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DC-DC variable voltage and current lab power supply

Discussion in 'Power Electronics' started by bengaltiger, Aug 8, 2012.

  1. bengaltiger

    bengaltiger

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    Aug 8, 2012
    Hi,

    looking for simple DC-DC variable voltage and current lab power supply circuit.

    Input power supply 12 volts , 2 amps.

    output variable voltage (3 to 12 volts) and variable current (50ma to 2A). (current limiting with LED indicator)

    Also I will add digital voltmeter and ammeter module.


    Regards,
    BT
     
  2. davenn

    davenn Moderator

    13,207
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    Sep 5, 2009
    Hi BT

    welcome to the forums :)

    For a 12VDc output, you are going to need a bit more than 12VDC input.
    There are fixed voltage DC to Dc converters that you would get away with 12 in 12 out
    Am not sure about contineously variable ones tho.
    Maybe some of the others will comment :)

    Dave
     
  3. bengaltiger

    bengaltiger

    6
    0
    Aug 8, 2012
    Thanks,

    Its easy to get variable voltage schematics, but I need variable current as well (with limit)

    And I need an indicator to see if the load wants more current / overlimit.

    BT
     
  4. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    Bengaltiger, Davenn's point was that most regulators require some extra input voltage, typically 2 volts or thereabouts to enable regulation.

    Without any overhead, you're looking at a double conversion SMPS, which first changes your 12V into something higher (say 18V) and then into the output voltage you desire.

    These are reasonably complex, but are actually available off ebay for next to nothing. Having said that, 2A is probably nearing the upper end of what these can commonly do and you need to start thinking about thermal management.

    The current limit is a little more tricky. If you were wanting output voltages up to (say) 10V, then a linear regulator and a some fairly simple current limit circuit may suffice. With a switchmode regulator it gets a little more complex and potentially messy.

    So, what you're asking for is possible, but unless it reflects actual requirements, you are probably producing a set of desired features that will complicate things.

    edit: Here is an example of such a power supply (without current limiting), and here is one with current limiting (but requires a higher input voltage). And here is a step up DC-DC converter.

    Using the second and third devices you could do what you wanted. I'd recommend that you limit the current to under 2A with these though.
     
    Last edited: Aug 8, 2012
  5. bengaltiger

    bengaltiger

    6
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    Aug 8, 2012
    Thanks.

    probably 3-12 v , is my mistake. I will prefer 3-9 V
    I dont want to draw more than the input.

    So I want a DC-DC step down voltage regulator with current limit.

    And adjustable current limit (50ma to 2A) with LED indicator for current over limit.

    Please help me for the circuit.

    BT
     
  6. davenn

    davenn Moderator

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    Sep 5, 2009
    read steve's last post he gave you links to some examples :)

    Far out !!! US$6 ( the 2nd link) ya couldnt build one for that low price nice find mate :)

    Dave
     
    Last edited: Aug 8, 2012
  7. bengaltiger

    bengaltiger

    6
    0
    Aug 8, 2012
    thanks to all of you and specially steve.

    Still I wanted to do little experiment with simple circuit.

    So if you know please let me know.

    BT
     
  8. bengaltiger

    bengaltiger

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    Aug 8, 2012
  9. BobK

    BobK

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    Jan 5, 2010
    Well, there is no provision for it in the chip, but I can think of two ways to do it.

    1. Compare the voltage across the current limit resistor to a reference and when it is at the limit light the LED.

    2. Compare the voltage on pin 4, the voltage sense, and if it is below Vref of the regulator chip, light the LED.

    Either would require a separate voltage reference and a comparator and a few resistors and would not really indicate that the chip had gone into current limit mode except by inference.

    Bob
     
  10. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,169
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    Jan 21, 2010
    If I were you, I'd cut my teeth on a simple series regulator using something like a LM317.

    This is an adjustable three terminal regulator that can give you a variable voltage from 1.25 to around 35V, but with a 12V input would be limited to 1.25 to around 9.5V to 10V depending on current.

    These do NOT have a current limit (other than to prevent their own destruction), but they are simple to build and you will learn lots that you can apply to something more complex.

    Current limiting can be a little more tricky, especially where you don't have a lot of voltage overhead to burn. For this, perhaps three terminal regulators are not the best solution and you may want to look at other more complex designs.

    One common approach is to place a current sense resistor (maybe as little as 0.1 ohm) in series with the unregulated input to the regulator. You then use an op-amp to compare this voltage to a voltage reference. When it exceeds some preset point you use the op-amp's output to pull down the regulators voltage. In the case of the LM317, you can't pull it down below 1.25V unless you have a negative power supply. Some other chips allow you to do this -- they invariably have more than 3 pins :)

    edit: there's another thread active at the moment which links to a description of a circuit using a more complex regulator.

    https://www.electronicspoint.com/want-add-led-indicator-current-limit-t250994.html

    and here is the datasheet which has a circuit having a variable current limit (yep, 0.1 ohm resistor :))

    Hey! that's you again. Any reason you started a new thread? (talk about wasting my time...)
     
    Last edited: Aug 9, 2012
  11. bengaltiger

    bengaltiger

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    Aug 8, 2012
    Steve, my gratitude for a nice reply.

    I will read few times to get it fully. And sorry for that thread.

    Have a Q.

    constant current vs current limit

    can I use adjustable constant current power supply when I need to limit the current?
    Is constant current power supply will always deliver max current or it depends on the load, I mean how much the load is drawing.

    BT
     
  12. BobK

    BobK

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    Jan 5, 2010
    A constant current supply attempts to supply the set current into the load by adjusting the voltage up or down. Of course there is a max voltage it can supply and it will fail to supply the set current if it requires more voltage than the max.

    A current limited supply normally is attempting to supply a set voltage, but reduces the voltage if the current limit is exceeded.

    There is no such thing as a supply that is constant voltage and constant current at the same time. Mr. Ohm declared this to be illegal.

    Bob
     
  13. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    BobK says exactly what I planned to say when I read your question last night.

    I would only add that it is far more normal to operate a power supply as voltage regulated and current limited than as current regulated and voltage limited.

    In a large amount of cases, the load determines what you actually need to do.

    One case where you would operate it in current regulated mode is when driving a high power LED, where you would set the current for (say) 300 mA and the voltage to something over the Vf of the diode (say 5V).

    In this case the voltage would drop to the Vf at 300mA for that LED, which might be 3.5V.
     
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