# Help with power source

#### art_12345

Joined Mar 26, 2015
4
Hi, I am new to this forum, and also I want to apologize for my bad english.
I am trying to buil a power source where I can get +12v dc, -12v dc, 5vdc, and a full rectified signal of 12v like the following:

I am using a transformer with central tap and the following circuit to get all the dc voltages:

but I want to know if there is a way to get also the full rectified signal without using another transformer?

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#### Papabravo

Joined Feb 24, 2006
12,301
Once the capacitors are placed on the output of the bridge rectifier the rectified AC is GONE.
You can run the output of the +12v to a 7805. If you pull significant current it will get very hot according to
(12V - 5V)* (0.5 Ampere) ≈ 3.5 Watts

A better solution is a multitap secondary or a third secondary winding.

#### cmartinez

Joined Jan 17, 2007
6,380
Bullseye Mike... I was trying to solve the OP's conondrum, until I saw your post.... it's met all of the KISS's requirements...
@art_12345, the full answer is in Mike's post.

#### ScottWang

Joined Aug 23, 2012
6,779
If the D5,D6,R4 used for the input of 5V regulator, do you think is that too high?

#### cmartinez

Joined Jan 17, 2007
6,380
If the D5,D6,R4 used for the input of 5V regulator, do you think is that too high?
The standard 7805 can handle up to 35V, I think. So he should be alright.

#### ScottWang

Joined Aug 23, 2012
6,779
20Vac is too high for a liear 7812,7912 regulator.
The standard 7805 can handle up to 35V, I think. So he should be alright.
I'm not talking about input rated voltage, it should care about the power consumption of 7805, when the input voltage is too high then all the waste power will turn to the heat in the 7805, it may reducing the using life of 7805.

#### cmartinez

Joined Jan 17, 2007
6,380
20Vac is too high for a liear 7812,7912 regulator.

I'm not talking about input rated voltage, it should care about the power consumption of 7805, when the input voltage is too high then all the waste power will turn to the heat in the 7805, it may reducing the using life of 7805.
Oh... I think I understand now. The 7805 is a linear regulator, so maybe he should be using a switching one instead?

#### art_12345

Joined Mar 26, 2015
4
Thanks Mike, this is what I was looking for, I will change the7812 for the switching regulator also, Thanks to all

#### cmartinez

Joined Jan 17, 2007
6,380
Thanks Mike, this is what I was looking for, I will change the7812 for the switching regulator also, Thanks to all
Yes, the same brand I posted handles the same switching regulator at other voltages. Just check it out.

#### ScottWang

Joined Aug 23, 2012
6,779
I just checked the R-78C5.0-1.0 online store in Taiwan, only one I can find and it charges NT$350 close to US$11.17, wow, it can be buy a small arduino to play.