I have a 28V-0-28V, 7Amps centre tapped transformer to supply voltage to my load. Load will be a cell (plates are connected in both series and parralel) used in Chlorinator. I have a voltage regulator L7805CV. The link is below. [http://www.farnell.com/datasheets/2307057.pdf?_ga=2.224828637.639084130.1536017950-602646683.1536017950][1]

What I want is to use the same centre tapped transformer for the voltage regulator to get 5V. From the datasheet I found that Vi for regulator has to be around 10V to get Vout 5V. But the transformer is rated at 28V-0-28V. How can I drop voltage after Bridge rectifier but before feeding on to voltage regulator, Vin?

  • \$\begingroup\$ Why did you choose the wrong transformer? Why not use a proper wall 5V supply? It is possible to use a carefully select light bulb or power resistor to drop 18 to 25Vdc depend on your cap size and load current but you Chlorinator load curve is undefined. \$\endgroup\$ Sep 4, 2018 at 1:40

1 Answer 1


Rectify the transformer output, and feed it into a switching regulator instead, such as one of these


  • \$\begingroup\$ I have kept that for my last option. At the moment, I am trying to feed the Vin at Voltage regulator using another transformer 12V-0-12V (2A). This transformer is solely for voltage regulator only. However, another transformer is used for providing power to load and zero crossing detection. I am not getting any good sign from my test. \$\endgroup\$ Sep 4, 2018 at 1:53
  • \$\begingroup\$ Vin is expecting a DC voltage, not AC. There is only 1 diode for reverse polarity protection, which is no sufficient for for AC to DC rectification, and insufficient (in my opinion) capacitance for input filtering. \$\endgroup\$
    – CrossRoads
    Sep 4, 2018 at 2:10

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct.

Not the answer you're looking for? Browse other questions tagged or ask your own question.