# Need help in converting a 12VDC 16.5A PSU to +-17.5VDC 2A PSU

#### Warloxk

Apr 13, 2023
1
Hello everyone,

I am in need of some help in converting a 12VDC 16.5A (old XBOX 360) power supply unit (PSU) to a +-17.5VDC 2A PSU. I have searched online for tutorials and guides but haven't found anything helpful yet.

I have some experience with electronics and basic circuitry, but I am not an expert. I am hoping that someone here can provide some guidance on how to modify the PSU to fit my needs.

Specifically, I need the PSU to output two voltages: +17.5VDC and -17.5VDC, with a maximum current of 2A for each. I am planning to use this PSU to power a small audio amplifier circuit.

Any advice or resources you can offer would be greatly appreciated. Thank you in advance for your help!

#### Harald Kapp

##### Moderator
Moderator
Nov 17, 2011
13,182
The positive voltage (17,5 V, 2 A) is the easy part. You need a DC/DC step-up (or boost) converter. Here's a link to an example.
The negative voltage would need a DC/DC voltage inverter. These are not so common.

A workaround can be made by using two step-up converters, one set for 35 V, the other for 17,5 V in this configuration:

The lower step-up regulator is set to 17,5 V
The upper regulator is set to 35 V.
The "-" outputs of both regulators are connected.
The "+" output of the lower regulator serves as GND (0 V) for the output voltages.
The "-" output of the lower regulator is then at -17.5 V with respect to GND_out.
The "+" output of the upper regulator which is at 35 V with reference to the "-" output is the at +17.5 V with reference to GND_out.

Last edited:

Feb 19, 2021
408

You possibly could change out one of the Rs to get 17.5 V.....

Regards, Dana.

#### Ylli

Jun 19, 2018
386
The positive voltage (17,5 V, 2 A) is the easy part. You need a DC/DC step-up (or boost) converter. Here's a link to an example.
The negative voltage would need a DC/DC voltage inverter. These are not so common.

A workaround can be made by using two step-up converters, one set for 35 V, the other for 17,5 V in this configuration:
View attachment 58732
The lower step-up regulator is set to 17,5 V
The upper regulator is set to 35 V.
The "-" outputs of both regulators are connected.
The "+" output of the lower regulator serves as GND (0 V) for the output voltages.
The "-" output of the lower regulator is then at -17.5 V with respect to GND_out.
The "+" output of the upper regulator which is at 35 V with reference to the "-" output is the at +17.5 V with reference to GND_out.
If he could find up-converters that have isolated outputs, he could just connect the outputs in series aiding and ground the center tap. On the above configuration, the lower converter would need to handle 4 amps.

#### Harald Kapp

##### Moderator
Moderator
Nov 17, 2011
13,182
If he could find up-converters that have isolated outputs,
If...

And these will surely be more costly than non-isolated ones.

#### AnalogKid

Jun 10, 2015
2,739
To expand on post #2 - Most boost converters, certainly most modules on ebay, are non-isolated; the input GND is tied directly to the output GND. If you start with two "normal" positive boost converters, there is a conflict where the output grounds are connected. If you want to stay with this approach, one of the converters will have to be fully isolated.

As mentioned, there is a true inverting converter (sometimes called a buck-boost) that would not have the GND conflict, but these are far less common. The approach seems simple enough - start with a typical boost converter circuit (with an NPN or n-channel switching transistor), and change the transistor to PNP or p-channel. With some circuit rearranging, the flyback pulse out of the inductor now goes negative, below GND, where it can be rectified and filtered into a negative voltage source. In practice there is a little more to it than that, but there are many example circuits on the innergoogle and in manufacturer's app notes.

Do you want to build or buy?

ak

Last edited:

#### kellys_eye

Jun 25, 2010
5,684
I am hoping that someone here can provide some guidance on how to modify the PSU to fit my needs.
Perfectly possible. Remove the secondary windings (count the turns) from the SMPS transformer and determine the turns-per-volt. Rewind the appropriate number of turns for 35V but take a centre-tap from the new winding i.e. if it's 100 turns to get 35V then wind 50 turns, take a wire out and wind another 50. Rectify the two halfs and smooth accordingly. You will, of course, have to modify the SMPS feedback circuitry to ensure the correct stabilised output.

All tongue-in-cheek (unless you are familiar with SMSP design then perfectly possible - but a little more complicated than I describe) or you could go to https://sound-au.com/project89.htm and just build it from scratch......

Use your Xbox PSU to supply it.

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