Repurpusing Laptop power supply

[camarada78]

I have a bunch of laptop power supplies lying around and wanted to make a power supply for testing.

My first though is to see if its possible to make a +18/-18v using two 19.2v toshiba PS i have here.

The idea was to put both negatives with a common ground, feeding a 7818 and 7918. Would that work or am I missing something?

thanks, have a good day.

 

[PermO]

I don't know if you can hook up two switch mode PSU's like that, and I don't think you'll need to.

In eurorack synth world it is very common to use a single Laptop PSU to achieve dual voltage +12 and -12 rails.

See what they do and convert that to 18V instead of 12.

 

[camarada78]

Yes, i know there ar plenty of buck converters that would take a simple 19v and convert to a symmetric +/- supply. But i was looking into something i can do with the stuff i already have on my workshop.

 

[ccaudle]

The idea was to put both negatives with a common ground

I assume you mean connect the negative of one supply to the output supply common to create the positive supply, and connect the positive of the other supply to output common to create the negative supply.

With the supplies unplugged measure from the positive and negative terminals to the protective earth pin on the power line. If the supply output negative is tied to earth you will not be able to do that, the outputs need to be completely floating to put them in series.
If they are completely floating it will likely work.

feeding a 7818 and 7918

Check the dropout voltage for 78xx and 79xx regulators, you will not get regulated 18V (at least not very well regulated) with 19V input. They might limp along at low output currents, but the datasheet value is 2V dropout, with the output being rated as 17.3V-18.7V. All the OnSemi datasheet specs are quoted with 21V minimum for the C rated part, or 22V minimum input for the B rated part.

 

[grid_stopper]

i just built a pair of these, using two laptop PSU's and they work fantastically

https://www.thingiverse.com/thing:5021081

 

[camarada78]

i just built a pair of these, using two laptop PSU's and they work fantastically

https://www.thingiverse.com/thing:5021081

Cool, but these seem 0-30v. Very useful but i was planning on doing a symmetric +18v/0/-18v using two 19.2v smps in parallel.

 

[grid_stopper]

yes, thats essentially what ive done but i connect them in series for bipolar. works very well.

 

[thor.zmt]

I have a bunch of laptop power supplies lying around and wanted to make a power supply for testing.

My first though is to see if its possible to make a +18/-18v using two 19.2v toshiba PS i have here.

The idea was to put both negatives with a common ground, feeding a 7818 and 7918. Would that work or am I missing something?

thanks, have a good day.

There are a few "gotchas" here.

First, if the mains side has a 3-pin plug then the negative pole connects to earth. If so making +/- XX Volt doesn't work, the negative voltage will be shorted out. If not, you have usually 2.2nF connected between negative and 1/2 mains voltage.

Second, these power supplies are no low noise. Typically 10's to 100's mV peak-peak noise at the switching frequency are normal. The actual noise will be inaudible, but can cause audio band intermodulation. Running two such supplies together with each having a similar but unequal and not phase locked switching frequency can create beat notes in the audio band. As switching frequency is variable with loading, these may only happen under specific conditions.

Third, efficiency legislation means most modern SMPS will operate in "cycle skip" mode at low load, to maintain efficiency. This means the switching frequency is modulated with an audio band frequency, which can cause what might be called "birdies".

There are of course solutions to all these, but it pays to be aware of these issues, guard against them and specifically look for them, to be sure the do not materialise in your project.

Personally I'd probably look instead for recent, GAN FET based USB-C 100W adapters, these give up to 20V/5A and are far more advanced in design than older laptop supplies. Plus commonly no earth is used.

Thor

 

[camarada78]

There are a few "gotchas" here.

First, if the mains side has a 3-pin plug then the negative pole connects to earth. If so making +/- XX Volt doesn't work, the negative voltage will be shorted out. If not, you have usually 2.2nF connected between negative and 1/2 mains voltage.

