# Full wave rectifier from a wall wart?

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#### steve_j83

##### New Member
I would like to build a variable +/- power supply from a 15volt wall wart.

Can i put in a full wave rectifier or just a half wave?

If i can where does the ground go?

#### blueroomelectronics

##### Well-Known Member
Is the wall wart AC or DC out?

#### steve_j83

##### New Member
sorry yeah ac ... for a dual supply

edit: thanks for the quick reply

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#### dknguyen

##### Well-Known Member
You need two half waves- to separate the positive and negative AC from each if you want a dual supply. A full-wave would just grab take both the positive and negative AC and make it one polarity.

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ok thanks

#### dknguyen

##### Well-Known Member
YOu could also use two full-wave rectifiers to do the same thing. Added complexity though and since you want a dual supply, you have a use for both positive and negative waveform. (If you only needed one polarity, using a half-wave would make no use of every half cycle and also increase output ripple).

Two full-wave rectifiers would produce a smoother output though. (Rather than grabbing every second cycle for positive or negative output like dual half-waves, it would take each half cycle whether it be positive or negative, and split it between positive and negative output.)

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#### Hero999

##### Banned
This circuit will work, for a ripple of 2V use 1000:mu:F per 100mA of load current.

The output voltage will be just over 20V.

If you want 15V add an LM78L15 and LM79L15.

The capacitor values shown below with the aforementioned regulators added will be able to supply up to 100mA with 15V in.

#### steve_j83

##### New Member
So that would be half wave yes? Is there and way of makeing that full wave without resorting to a centre tap transformer?

#### steve_j83

##### New Member
Im thinking i should just forget the wall wart... wanted to go that route as i didnt want to dable in 230v...

Hot does this one look to you guys? Seems simple enough

C1, C2 2 2200uF 35V Electrolytic Capacitor
C3, C4, C5, C7 4 1uF 35V Electrolytic Capacitor
C6, C8 2 100uF 35V Electrolytic Capacitor
R1, R4 2 5K Pot
R2, R3 2 240 Ohm 1/4 W Resistor
BR1 1 2A 30V Bridge Rectifier
U1 1 LM317 Adjustable Positive Regulator
U2 1 LM337 Adjustable Negative Regulator
T1 1 30V Center Tapped 2 Amp Transformer
S1 1 SPST 2 Amp Switch

It seems simple enough

#### Hero999

##### Banned

You can't use a centre tapped transformer because I thought you were using a wallwart and those things don't have a centre tap.

#### tcmtech

##### Banned
No center tap on a wall power pack?
It must be a UK thing. I have taken dozens of them apart for specialty projects and I would confidently say at least 3/4 of them have center taps around here.

I learned something new too!

#### audioguru

##### Well-Known Member
The circuit you posted from Aaron Cake's site has the wrong values for its voltage-setting resistors and pots. It has the values for the more expensive LM117 and LM137 regulators. Using those values with LM317 and LM337 regulators will cause the output voltages to rise when the load current is low.

#### steve_j83

##### New Member
ooohhh not good, thanks for the heads up Mr audio

#### tcmtech

##### Banned
If your planning to run other things off of it you may want to think about building it with two transformers. That way you can stack them if you need dual voltage or a higher voltage and they could be or put in parallel for more current. Plus your total available power is higher.
The total parts are about the same but you would gain some added versatility. If you needed two independent power sources you would have them available too.

#### Hero999

##### Banned
No center tap on a wall power pack?
It must be a UK thing. I have taken dozens of them apart for specialty projects and I would confidently say at least 3/4 of them have center taps around here.

I learned something new too!
So do US wallwarts have three core cable and a three pin plug connecting you the device being powered? I doubt it.

The only wallwarts I've seen with tapped transformers are those with multiple voltage settings. These are normally also DC and you have to break it open to get access to them.

#### Mikebits

##### Well-Known Member
Most wall warts I have opened here in US are garbage.

#### steve_j83

##### New Member
Hey hero

Am having a look at your psu... Looks like it could be the trick.. I might make two... one for putting in the project (might have to do something about the heat) and one for a desktop.

Im a little confused about transformers though... Rapid has all their ratings in VA not Amps

So i gather i should use the equation VA x PF / V

What would the PF be?

#### Hero999

##### Banned
VA is volts times amps, just divide the VA by the voltage to find out the current rating.

If you have a choice then buy a transformer with two secondaries connected in series or a centre tap and a full-wave rectifier.

The half wave rectifier idea is only good for an AC wallwart.

#### forumlicker007

##### New Member

It seems simple enough
This is the way you've to go for a dual supply.

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