Dropping PSU voltage from 35volts to 32volts

Discussion in 'General Electronics Chat' started by Mad Professor, Oct 19, 2009.

  1. Mad Professor

    Thread Starter Active Member

    Apr 15, 2009
    133
    1
    Good Day All.

    I would like you advice on how I can go about using a power supply taken from a fruit machine.

    The mains transformer has muliple inputs and outputs, but I am only intrested in the 240vac input and the 27vac output.

    This power supply also has a small PCB with a BP35-02W Bridge Rectifier, a 4700uf 80wv Cap, and a 2.2kohm resistor across the power output.

    As said the transformer is pushing out 27vac unloaded, and after the bridge rectifier, cap, and resistor it is pushing out 35 to 36volts dc unloaded.

    The board that I need to connect this psu to can take upto 40volts, apart from two voltage regulators, L7812CV & L7805CV, and the L7800 range of voltage regulators has an absolute max working voltage of 35volts.

    So with this in mind what is the best way to go about getting a stable 30 or 32volts dc from this but still being able to use the full power of the transformer in this case 10amps.

    Thanks for your time.

    Best Regards.
     
  2. t06afre

    AAC Fanatic!

    May 11, 2009
    5,939
    1,222
    32 volt to 12 volt and 5 volt is quite a drop. Have you proper heat sinks?
     
  3. MikeML

    AAC Fanatic!

    Oct 2, 2009
    5,450
    1,066
    Two ideas:

    How stable is the load? If the supply is always loaded, that will drop the average unregulated DC voltage. If most of the load can be disconnected, that will cause the unreg voltage to spike up.

    Reduce the input voltage. Any taps on the primary? Get a small 240V to 24V control transformer, and wire it as a buck auto-transformer to reduce the big transformer's primary voltage by ~10%
     
  4. Mad Professor

    Thread Starter Active Member

    Apr 15, 2009
    133
    1
    t06afre: The voltage regulators do indeed have there own heatsinks, each one is aprox 50x35x12mm, I don't know if this size of heatsink is suited or not, but that is what is fitted.

    MikeML: 90% of the time the power supply will indeed be loaded, with a min of 1-2amps being pulled wile in use, but when the board is in standby it will be pulling next to nothing, and this is where the problem lies.

    What can I use as a dummy load to see what the voltage drops to with load?
     
    Last edited: Oct 20, 2009
  5. t06afre

    AAC Fanatic!

    May 11, 2009
    5,939
    1,222
    You could do something like this. It will give you regulated output equal to Vz-base emitter voltage drop. The schematics is just for principal method
     
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