Second, these power supplies are no low noise. Typically 10's to 100's mV peak-peak noise at the switching frequency are normal. The actual noise will be inaudible, but can cause audio band intermodulation. Running two such supplies together with each having a similar but unequal and not phase locked switching frequency can create beat notes in the audio band. As switching frequency is variable with loading, these may only happen under specific conditions.

Third, efficiency legislation means most modern SMPS will operate in "cycle skip" mode at low load, to maintain efficiency. This means the switching frequency is modulated with an audio band frequency, which can cause what might be called "birdies".

There are of course solutions to all these, but it pays to be aware of these issues, guard against them and specifically look for them, to be sure the do not materialise in your project.

Personally I'd probably look instead for recent, GAN FET based USB-C 100W adapters, these give up to 20V/5A and are far more advanced in design than older laptop supplies. Plus commonly no earth is used.

Thor

Thanks for the answer. I suspect these "birdies" are the noise i hear when clients bring bad quality smps with their guitar pedals.

But ive made simple pedal supplies with old laptop smps (19v with no earth) with an extra filter stage (usually 7809 and filter caps) to make 9v pedal supplies with good results. no audible noise. I am searching for a new project to use these 19vdc smps (i have 4 of them laying around). Since i posted i did a research and i think a "dc to symetrical dc circuit" may be what i am looking for. Maybe it even has an of the shelf solution already in Aliexpress.

 

[k brown]

How would SMPSs respond to having two in series (in the OP's example, to achieve 38.4v), then feeding that to a filtered voltage divider, like these?:
https://sound-au.com/project43.htm

 

[Tubetec]

Ive seen a lot of unwanted noises due to multiple Smpsu's on guitarists pedal boards ,
grounding is typically a rats nest too which makes matters worse .

On one ocassion connecting the bass players brand new switcher Genz Benz amp to a hard disk recorder via XLR and it stopped working ,refused to make any sound at all .

 

[thor.zmt]

Thanks for the answer. I suspect these "birdies" are the noise i hear when clients bring bad quality smps with their guitar pedals.

Possible, guitar pedals draw jack all current, ideally you pre-load the PSU enough to waste enough power to get the unit out of "green mode".

But ive made simple pedal supplies with old laptop smps (19v with no earth) with an extra filter stage (usually 7809 and filter caps) to make 9v pedal supplies with good results. no audible noise.

Yes, the cycle skipping noise is audio band and differential (and kinda random and signal dependent (current draw)).

So a regulator with enough overhead Voltage will reduce it significantly.

An LM317, perhaps with enhanced HF rejection and a 12V/2A SMPS would be my take, 1.5 A (given the right heatsink) can drive a LOT of pedals.

I am searching for a new project to use these 19vdc smps (i have 4 of them laying around). Since i posted i did a research and i think a "dc to symetrical dc circuit" may be what i am looking for. Maybe it even has an of the shelf solution already in Aliexpress.

I would go with a suitable switcher in the "1.2 MHz class" and use the classic greinacher cascades from the switching node to make +/-18V and +48V.

Probably make a bit more and use linear regulators plus LC filtering.

I doubt this exists, but it might be cute as community project, with an SMD based PCB that can take in 12V - 24V DC via DC Jack or connect to a USB-C charger and output a lot of volts and AMP's for +/- 12/15/18V and 48V or 68V Phantom power.

If we use my preferred richtek part in push-pull with 180 degree switching polarity and 12V minimum input, we can get as much as 82VA total in a very compact module.

Linear output regulators, using discrete Sziklai based followers with RC filtered Zenner references satisfies both DC accuracy and offers low noise competitive with any IC regulator and really low output impedance, with much greater PSRR than any IC regulator.

I can help on the Design (for free if it's an open sauce project), but I'm not designing PCB's or do the legwork for fabbable (JLCPCB) project.

Thor

 

[camarada78]

It would be a cool project indeed but I think that would be a little overkill just to repurpuse old smps. there are cheap smps for mixing consoles with acceptable quality on aliexpress/ebay that would solve most power supply problems (at least for me).