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Salvaged SMPS to bench supply

Discussion in 'General Electronics Discussion' started by RubiconJoe, Oct 19, 2014.

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  1. RubiconJoe

    RubiconJoe

    22
    1
    Oct 7, 2014
    I salvaged what seems to be a pretty high end switch mode power supply. It is rated for 12V DC, 5A. Aside from trying to reverse engineer it to see how it works, I would also like to convert it into a variable bench supply for my lab. It is a relatively small unit and think it would be perfect for my bench.

    What would be some things to consider for designed/modifying this supply?


    2014-10-18 20.03.15.jpg
     
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,482
    2,830
    Jan 21, 2010
    You face a number of difficulties:

    1. You need to be able to change the output voltage. Because this is designed as a fixed output voltage regulator any adjustment (if already present) will typically be limited to +/- 20% or so.
    2. There is possibly no current limiting on the supply (there may be some form of protection against gross overload though). In a bench power supply you typically want the output to be well protected and also to have a variable current limit.
    3. Output may not be floating. Often in a bench power supply the output is floating and mains ground is brought out to a separate terminal. This allows either rail to be grounded, or for the output to be fully floating.
    4. Safety. In this supply there is obvious easy access to mains potential. It would require a casing that prevents accidental electrocution. The other issue is that the control circuitry operates at mains voltages (i.e. not on the low voltage side) so modification needs to be carried out with extra care.
    I may be able to reply later to address how you might get around some of these limitations.
     
  3. RubiconJoe

    RubiconJoe

    22
    1
    Oct 7, 2014
    Thank you for your input, Steve. If you believe deigning a circuit from scratch as a lab supply would be a better route to take, that was the original plan before coming across this one
    .
    There is a case that completely encloses the circuit, that can be potentially mounted in another enclosure as the lab supply.

    I would definitely like to hear your ideas about getting around these limitations.


    Thanks again!

    Joe
     
  4. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,482
    2,830
    Jan 21, 2010
    Changing the output voltage:

    Your options here are to leave it fixed, or to "hack" the feedback network to allow the output voltage to change.

    If you decide to make it changeable, you'll need to determine how voltage sensing takes place. This is typically done using an optocoupler to interface between the low and high voltage sections. The optocoupler is connected to the output using a zener diode and resistor, or sometimes a TL431 or equivalent chip.

    [​IMG]
    Here is an example using a TL431.

    [​IMG]
    Here is an example using a zener diode.

    Once you've located this in your power supply, you may be able to either modify it or to replace it with the TL431 method -- which can be made adjustable over a reasonably wide range.

    Current limit:

    Many SMPS's have overload protection be limiting the current switched by the main pass element. This provides some level of protection for the power supply, but doesn't limit it to a particular current, is not usually variable, and is implemented on the high voltage side of the circuit.

    An alternative is to insert a current sense resistor in the output lead (before the voltage sensing) and use this to provide an alternate method of turning on the optocoupler in the feedback network.

    Without some finessing, this will not work all the way down to zero volts, so it's not perfect.

    Floating output:

    Typically you will find that either the negative output is connected to mains earth, or it is connected to mains earth (or neutral) via a high voltage capacitor.

    If your power supply has continuity between the negative output and the mains earth then you have the former.

    It may be possible to locate where the ground is connected to the low voltage side and to cut this track (bridging it with a high voltage (typically 3kV) capacitor and bringing out the mains earth to another connector.

    Safety:

    A case will obviously be essential, but also you need to ensure that live parts of the circuit are kept away from low voltage wiring, and that any mains wiring is safe and the mains lead is secured.

    [​IMG]

    Using an IEC socket (actually a plug) on your device means that someone pulling on the mains cable will simply unplug the unit, not threaten to pull the wires out.
     
    abuhafss and KrisBlueNZ like this.
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