Home made Lab power supply

Discussion in 'General Electronics Chat' started by riko, Dec 12, 2014.

  1. riko

    Thread Starter New Member

    Sep 7, 2014
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    Hello everyone.
    I am working on a project that aims to create Lab power supply, with the ability to adjust the output voltage and current. I was able to make the schematic to adjust the voltage at the output, but I encounter some problems creating the current regulator. I hope You can help me to achieve the current regulation. I need simple and reliable schematic that doesn't change critically the current one. I hope it would be possible by upgrading the schematic bellow.https://drive.google.com/file/d/0B5Y7MPFIDn9dZzh0LW93d3JKX1k/view?usp=sharing
    Sorry about bad English
     
  2. bertus

    Administrator

    Apr 5, 2008
    15,648
    2,347
    Hello,

    The schematic does not show.
    Please upload the schematic to the forum using the "Upload a File" button.

    Bertus
     
  3. Papabravo

    Expert

    Feb 24, 2006
    10,148
    1,791
    I can see the current regulator just fine.
    Question: Are you trying to control both current and voltage at the same time?
    Question: What is wrong with the schematic that you have?
     
  4. riko

    Thread Starter New Member

    Sep 7, 2014
    11
    0
    Yes. I want to be able to adjust the current and the voltage at the same time. Let say I want to charge a battery with a constant current - for example 1A. But I don't want the voltage to exceed 5V. So if the battery is discharged I would have 1A at 4V. With the battery charging I expect the current to be a constant so the voltage would grow with the time, right? But It should not grow above certain voltage threshold, in my case 5V. When the voltage reach 5V the current should drop down with the time. This is just an example how the power supply should operate. I hope you understand me :)
     
  5. MrChips

    Moderator

    Oct 2, 2009
    12,447
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    That is how constant voltage/constant current supplies work.
    Note that constant voltage and constant current are simply limits, i.e. you do not want the supply output to exceed those limits. At any one instant, only one limit is activated, i.e. constant current or constant voltage, whichever comes first, never both at the same time.
     
    absf likes this.
  6. ScottWang

    Moderator

    Aug 23, 2012
    4,855
    767
    There are two circuits to adjust current as below, you can see the output of LM317:
    [​IMG]

    [​IMG]
     
  7. crutschow

    Expert

    Mar 14, 2008
    13,024
    3,236
    Scott I have an issue with your current limit circuit. Normally you use the voltage drop across a shunt resistor to determine the current limit point but you circuit doesn't have that, just a 20k pot. It would seem the current limit point is determined by the current gain of the two transistors for a given pot position, which is generally not desirable, since current gain has such a wide tolerance.
     
  8. ScottWang

    Moderator

    Aug 23, 2012
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    Remember that the bjts and the VR1 were in series with the output, voltage drop is not the way that I want to use, but when I adjust the base current of bjt and then the voltage drop will be happened.

    I was used two transistors as the current limiting resistor, the VR1 20K was to adjust the base current of bjt, normally when we using the resistor to limiting the current has some problems as power consumption, the power of normal pot is not enough and then we have to using wirewound resistors, it's more expensive, so I don't want to use it, and change the way to using two bjts to replaced the wirewound resistors.

    The pin1,2 of VR1 and R2 as a Rbe to limited the Ic current, if we didn't use Rbe then we need to used a more bigger resistor to limiting the base current, but I don't like that way.

    Maybe you can do the real testing whatever with breadboard or with PCB.
     
  9. riko

    Thread Starter New Member

    Sep 7, 2014
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  10. riko

    Thread Starter New Member

    Sep 7, 2014
    11
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    Absolutely. This is what I want to achieve.
     
  11. crutschow

    Expert

    Mar 14, 2008
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    The problem is, that current regulator cannot drive a grounded load, which you need.
    You could invert the circuit using a P-MOSFET to drive a grounded load.
    How much current adjustment range do you want?
     
  12. riko

    Thread Starter New Member

    Sep 7, 2014
    11
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    The load won't be grounded. I meant to implement the schematic as follows:
    +30V -> Voltage regulator -> LOAD -> Current regulator -> GND.
    I am not able to draw the schematic right now, but I willl ASAP.
    The current range is to be 0 - 2A.
     
    Last edited: Dec 13, 2014
  13. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    This is not how a bench PSU operates.
     
  14. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    You want the current regulator inside the voltage feed-back loop; not outside of it...
     
  15. crutschow

    Expert

    Mar 14, 2008
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    Note that the voltage regulator circuit you posted is simply an emitter follower with no other feedback from the output, so it will have a poor voltage regulation with changes in the load current.
     
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