Increase current output of a voltage regulator

Discussion in 'General Electronics Chat' started by Jasonh74, May 18, 2015.

  1. Jasonh74

    Thread Starter New Member

    May 18, 2015
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    Here is my scenario. I have a 12v DC power supply capable of outputting 6A. I am using this to power a led light bar and small DC water pump. I also have a battery powered aerator (running off 2xD size batteries, batteries are in parallel)
    I want to run this aerator from my 12V supply. To do this I bought a variable voltage regulator from Jaycar, put it together, verified that I can get 1.5v out of this device when it is connected to my 12V supply. Unfortunately this will not run the aerator motor.

    I have measured the current in the circuit when the batteries are powering the aerator and it was approx. 250mA. When my voltage regulator is trying to power the aerator the measured current is only 7mA. So it looks like I am not getting enough current out of my regulator.

    Is it possible to increase this current or have I done something wrong

    Thanks,

    Jason
     
  2. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    Can you post a link to the reg or its datasheet?
     
  3. Jasonh74

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    May 18, 2015
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  4. DickCappels

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    Aug 21, 2008
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    It looks like a simple LM317 adjustable regulator. Do you know how many amps you need for that motor?

    In the mean time, your solution might look a lot like this:
    [​IMG]
    There are simpler circuits without over current protection, but it only takes a moment for an unprotected power supply to become a dead power supply.


    It is the last circuit on this page:
    http://www.brighthubengineering.com...317-enhanced-power-supply-circuits-explained/
     
  5. crutschow

    Expert

    Mar 14, 2008
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    I don't see how that circuit has over-current protection. :confused:
    The extra transistors are just a compound stage to add current capability.
     
  6. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Regulated supplies are a little overkill for motors, a simple suitably sized 9v AC transformer, bridge and electrolytic are all that are needed.
    More rugged too.
    Max.
     
  7. ian field

    Distinguished Member

    Oct 27, 2012
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    Presumably the Jaycar kit has a schematic you could post, then someone might be able to advise how to beef it up.

    The method used with the TO220 style 3-terminal regulators as shown in various appnotes, is to put a current sensing resistor in series with the regulator input, the voltage developed across that turns on an external bypass transistor.

    It would save me a lot of typing if you download and study the appnotes for regulators such as the LM78xx and LM317 etc.
     
  8. DickCappels

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    Aug 21, 2008
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    You are absolutely correct, Crutschow -I had not noticed that that the two transistors were of opposite type. I was thinking of the configuration below, which is not optimum but shows the basic idea.
    correctone.png
    http://www.qsl.net/mi0rtx/12VOLT_POWER_SUPPLY.htm
     
  9. ian field

    Distinguished Member

    Oct 27, 2012
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    That configuration will have virtually non-existent regulation.

    The appnote shows a current sense resistor in series with the LM317 input, the B/E junction of a PNP external bypass transistor is biassed by the volt drop across that sense resistor.

    AFAICR: current limit can be a bit more convoluted with the external bypass.

    Not sure - but I think some appnotes show an example of the external PNP driving a bank of parallel NPN, I think that puts you back to the original position regarding current limit.
     
  10. WBahn

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    Mar 31, 2012
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    The TS says that the aerator draws about 250 mA. He is also saying that the regulator he put together from the kit is only supplying 7 mA. Seems like nearly any regulated supply should be able to deliver much more than 7 mA and, indeed, the link he provided says that the one he built should be able to deliver 1000 mA, more than enough for his purpose. So instead of switching to one of a host of alternatives, perhaps the TS needs to first try to figure out (with our assistance) why the one he built isn't putting out the current it is rated for.

    EDIT: Typo -- I said "with out assistance" when I meant to say "with our assistance". Quite a difference a typo can make.
     
    Last edited: May 20, 2015
    DickCappels likes this.
  11. WBahn

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    I would say that you have done something wrong since the link you provided indicates that the regulated supply should put out 1000 mA.

    Did you build this supply from a kit, or buy it pre-made?

    If you measure the output voltage with no load attached can you adjust the output voltage over the entire range that it is supposed to be adjustable over? Note that I'm assuming that there isn't some minimum load requirement.
     
  12. dl324

    Distinguished Member

    Mar 30, 2015
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    The LM317 has built-in current limiting. If the external pass transistor circuit is properly designed, protection in the LM317 can also protect the external circuitry.

    EDIT: The circuit in question uses the fact that LM195 are almost indestructible. So the regulator and pass transistors will both self protect.
     
  13. ian field

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    Apparently the TS mentiones a water pump and a bank of LEDs alongside the aerator.
     
  14. WBahn

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    Mar 31, 2012
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    And, yes, it would be nice to know what the total voltage/current needs of everything is. But that doesn't change the issue that he has a power supply that is supposed to be capable of putting out 1000 mA and it is only putting out 7 mA.
     
  15. Jasonh74

    Thread Starter New Member

    May 18, 2015
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    When the batteries are being used to drive the motor, the circuit is running approx. 250mA
     
  16. Jasonh74

    Thread Starter New Member

    May 18, 2015
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    This was a kit I had to assemble. With no load, the voltage is variable from 1.5V to 15V as measured by my multimeter.
     
  17. WBahn

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    Mar 31, 2012
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    Can you post the schematic, even a sketch of one, for the kit?

    How did you determine that it is only putting out 7 mA? What were the conditions when you made that measurement?

    What kind of resistors do you have available? Do you have some 100 Ω resistors (or anything close)?
     
  18. Jasonh74

    Thread Starter New Member

    May 18, 2015
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    Here is the schematic
    image.jpg

    I measured the current by putting my meter in series with the supply.

    And I found a problem. It was with the interface between the soldering iron and the pcb. AKA me. I had connected the ground for the output to the top pin on the pcb that is labelled which is also labeled grnd. But when I sat down tonight I saw that this was wrong.

    I now have the unit outputting 1.5v and 250mA with the regulator power being supplied from my 12v power supply and with the aerator not under load. When I drop the airstone into water the current goes up to 350mA but the motor stalls. I get the same response when the power source is batteries but the motor doesn't stall
     
  19. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    What is the nature of the 12V input? Is it AC. If so, C1 is mega too small.
     
  20. Jasonh74

    Thread Starter New Member

    May 18, 2015
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    It is a omegalec 240v to 12v power supply p/n sps-0512-dr
     
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