few questions regarding linear PSU

Discussion in 'General Electronics Chat' started by gigabyte091, Dec 15, 2015.

  1. gigabyte091

    Thread Starter New Member

    Dec 15, 2015
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    Hi everybody, I'm new on this forum :)

    I'm making PSU from scratch using my schematics. I plan to use 2 OP07 op-amps, one for voltage, one for current control and TL431 as reference voltage source.

    I will control output voltage using 10k pot as voltage divider connected across reference voltage and gnd, and that voltage will be sent to opamp as reference voltage.

    Is this a
     
  2. dl324

    Distinguished Member

    Mar 30, 2015
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    Welcome to AAC!

    When you complete your question, include a schematic.
     
  3. gigabyte091

    Thread Starter New Member

    Dec 15, 2015
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    Thank you :)

    uhh sorry, i forgot to upload schematics.

    Here it is:

    Shema napajanje.png

    Few notes: I didn't yet put TL431 reference as i need to install TL431 spice model.
    also i have to put reverese polarity protection diode, reverse bias protection, and pass transistor protection diode and other protections.
     
  4. dl324

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    What are your questions?
     
  5. hp1729

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    Nov 23, 2015
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    You are looking for an output of around 14 or 15 V?
    What current?
     
  6. Papabravo

    Expert

    Feb 24, 2006
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    You do know that you cannot control voltage and current independently. You can control one of them and the other is determined by the load.
    The usual way to design a power supply is constant voltage mode. You can have current limiting, which reduces the output voltage when the current limit is reached.
     
    cmartinez likes this.
  7. hp1729

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    Nov 23, 2015
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    Current limiting is not current control?
    Is current control setting the current to a specific value, Like a constant current circuit?
    I think the design intends current limiting. At a certain current level voltage is shut off. As voltage is shut off current decreases and the current limiting stuff is shut off. Current flows again, if the limit is reached it shuts off again. Current limiting but not current control?
     
    Last edited: Dec 15, 2015
  8. #12

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    Nov 30, 2010
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    Everything has a current limit, but the principle by which it works is that the voltage MUST decrease in order to limit the current. If you try to say, "I'm going to set my power supply for 10 volts and 1 amp, then connect a 100 ohm resistor as the load, you will have 1/10th of an amp flowing, not a whole amp. If you connect a 1 ohm resistor, you will have 1 amp flowing, but you won't have 10 volts, you will have one volt. You can set a LIMIT but you can't set both limits at the same time, except for exactly one resistance, which is 10 ohms in this case. All other answers are not compatible with both goals.
     
  9. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    And that is why Giga has D1 and D3 in his schematic...
     
  10. #12

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    Tell Papabravo.
     
  11. Papabravo

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    My interpretation of the TS/OPs original post was that he wanted to control both voltage and current independently at the same time. My answer to your question: "Current limiting is not current control?" is no it is not. As has been pointed out current limiting is often done by reducing (controlling) the output voltage.
     
  12. #12

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    There is more than one way to do current limiting. One way is to set a current limit and, if the load tries to use too much current, only allow the current you set as the limit. This is called a constant current supply. In this method, the voltage goes down smoothly and the current you set is what gets delivered to the load. When your load needs less than the current limit you set, the power supply goes to the highest voltage limit which you set, and stops at that voltage while delivering less current than you set as the maximum. This is a sort of automatic change over between constant voltage and constant current modes of operation.

    Another way is fold-back current limiting as shown on the LM723 datasheet. When the load tries to use too much current, the voltage is drastically reduced, but it never actually gets to zero. As soon as the excess load is removed, the voltage pops right back up where you set it.

    A third way is to detect excess current and shut the whole power supply off, like a fuse might work. This usually requires a manual reset. I have done this with an AC power supply, and it automatically checked every 1 second to see if the load was still shorted. I could probably do it with a DC supply, but I haven't tried that because the other methods work so well that I have not found a need for an automatic reset on a DC supply.

    I probably missed a few methods.
     
