Charger for 12V / 400mA - Z-Diodes?

Discussion in 'General Electronics Chat' started by Conrad007, Aug 22, 2012.

  1. Conrad007

    Thread Starter New Member

    Oct 24, 2011

    I am trying to build a charger with an output of 12V for my radio receiver, using a classic 4-diodes bridge rectifier followed by a 7812 voltage regulator. I will also use capacitors in parallel with the diodes, in order to reduce the mains hum as much as possible. I have now 2 problems / questions:
    1. the transformer I have (220V AC input) has 2 secundar outputs, which are giving: the first one about 6.5V and the second one about 11.8V, which is not enough to obtain 12V DC... Can I connect the 2 output coils in serial, in order to obtain about 18.3V AC ?
    2. if I can connect the secundars in serial, can I use 1N4001 for the bridge rectifier? Would it be somehow dangerous for the diodes or for the voltage regulator? According to the datasheets of 1N4001, they have a Vrrm of 50V (peak repetitive reverse voltage) and the 7812 has a specification for the "input voltage" of 19V, although the "input voltage required to maintain line regulation" (???) is only 14.6V.
    I want to build a robust charger, which will not put in danger my (expensive) receiver.
    Could somebody help me with an advice?

    Thank you in advance!
  2. maanga

    New Member

    Aug 20, 2012
    Your 11.8 volts a/c winding is enough. This will give 16 volts DC after the rectifier diodes. 7812 needs another 2.5 or 3 volts to give regulation, which amounts to 15 volts. The 16 is well above the requirement.
  3. wmodavis

    Well-Known Member

    Oct 23, 2010
    1. Yes

    2. Yes

    Comment: As maanga said though it might me marginal with line voltage and/or load variation.
  4. Conrad007

    Thread Starter New Member

    Oct 24, 2011
    Thanks a lot maanga and wmodavis. I will build it and carefully make measurements, to have the confirmation of the amount of voltage and current drops, etc. I think I will also measure the peak of output voltage during the plug-in and plug-out of the charger to the main.
    Thanks again for your quick answers, it's always good to get 2 similar opinions ;-)
  5. #12


    Nov 30, 2010
  6. cork_ie

    Active Member

    Oct 8, 2011
    Yes Manga is correct, follow his advice.

    A simple resistive test with an ohmmeter will generally give you a fair idea of how a secondary winding is wound in a transformer. Centre taps are generally easy to find, and can be ignored if you need a higher voltage.