Using Laptop Power Bricks as power supply input

Thread Starter

Michael Schwager

Joined Oct 8, 2019
2
Hello,
I am looking to cobble together a cheap bench power supply. As you're probably aware, basic buck or boost/buck circuits are readily available from China via eBay or direct via AliExpress. I'm looking to create a supply with steady 5v and 3.3v outputs, and a a variable 0-30v output with constant current control. I want to use spare laptop power bricks as my input source to the voltage regulators.

I see Earth ground as a separate banana socket on most bench power supplies. Assuming that somewhere in the input path there's an isolation transformer, what's the use of the ground?

I would like to have a common circuit ground between my 3 regulators. The problem is, the constant current regulator has the sense resistor on the ground side. Thus I can't tie them all together. However, if I use 2 separate bricks as input, my problem is solved (assuming the two power bricks are isolated from earth ground). Is there any danger with tying together the grounds of two laptop bricks? Of course, in my idea one of the grounds appearing at the output will be through the sense resistor, so I'm actually tying together the grounds through a 0.5 ohm resistor. I wonder if I could end up creating a ground loop or something and fry the resistor out.

Thanks.
 

Hymie

Joined Mar 30, 2018
762
Most dc bench power supply outputs are isolated from earth, some provided with an additional earthed banana socket.

The main purpose of this accessible earth connection is to allow the dc output voltage to be referenced to earth if required (by connecting either the positive or 0V terminal to earth).

In my experience, most brick power supply outputs are isolated from earth. You could test this on your chosen power supplies using a continuity meter. Alternatively you could use class II supplies that have no connection to earth to guarantee the isolation.

Even if there is some earthed reference in the PSU outputs, there should be no safety issue in commoning the 0V outputs, but it might cause one or both of the PSUs to shut down especially if there is a load directly connected between the positive outputs of the supplies (which might occur in use, making load connections across your dc bench output terminals)
 

dendad

Joined Feb 20, 2016
3,332
I have used laptop power bricks like that before. The 19V out is a good starting point for a 0 to 15V supply.
Another power brick to look at is those from some HP ink jet printers. They are 30, or it may be 33V out. I forget. Lots of old printers are tossed out. Then you can just use buck converters.
 

Dodgydave

Joined Jun 22, 2012
8,880
You can use fixed regulators like 7805, 7803, and use a LM317 for Constant Current control, then you can use the same ground,

The laptop brick can be made variable.

If you want to use Current Limit, then you need an op amp and series resistor in the Ground or Positive supply.
 

crutschow

Joined Mar 14, 2008
24,410
The ground resistor for current limit can be placed in the ground going only to the supply you want to limit.
The other grounds can go directly to the other supplies (or to their own current-limit resistor).
 

RIKRIK

Joined Oct 11, 2019
60
I'm currently using a old pc power supply I took from a scrap computer , 12v,5v,3.3v, and a cheap voltage regulator on the 12v output.

if you google "diy atx to psu" and have a look though Google images there's a few options, I brought a cheap board from aliexpress, not the best ,but gets the job done, however all fuses are 20a so I'd recommend changing them.

I did try a laptop charger but it had some smart chip in there to tell it i wasn't using it as a charger.
 
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