240vac to 12vdc transformer Help!

Discussion in 'The Projects Forum' started by DinoDave, Feb 17, 2008.

  1. DinoDave

    Thread Starter New Member

    Feb 17, 2008
    2
    0
    Hi, am workin on project wich is a revision of something i have already made.

    Basicly i have a float switch connected to a relay and mains powered device, for safety with water i run the float switch connected to the coil of the relay on a 12v dc supply.

    So currently I have two cables running into my project, 240v ac mains (UK!) to the contacts of the relay.
    A 12v power adaptor(the sort you plug into mains power and in adapts to 12vdc) to coil side of relay.

    I would like to get away from the two supplys on my latest revision. Explained..... "240vac entering my project on one cable, split, to contact side of raley and other to a transformer that will give me a nice 12v output that i can then run to the coil side of the relay and float switch.

    Basicly would be nice to crack open my mains adaptor and put it's guts inside my project.

    I hope I have managed to explain myself and any component sugestions from maplin, farnell rs etc would be greatly appreciated(UK!)

    Thanks Kindly
    Dave.
     
  2. KMoffett

    AAC Fanatic!

    Dec 19, 2007
    2,574
    230
    Dave,

    I've done that many times. It's cheap source of an internal line voltage to low voltage DC supply to put inside some project. I usually use a bandsaw/ hacksaw/coping saw to cut along the glue-line on the wall-wart's case. Be vary careful not to cut too deep, or you start cutting components inside. The biggest challange is a way to mount the transformer/PCB into your case. There is no frame or mounting tabs on the transformer. Never found the universal solution, but still looking. :) Mostly these are unregulated transformer/rectifier/capacitor versions. OK for your relay. But, I've now found SMPS cellphone chargers at Goodwill that are regulated and much smaller and lighter for the given output current. And I've found some that have name brand ICs (with online datasheets) inside, so am able to change component values to tweak the outputs to a higher or lower voltage (observing other components' max voltages).

    Ken
     
  3. theblackpanther

    New Member

    Feb 8, 2008
    8
    0
    Hi.

    Your 12V adapter will have the following inside it:

    1. A 240-15V step down transformer
    2. A rectifier unit to convert the AC to DC
    3. A filter mechanism to remove the ripples in the rectified output
    4. A regulatory mechanism, which ensures that the output voltage is maintained at 12V.

    You can easily incorporate all the above in your circuit, using the simple schematic attached. You may find other versions if you google "12V power supply circuit".

    Regarding the components, you can either buy a transformer or remove one from any discarded electrical apparatus. The secondary voltage is usually chosen slightly greater than the required DC output. So go for a 240-15V transformer. Take care of the curent rating of the transformer. Choose one depending on the load current of your application.

    For the rectifier, you can either connect 4 1N4007 diodes as shown, or use an integrated rectifier package (like RB156) if available. Take care of current again. 1N400x series is rated for 1 A.

    The 470 uF capacitor sufficiently filters out the output of the rectifier. And finally, you have the 7812 regulator IC, which gives you a nice and clean 12V output.


    All the best.
     
  4. JoeJester

    AAC Fanatic!

    Apr 26, 2005
    3,373
    1,157
    theblackpainter,

    Not to complain, but you didn't put the transformer in the diagram. You left the impression that you connect the diodes directly to the 240 mains.
     
  5. KMoffett

    AAC Fanatic!

    Dec 19, 2007
    2,574
    230
    blackpanther,

    I can't wait for the smoke. :eek:

    First, your bridge rectifier is hooked up as a half wave rectifier instead of a full wave. Did you mean to do it that way?

    Second, unless his float switch in designed to be isolated enough to operate on line-voltage this circuit can make the switch an electrical hazard.

    Third...and here comes the smoke...the regulator is spec'ed for an absolute input/output differential voltage of 40vDC. Your peak input voltage is 339 volts and the output is 12 volts. ???

    I'm not even going into how big the 470uF/450vDC input capacitor would be.

    Ken
     
  6. DinoDave

    Thread Starter New Member

    Feb 17, 2008
    2
    0
    Cheers guys, I think I will have a bash at butchering a wall adaptor, Where there is a will there is a way and all that! Shame I can't find the datasheet for my float switch as I may have been able to get away with a smaller power supply i.e mobile phone charger etc! Never mind!
     
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