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Using Transistor With LM317 for increasing current

Discussion in 'General Electronics Chat' started by Ayne, Dec 5, 2006.

  1. Ayne

    Ayne New Member

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    I am using LM317 and taking about 1 Ampere from it's output now i want to increase output current about 4 Ampere.
    In the circuit below,
    i have attached a transistor 2N3055 for increasing current.
    Will this work?? or any suggestion.

    And
    I also want to connect a voltage indicator that will display output voltage comming from Power supply.
    Supply Range 1.5 ~ 30 Volts DC.
    Plz tell me the easiest and the cheapest way for doing this???
     

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  2. stevez

    stevez Active Member

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    Keep in mind that if you plan on using the supply at 1.5 volts and 4 amps, that you'll need to dissapate quite a bit of power - most in the 2N3055 but some in the LM317.

    Is the 30 volt input well filtered?
     
  3. audioguru

    audioguru Well-Known Member Most Helpful Member

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    Hi Ayne,
    Your circuit will have poor voltage regulation because the output voltage is not fed back to the LM317 for error correction.
    Also, the LM317 regulates very well. then the very high value of C1 in your circuit isn't needed.

    You should look at the datasheet for the LM317. It shows that your R2 should be a max of 120 ohms and the extra-priced LM117 should be 240 ohms max, or some of them will have their output voltage rise without a load.
    The datasheet also shows a High Current Adjustable Regulator circuit but it uses National's expensive unusual transistors.

    The datasheet for the 78xx fixed voltage regulator shows a similar but simple circuit for a High Current Regulator using a booster transistor. You could connect a booster transistor to the LM317 the same way. Then you will have high current with excellent voltage regulation.
     
  4. dave

    Dave New Member

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  5. justDIY

    justDIY Active Member

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    or, leave the ancient LMx17 family behind and step up to the more modern LM1084 ... drop in replacement for the x17, but with significantly lower dropout voltage, and greatly enhanced power handling ... the TO220 packaged 1084 is rated at 5 amps.
     
  6. eblc1388

    eblc1388 Active Member

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    Will LM1084 works? What about max input/output voltage differential?

    The LM317H is rated at 60V and the normal LM317 is 40V.
     
    Last edited: Dec 5, 2006
  7. audioguru

    audioguru Well-Known Member Most Helpful Member

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    Nope. It has a plastic package and is too small for high power. A big metal-cased power transistor is needed to increase the power of an LM317.

    The LM1084 has a max dissipation rating of 37W on a huge perfect heatsink but you will be lucky if it doesn't overheat (shutdown) if it dissipates 27W. With a 30V input and a 5V output then its max temperature is reached with a load current of only 1.08A.
     
  8. Ayne

    Ayne New Member

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    Infact i want to charge 12 volt and 24 volt DC batteries and i have a variable power supply (usning LM317) for my work bench. I want to increase it's currnet.

    Second qestion
    I also want to connect a voltage indicator that will display output voltage comming from Power supply.
    Supply Range 1.5 ~ 30 Volts DC.
    Plz tell me the easiest and the cheapest way for doing this???
     
  9. sam2

    sam2 New Member

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    What about a LM 338, good for 5 amps( TO-3) ? The data sheet shows 5,10. and 15 amp regulators using 1,2 and 3 LM338 together.
     
  10. audioguru

    audioguru Well-Known Member Most Helpful Member

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    The LM317, LM338 and LM350 adjustable regulators reduce their output current if there is more than 15V across them to protect themselves.
    The circuit with three LM358 regulators has 0.1 ohm output resistors to reduce their regulation so that their output voltages are nearly the same. It also uses an obsolete opamp that might be difficult to replace because its inputs work when at the supply voltage.
     
  11. sam2

    sam2 New Member

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    Audioguru knows far more than I ,but, I have built a power supply using a single LM338 and it supplies at least 5 amps at over 30 volts, So at least that should work for you.I have had no problem with heat , but I used a large heat sink. There are a few schematics if you look around or use the data sheet.
     
  12. stevez

    stevez Active Member

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    I have wondered if it would make sense to provide some kind of pre-regulator or pre-voltage drop ahead of the regulator on a variable supply -in order to limit the voltage drop across the regulator easing the burden.
     
  13. audioguru

    audioguru Well-Known Member Most Helpful Member

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    This regulator is floating so it can have a high output voltage and an input voltage a little higher. Then it will have only a few volts across it so it will have max output current and low heat dissipation.

    The problem here is that a 30V input will be used to charge a 12V battery so the regulator will have at least 16V across it and maybe 5A= 80W of heat.
     
  14. sam2

    sam2 New Member

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    I used a center tapped transformer for high/ low settings.
     
    Last edited: Dec 5, 2006
  15. Hero999

    Hero999 Banned

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    Why mess around with a circuit that will give you poor performance when the real circuit is on the LM317 datasheet?
     
  16. sam2

    sam2 New Member

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    Probably because I dont know any better:)
    I wanted a battery charger/power supply that I could charge 1-5 cell lipoly batteries at up to 4 amps.
    I found the schematic on line, and the reason given for using the center tapped transformer( high / low voltage) was so the LM338 didnt have to dissipate all that energy at low voltage and high current.
    Is it a crappy design? I dont know ? It seems to work well enough.
     
  17. Ayne

    Ayne New Member

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    Is there any solution with using transistor & LM317
     
  18. audioguru

    audioguru Well-Known Member Most Helpful Member

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    You can use the 1st circuit you posted. Move the 1000uF capacitor to the 30VDC input if there isn't enough capacitance there.
    The circuit will have lousy voltage regulation but lead-acid batteries don't care. The 2N3055 needs a huge heatsink and maybe a fan because 80W is a lot of heat for it.
     
  19. Ayne

    Ayne New Member

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    I have two 3300uF capacitor for filtration after bridge. It means total 6600uF.

    What u mean.
    Can u explain what is the negative factor of the circuit that is in 1st post??
     
  20. audioguru

    audioguru Well-Known Member Most Helpful Member

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    Then the ripple will be about 4V p-p with a 4A load.

    The LM317 regulates very well if its R2 in your circuit is 120 ohms or less and its input voltage is 2.5V or more higher than its output. But the base-emitter voltage of the 2N3055 transistor will increase when the load current increases so the output voltage will drop about 0.4V up to 0.9V when a 4A load is connected.

    The circuits that use a booster transistor have the LM317 regulating the output voltage much better.
     
  21. Ayne

    Ayne New Member

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    I found this circuit on LM350 datasheet.
    What r ur views about this circuit??
    Can i use any opamp???

    If some one already have make a PCB layout of it then plz share.
     

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