard at the outset, until they can finally realize the power dissipation in the regulation stage at low output voltages will be over 400 Watts! :eek: (assuming the filter caps have an average 40VDC with ripple)

    You'd need a heat sink the size of a Buick V6 engine to keep the transistors from vaporizing - or lots of forced cooling with plenty of surface area.

    There are very old application notes in National Semiconductor's LM117/LM317 datasheet for higher output supplies. However, if someone really needs more than a couple of amperes out over a wide range, they really should be looking at a switching supply.

    The cheap, easy, quick and efficient way to go is to simply convert an ATX form factor supply to a bench supply; it's an afternoon project, and maybe $20 in parts.
     
  6. Wendy

    Thread Starter Moderator

    Mar 24, 2008
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    I understand what your saying, but there are cases where you can connect a transistor to a massive heat sink, and it won't be a problem. Given that the cooling issue is solved (remember, hypothetical design), how would you design a really simple stable power supply?
     
  7. MikeML

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    Oct 2, 2009
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    A long forgotten trick is to use a series/parallel switching in the primary/secondary of the power transformer to keep the unregulated voltage ahead of the regulator at the minimum required to keep the regulator from dropping out. I seek out transformers which have lots of taps on the primary and secondary. A PIC could automate some relay switching.

    I still use a 15A, 3V to 20V uA723 2n3055s linear supply with fold-back current limiting that I originally built in the 1970s which uses a small Variac on the primary of the power transformer. I added a voltage comparator circuit that automatically detects if the voltage at the input of the regulator is set too low. This is my feedback to turn-up the Variac. However, this lets me set the supply up to deliver say 10A at 12V while keeping the dissipation within reason; not needing fans...
     
  8. k7elp60

    Senior Member

    Nov 4, 2008
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    I have built a lot of linear power supplys. I have used three terminal regualators like the 338K and a little known LT1083CP(adjustable @ over 7A). For a fixed supply or one that the voltage doesn't have to vary much they work great. If they are configured to have a wide adjustment of voltage, the internal circuitry will shut them down even when the output voltage and current are within limits.

    I am like mike, I like the 723. I use an NPN darlington so I can regulate more current with out an additional driver for the 723. I also like mikes idea of using the CT grounded for lower voltage and more current. Full bridge for the higher voltage and less current.

    On other thing to consider with brute force filtering(rectifier connected to the rectifier)is the maximum continious current one should not exceed is 0.566 of the RMS current for the total load current. Doing so can overheat the transformer, as the charging current for the filter can be excessive.
     
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