In the following power supply circuit, there is a full wave diode bridge (full wave rectifier?) after the DC Input. I can see how we need a full wave rectifier after an AC input, but why after a DC input? Is it to smooth out power signal?


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Circuitlib Schematic


2 Answers 2


Looks to me like it's purely for the convenience (and maybe safety) of the user. It allows you to connect the input using any polarity you choose rather forcing a specific polarity on you.

  • \$\begingroup\$ Ah, makes a lot of sense! \$\endgroup\$
    – Adam Lee
    Nov 6, 2014 at 16:05
  • 2
    \$\begingroup\$ +1 Yup, we switched polarity on wall warts, and went from a series protection diode to a bridge for backward compatibility. \$\endgroup\$ Nov 6, 2014 at 16:07

Majenko is correct; the bridge is there so the end user can ignore polarity when applying DC voltage to the circuit.

Another option is to use a single diode in series. This will protect the circuit from accidental miss wiring but the circuit will not be functional unlike the full bridge solution.

  • \$\begingroup\$ I like that you expanded on the alternative/usual approach when you do not do the bridge method, or a P-FET (for higher power, same result as a single diode) \$\endgroup\$
    – KyranF
    Nov 6, 2014 at 21:39

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