    Last edited: Dec 15, 2015
  13. gigabyte091

    Thread Starter New Member

    Dec 15, 2015
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    I just saw that questions are missing

    I wanted to ask where is the best to put voltage adjust pot ? in my schematics i put pot across voltage reference and feed voltage to op-amps, but is this a good way ? pot isn't temperature stable element.

    also if anybody have literature about floating regulators that would be great :)

    In my circuit, there is voltage control amplifier, and current control amplifier.

    Maximum current is 1.15A and is adjustable down to 0A

    You can call it curent limit if you want. Let say i adjust PSU to 20V and current limit is set at 500mA.

    When load current reaches 500mA voltage starts to go down as current control op amp is kicking in and regulate current. If you short output there will be 500mA at few milivolts. Power supply is working in constant current mode or in constant voltage mode.

    max output voltage will be about 25 to 30V
    max output current will be about 2A
     
  14. hp1729

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    Nov 23, 2015
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    One place to put voltage adjustment pot, on the output you have the 1K and 4.7K with the tap heading to the op amp. You could put the pot there.

    A question, both op amps have the two 1K resistors between "V Ref" and ground. Could you run them both to the same set of resistors? I assume V Ref" is Vcc?
     
  15. #12

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    The usual method is to work out the voltage controlled part first and the current limiting second because there are several ways to measure the current and you can usually find a way that does not interfere with the voltage design or it only requires a couple of details be changed to get it all working smoothly.

    I do have some problems with your first approach. hp1729 named one: dual voltage splitters when one will be sufficient. R6 doesn't even need to exist. Using a constant current supply into a zener diode when MUCH more precise voltage references are now available. A sense of proportionality like using a 5% tolerance zener diode then worrying about a 100 ppm variable resistor. You are going to learn a few things this week, but I don't know of a better place to learn them.
     
  16. gigabyte091

    Thread Starter New Member

    Dec 15, 2015
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    For the Voltage adjust instead of two resistors there will be potentiometer.

    For Current adjust, 39k is fixed. This resistor set max current to about 1.15A and 1k resistor will be replaced with potentiometer.

    Vref isnt Vcc, Vref is taken from the cathode of zener diode.

    It is good idea to put volt adjust pot on the output, leaving original divider and tapping with pot to middle point of divider but in that way i wont be able to adjust Voltage all way down ?
     
  17. gigabyte091

    Thread Starter New Member

    Dec 15, 2015
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    I will use TL431 as a reference Voltage source, as I said in the previous post as i dont have TL431 in LTspice yet, this is a Mark I or version 1.00 of the power supply hehe.

    I want to learn more :) so any advice is more than welcome.
     
  18. #12

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    You just noticed a bad design choice. Look at my blogs about voltage regulators. They show how to get to a zero output voltage and how to float the regulator. You can get to zero with an LM723, but it requires a dual voltage supply to get there. The OP-07 is an accurate chip, I just can't remember if it will work with a single supply.
     
  19. SLK001

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    Nov 29, 2011
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    You should think about parallelling two 2N3055s as your pass transistor. Tie the collectors and the bases together, and add a 0.1 ohm resistor on the emitters of each device.

    Your schematic is difficult to follow. Draw it in Eagle, or some other capture program. Not everyone is familiar with LTSpice (I use PSpice). I don't see how you are sensing your current. The sense resistors should be in your main path and before you take your voltage sense. Is that 100 ohm in your schematic a load for the simulator? If not, what is its purpose?
     
  20. gigabyte091

    Thread Starter New Member

    Dec 15, 2015
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    @SLK001 I know, i will add 2-3 BD911 in parallel, and BDX53C as a driver transistor.

    This is just a test setup, alpha version :)

    I will redraw schematics in eagle then, but i can answer your questions now.

    Current sense resistor is in negative line of PSU, it's 0.1 ohm resistor. Voltage sensing is put before shunt resistor so voltage drop across him isn't affecting voltage regulation. 100ohm resistor is there as dummy load.

    Maybe i change that and measure current from one of the emitter resistors.
     
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