Hello,

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!
Conrad

 

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.
regards,
Maanga

 

1. Yes

2. Yes

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

 

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 ;-)

 

I calculate 15.28 volts before the regulator but you really can't know until you build it. You'll need about 4700 uf at 25 volts. Something like this at $1.83 plus shipping.

http://www.mouser.com/ProductDetail...=sGAEpiMZZMtZ1n0r9vR22YQlrerufNMZ7Kpt5viZcHQ=

 

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.