Designing a Voltage Regulator

Discussion in 'The Projects Forum' started by darin_cheong, Nov 20, 2008.

  1. darin_cheong

    Thread Starter New Member

    Nov 20, 2008
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    Hi,

    Does anyone have any regulators or circuits to recommend a design of a current/ voltage regulator that is able to produce a Current with a range of 0mA to 20mA and a Voltage range of 0V to 5V. Adjustments of the values can be done using a potentiometer.

    regards
    Darin Cheong
     
  2. Wendy

    Moderator

    Mar 24, 2008
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    Actually it is pretty easy, but it might help if we know the application?
     
  3. darin

    Active Member

    Nov 20, 2008
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    Sorry was having problems with my log in account.

    The application is more for an Analogue Input module which has a input voltage of 0V to 5V and a 0mA to 20mA. I have a few common regulators like the LM317 and the 7805. But i need help in setting to how i can bulid the circuit and also able to adjust the current range from 0mA to 20mA.
     
  4. beenthere

    Retired Moderator

    Apr 20, 2004
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    The 7805 can easily give the 5 volts, and the data sheet on the LM317 shows how to configure it as a current regulator.
     
  5. SgtWookie

    Expert

    Jul 17, 2007
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    The LM317T/K has a minimum current specification of 10mA, max of 1500mA.
    The LM317L has a minimum current specification of 5mA, max of 100mA.

    An LM317's minimum output voltage is the same as it's Vref, which is nominally 1.25v, but can vary from 1.2v to 1.3v. You CAN get it to go to 0v, but you will need a -1.3v source.

    If you are flexible enough in your specifications to allow from 1.3-5v output and 5mA-100mA current, the LM317L could be used - providing you had a minimum 10v 100mA source.
     
  6. darin

    Active Member

    Nov 20, 2008
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    Hi,

    Thanks alot for the information. I am testing out some applications on the 7805 and trying to see if i am getting any results from it.

    Currently i am trying out the high current volatge regulator and the High output current with short circuit protection while using a potentiometer at the load to adjust the current and the voltage. Any feedback u guys can give me with circuit i am testing. (Heard something about High Current that can cause the potentiometer or the LM7805 to burn out)

    Anyway for the LM317, what are the recommended application that will fit the requirement of 0V to 5V and 0mA to 20mA like what you have mentioned.

    http://www.national.com/ds/LM/LM117.pdf (I have the LM317T available at the moment)

    regards
    Darin Cheong
     
  7. Wendy

    Moderator

    Mar 24, 2008
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    The problem is those parts can't go to 0 volts or 0 current. It can be done with descrete circuitry, but the minimums with a LM317 as a voltage regulator or current regulator (it can do both) is 1.25V or 10ma, as stated above. Want to see some alternates, or do you want to experiment with this first?
     
  8. darin

    Active Member

    Nov 20, 2008
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    Thnaks alot for your help!! I will definitely love to see some alternatives that you can provide.
     
  9. Wendy

    Moderator

    Mar 24, 2008
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    Here is something I sketched up a while back. Never built it, but it should work. Vcc should be regulated, and you can put a variable regulator (which I will add later) after it.

    [​IMG]
     
  10. darin

    Active Member

    Nov 20, 2008
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    Hi thanks for the diagram. But what are the recommended values which is needed for the devices in order to provide a 5V and a current of 20mA.
     
  11. Wendy

    Moderator

    Mar 24, 2008
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    I'll draw something out when I have a chance, but I suspect other people will have good ideas also. How knowledgable are you in electronics?
     
  12. mik3

    Senior Member

    Feb 4, 2008
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    You cant provide any voltage and any current you want on a load. If you regulate the current then the voltage across the load will depend on its impedance. If you regulate the voltage the current through the load will depend on its impedance. You can regulate one at a time but not both.
     
  13. darin

    Active Member

    Nov 20, 2008
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    I have been trying out this research for quite sometime, but according to ohm's law i have dsicovered that i am not able to get any the current ranges (0 to 20mA) and the voltage ranges(0 to 5v) that i want at the same time unless i am having 2 regulators. A separate current regulator and a separate volatge regulator.

    But seeing some application by using regulators, i have seen application that are able to produce the ranges of volatges and current values which i need. I was just wondering if there is any way of any calculations or just by replacing resistances value will help.
     
  14. SgtWookie

    Expert

    Jul 17, 2007
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    See the attached; it's a slight variation on Bill Marsden's constant current schematic. A resistor was added below the pot to provide a limit to the maximum current possible.

    It provides a constant current variable from nearly 0mA to 20mA. It's not really possible to get ALL the way to zero current due to leakage via the transistor, but it's on the order of microamps.

    If you download LTSpice/SwitcherCad from Linear Technology's website, you will be able to experiment with the simulation I created.
     
  15. Wendy

    Moderator

    Mar 24, 2008
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    Thanks Wook. That is half of it. I'll sketch up a voltage regulator using something like a LM324 and a transistor.

    Thing to remember is Mik3 is correct also. A circuit takes the current it needs, you can't force it to use an exact current except by design of the load circuit.

    What this circuit will do is provide the voltage required unless the current setting is exceeded, at which point the current regulator takes over and the voltage drops to what ever is required to provide the current setting.

    This is basically a course in Power Supply 102 (the one after 101).

    *************************************************

    I'll be adding values for the parts in a bit.

    [​IMG]

    Parts List:

    U1 : 7809
    U2 : LM324
    Q1 : 2N2222
    Q2 : 2N2907
    C1 : 100µF
    C2 : 0.1µF
    CR1,3,4 : 1N4001
    CR2 : 5.1V Zener
    R1 : 91Ω ¼W
    R2,5 : 2KΩ Variable
    R3 : 6.2KΩ ¼W
    R4 : 390Ω ¼W
    R6,7 : 10KΩ ¼W
     
    Last edited: Nov 23, 2008
  16. Wendy

    Moderator

    Mar 24, 2008
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    Logic check, the op amp will be trying to source voltage through the BE of Q2. Does this mean that the voltage regulator should be in front of the current limiter?

    I think I'll start another album for more complex circuits that are likely to work, in addition to the concept stuff.
     
  17. mik3

    Senior Member

    Feb 4, 2008
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    Bill,
    After a quick though, I believe the circuit will work as it is.

    If the load current is less than the current the current regulator is set to regulate, transistor Q1 will fully conduct trying to reach the value of its set current. Thus, the current regulator in this case does nothing and the voltage regulator takes over and the current through the load is determined by the output voltage of the voltage regulator and the load resistance.
    However, if the load tries to draw more current than the current regulator is set to regulate, the current regulator will keep it fixed at the set value. Also, the voltage regulator's transistor Q2 will conduct more trying to increase the current through the load and regulate the voltage across it. But because the current is held fixed by the current regulator, the voltage regulator transistor Q1 will fully conduct (in its try to keep the voltage fixed) and thus the current regulator takes over.
     
  18. darin

    Active Member

    Nov 20, 2008
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    Hi, thanks for the information. It is really useful. I will try getting it bulit and tested. But another question is it possible if i made use of a addressable potentiometer DS1805 to connect to one of the potentiometer in the circuitry so that i am able to communicate the circuit with a microcontroller.

    Making use of a ATMEGA32 using 2 wire serial communication with a DS1805 potentiometer which is 10K Ohms and have +-20% tolerance
     
  19. Wendy

    Moderator

    Mar 24, 2008
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    I'm not familiar with the DS1805, but if it is a potentiometer the voltage side won't care. Let us know how it works out.
     
  20. darin

    Active Member

    Nov 20, 2008
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    Ok thanks alot, will get it tested out..
     
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