# Why bridge rectifiers are used in case of DC power supply.?

I was searching for 12v to 3.3v/5v DC voltage conversion. I found many schematics and the voltage regulators. But in most of the schematic, they have included a bridge rectifier at the starting. As I am a beginner, I thought bridge rectifiers are only used in case of AC to convert AC to DC. I found this similar question but they are discussing about AC volts.

So why bridge rectifiers are used in case of DC power supplies.

Here is the schematic which I found:

• You don't normally sice you loose your ground reference. A single diode for blocking reverse polarity can be used if that's what you are after. Some industrial equipment use diode bridges on the input to make sure it runs even in the event of reverse polarity somewhere upstream. – winny Jul 6 '16 at 7:22
• A single diode protects from reverse polarity, a full bridge let the device actually work in the event of reverse polarity. – Vladimir Cravero Jul 6 '16 at 7:40

As Vladimir Cravero mentioned in the comments, using a bridge rectifer in a DC application would be make the input polarity insensitive.

The image below is a single diode used for reverse polarity protection. If V1 < 0.7, R1 sees no voltage/current, and the load is protected.

simulate this circuit – Schematic created using CircuitLab

simulate this circuit

In the image below, if the V1 is wired correctly and V1 > 1.4 , then D1 and D2 conduct.

If V1 is wired incorrectly, meaning that + and - are accidentally reversed and V1 < -1.4, then D5 and D3 conduct.

When V1 is between +/- 1.4, there is not enough voltage to overcome the diode forward voltages, so R1 sees no voltage/current.

So while this does protect against polarity reversal, I like to think of it as polarity insensitive at the cost of a reduced output voltage of 2*Vd.

You are right, bridge rectifiers are useful for AC to DC conversion, and can be omitted when the input voltage is DC already.

The examples you have found certainly included it to be on the safe side, in case the wall-wart has the wrong polarity, or if a simple AC transformer is used instead of a DC output wall-wart.

In your case, you can omit it if you don't require the device to work whatever the input jack polarity.

Last advice: the schematic you have here is a linear regulator, not a DC converter. It works well, but will waste a lot of power if you need to draw significant current. If you need to draw more than about 100-150 mA, I'd use a DC-DC buck converter instead (there are modules readily available for this).

You have just found schematics that describe AC to DC power supplies. Bridge rectifier is the most obvious way to do rectification.

It does not hurt if you provide DC at the input of the power supply you have shown (beside small voltage drop and slight heating of the bridge).

If you use a bridge at the input of a DC to DC power supply you gain reverse polarity protection and it will also work no matter where you connect input + and - .

If you want just to make 5V DC from 12V DC look at the datasheet and reference design of 7805 or LD1117.

• Is it possible to use single voltage regulator like LM317 to get two output voltage like 3.3v and 5v. – S Andrew Jul 6 '16 at 8:21
• No. A single output regulator will give you only one regulated voltage. – filo Jul 6 '16 at 9:36

A bridge rectifier is required to convert AC to DC. In you had 12V AC which has to be converter to 5.5V DC, you needed a bridge rectifier. The rectifier part of the circuit actually converts 12V AC to 12V DC and the rest converts 12V DC to 5V DC. For your case, since you already have 12V DC supply, you can simply eliminate the bridge rectifier part of your circuit and start from C3 on the left. 2 very simple 12V-5V and 12V-3.3V circuit schematics are given below.

simulate this circuit – Schematic created using CircuitLab

• Well, you can use LM317 for 5V generation as well. Just use the second circuit and replace R2, R1 and R3 as this: - R2=220Ohm, R1,R3 = 330Ohm – sribasu Jul 6 '16 at 8:16
• Is it possible to use single voltage regulator like LM317 to get two output voltage like 3.3v and 5v. – S Andrew Jul 6 '16 at 8:20
• Absolutely. I have added 2 more schematics. See 3rd and 4th Schematic. These are the 2 ways with which you can do this. Either use a switch to toggle between 2 different values of R3 - 33 Ohm or 330 Ohm. Or else, you can use a Variable Resistance (Potentiometer) to control the output voltage. For this approach, you have to measure the output voltage everytime you turn the potentiometer and change its resistance. – sribasu Jul 6 '16 at 8:36
• Thanks but I have two circuits which I have to run using 3.3v and 5v. I have a HC05 module and microcontroller. HC05 need 3.3v and Microcontroller needs 5v for operation. So I cannot use a switch or pot in this case. I also dont want to use another regulator. Is it possible using single regulator? – S Andrew Jul 6 '16 at 8:45
• Your discussion is going in wrong way because the question is about diodes. Consider asking another question. – Aircraft Jul 6 '16 at 9:17