# Full bridge rectifier with voltage regulation

#### PsySc0rpi0n

Joined Mar 4, 2014
1,706
Hello.

So, along with the LEDs thing, I'm also trying to design and build a full bridge rectifier with voltage regulation.
I already have a circuit and I just want some insights to understand if it is good enough for simple applications (home made) or if anything else is needed.

This is what I have in falstad online simulator!
The input is 230V, 50Hz (EU mains). The output is set to about 10V by the 240Ω and the 1.7kΩ resistors!

I attached the txt file so that anyone that wants to open it in the online simulator, can do so.

The datasheet I used is here (page 13):

Note: That 10Ω resistor is just because falstad doesn't allow capacitor loops without resistance. So, I added that resistor!

#### Attachments

• 5.8 KB Views: 2

#### AlbertHall

Joined Jun 4, 2014
12,293
If the input to this circuit is 230VAC then the input voltage to the LM317 will be around 330V. The absolute maximum differential across the LM317 is 40V so it is going to release the magic smoke double urgent. You need a transformer to reduce the LM317 input voltage.

Also you show a connection to mains earth on the negative bridge output and that will blow up the bridge rectifier. Again the transformer input will prevent that.

#### PsySc0rpi0n

Joined Mar 4, 2014
1,706
I knew I was forgetting something and that it was being too easy.
Despite of the need for the transformer, could you please explain the second part you mention about the connection to mains earth on the negative bridge output that will also blow the bridge rectifier?

#### AlbertHall

Joined Jun 4, 2014
12,293
The AC input pins vary between +330V and -330V. When either input is at -330v you have a forward biased diode in the bridge to earth (0V).

Correction to the correction: One input pin will be at approximately 0V but the other will swing between +330V and -330V. The result is the same - exit one bridge rectifier.

Last edited:

#### sghioto

Joined Dec 31, 2017
5,145
As AlbertHall mentioned a stepdown transformer is required in this application. A 230 VAC to 12VAC would be ideal.
It's fun to build but keep in mind you can probably buy a 12 volt power supply required for the price of the transformer.

#### sghioto

Joined Dec 31, 2017
5,145
Also you show a connection to mains earth on the negative bridge output
Is that an earth ground or just a common ground for the circuit?

#### Papabravo

Joined Feb 24, 2006
20,626
Hello.

So, along with the LEDs thing, I'm also trying to design and build a full bridge rectifier with voltage regulation.
I already have a circuit and I just want some insights to understand if it is good enough for simple applications (home made) or if anything else is needed.

This is what I have in falstad online simulator!
The input is 230V, 50Hz (EU mains). The output is set to about 10V by the 240Ω and the 1.7kΩ resistors!
View attachment 257320

I attached the txt file so that anyone that wants to open it in the online simulator, can do so.

The datasheet I used is here (page 13):

Note: That 10Ω resistor is just because falstad doesn't allow capacitor loops without resistance. So, I added that resistor!
A more reasonable way is to include the ESR of each capacitor. That way no power will be dissipated between the bridge rectifier and the LM317. Remember if you draw any significant current through the regulator that resisitor will get finger burning hot!

Joined Feb 20, 2016
4,378
As mentioned above, a transformer is needed.
And, do not connect the mains Earth to either side of the DC out, but have a third terminal available that is the Earth so, if needed, it can be connected.

Without the transformer, as well as blowing up the regulator, the bridge will go too as one side of the mains AC is already connecte to earth by the power utility.

#### crutschow

Joined Mar 14, 2008
33,370
I suggest you use a lines-powered switching regulator, such as an old laptop supply which typically has an output of about 19Vdc. to provide DC to the LM317.
That will be isolated and efficient.

Using power directly from the mains without isolation is a no-no, unless you are striving for a Darwin Award.

Last edited:

#### Audioguru again

Joined Oct 21, 2019
6,462
I converted the negative image of the schematic to a normal positive and cropped it:

#### Attachments

• 65.1 KB Views: 10

Joined Feb 20, 2016
4,378
As @crutschow mentioned, a laptop 19V supply is ideal as the basis of a power supply.
Then design your regulator to give a max 15V out. That will be plenty of headroom. Make sure there is a decent heat sink for your LM317, and note that the tag needs to be insulated from the heat sink.
It may be also an idea to add a fixed switch mode 5V reg and a fixed 12V out, all with the 0V common.
Use a switch mode for the 5V supply as there will be 14V dropped across the reg so a linear reg will dissipate quite a bit of heat under load.
The connections of the LM317 and LM78xx series regs are not the same!
Many new folk fall into that trap.