PSU question

Having a blank moment and want to check something. I am aware with the old ATX supplies you cant use two together because the GND being tied to earth. If you want to use two you cut the earth on one (bad idea).

How about laptop bricks with no earth to start with? Is it ok to tie two of these together to make a +/- supply? I have a couple around the 30V mark that are the same make, they only have two wire line inputs on them.

I am desperately trying to find a way to help someone out with very little access to anything. I cant even send stuff by post to them.

large_ghostman said:

Having a blank moment and want to check something. I am aware with the old ATX supplies you cant use two together because the GND being tied to earth. If you want to use two you cut the earth on one (bad idea).

Click to expand...

Are you sure about that? Because I've put wall warts (3-pronged) in series with each other just fine. Maybe what you're talking about is something specific to ATX supplies. Easy enough to check with a continuity check.

dknguyen said:

Are you sure about that? Because I've put wall warts (3-pronged) in series with each other just fine. Maybe what you're talking about is something specific to ATX supplies. Easy enough to check with a continuity check.

Click to expand...

Yes its a definite with the old ATX computer supplies, bitter experience then research . Most wall worts are actually 2 pin though arnt they? the third pin being unconnected inside?

large_ghostman said:

Yes its a definite with the old ATX computer supplies, bitter experience then research . Most wall worts are actually 2 pin though arnt they? the third pin being unconnected inside?

Click to expand...

Oh wait, you're right. The wall warts are two pin. They're just so big I forget what's on the other side lol =/

At least it sounds like the laptop two pin ones should be ok to use.

large_ghostman said:

How about laptop bricks with no earth to start with? Is it ok to tie two of these together to make a +/- supply?

Click to expand...

Yes.

Noisy, and maybe not the best regulation in the world, but double/reinforced insulation safe (assuming the safety certifications are valid).

ak

I think I've seen laptop supplies where the output -ve is earthed, so best check if you can.

The secondary side of small, two-prong on the AC side, wall-warts are transformer-isolated between the AC side and DC side. You can check that with an Ohmmeter. They are tested for leakage at some huge Voltage, like +-4kV.

When I needed to detect presence or absence of 240Vac line Voltage from an Arduino, I used an old cell phone charger I had laying around. 100 to 250Vac input gives an isolated 5Vdc output which I sense in the Arduino. There is no Ohmic connection from the 0V side of the charger to either AC prong.

You could connect two such wall-warts together on their output side and make -5V,0V,+5V or 0V,5V,10V or whatever you like...

large_ghostman said:

Having a blank moment and want to check something. I am aware with the old ATX supplies you cant use two together because the GND being tied to earth. If you want to use two you cut the earth on one (bad idea).

Click to expand...

Not really. You can isolate the PCB from Ground, not the metal case.

MikeMl said:

The secondary side of small, two-prong on the AC side, wall-warts are transformer-isolated between the AC side and DC side. You can check that with an Ohmmeter. They are tested for leakage at some huge Voltage, like +-4kV.

When I needed to detect presence or absence of 240Vac line Voltage from an Arduino, I used an old cell phone charger I had laying around. 100 to 250Vac input gives an isolated 5Vdc output which I sense in the Arduino. There is no Ohmic connection from the 0V side of the charger to either AC prong.

You could connect two such wall-warts together on their output side and make -5V,0V,+5V or 0V,5V,10V or whatever you like...

Click to expand...

Thanks Mike that confirms what i was hoping. Those laptop chargers are difficult to get into without alot of surgery and i wanted to avoid that. Some have 3 pins but i have some that are definitely two pin into the brick, they are also a pretty good voltage and around 3A (these are oldish ones).

Its difficult trying to use things i think the other team will be able to get hold of as well, although its looking like the infrastructure over there is just about collapsed in some places.

I am not being political about it, but they came up with all the original ideas, for as long as i can i want to work with them. But if it gets to the point they cant continue then reluctantly i might have to go my own way. I really dont want to say where they are for there sake, but there is alot of civil war going on and academics are targeted. Shame as alot of the stuff they wrote 8 years or so ago was bleeding edge at the time, now they scratch around for jam jars as flasks in make shift labs.