PC PSU into a Lab PSU

[Pontiacg5]

Usually they'll power up, but then cut off once the capacitors charge up. I'm not sure if it causes damage however - I suppose it could if your rails started floating too high.

From what I understand they are switchmode power supplies. They don't like running without a load, just like any switchmode laser diode driver.

 

[Cyparagon]

The difference is that laser drivers are constant current. You can't maintain constant current without a load, but you can maintain constant voltage without a load.

I've found they tent to shut down with sudden changes in load. So putting a power resistor on one of the rails makes any additional load less of a change.

 

[Things]

^ Found that out yesterday, when I went to plug my CNC driver board into the computer, the large capacitors on the board obviously wanted too much inrush, the computer turned off instantly

 

[Bionic-Badger]

I've got an old 300W Dell ATX power supply and converted it into a bench power supply. I added an LED + resistor across the PSU-is-plugged-in lines for an indicator and used a simple rocker switch to turn on the PSU. It doesn't care about having a load. Many of the older switched power supplies did care, but many newer ones have resistors built in that allow them to turn on without load.

 

[LaZeRz]

Also, some psu's have sensing lines on the 3.3v rail (orange) so if all the orange cables aren't connected it won't switch on.

 

[Cyparagon]

What if you don't use all the cables in your computer?

 

[bennett326]

so im working on a project similar to yours and i cant find how you make it cc and cv i can only find one or the other

help??

hahhah im a noob when it comes to electronics, i only know the very basics.

so take it easy please:thanks:

 

[LaZeRz]

IIRC you will have two make two circuits. one with a current regulator and one with a voltage regulator

I might be wrong though, im pretty tired

What if you don't use all the cables in your computer?

The 3.3v sensing cable is already bridged to another 3.3v cable in your 20 or 24pin connector

 

[bennett326]

IIRC you will have two make two circuits. one with a current regulator and one with a voltage regulator

I might be wrong though, im pretty tired



The 3.3v sensing cable is already bridged to another 3.3v cable in your 20 or 24pin connector

would it be as simple as making 2 lm850 circuits 1 for current and 1 for voltage

:yh:

 

[Bionic-Badger]

What if you don't use all the cables in your computer?

He means the orange wire connections in the ATX motherboard connector. There aren't any orange connectors in any of the other cables.

 

[bennett326]

so yesterday i bread-boarded some circuits a cc and a cv using the lm317 while waiting on my 338's i found that under a load w/ the multimeter still attached i don't get the correct reading so my question is how do you make it like the ones you would buy are ( where the meter displays the correct current fed to the load) ?

 

[Cyparagon]

under a load w/ the multimeter still attached i don't get the correct reading

You need to be way more specific.

 

[bennett326]

You need to be way more specific.

ok so i set up a voltage regulator w/ a 1m117 i had laying around i used a pot and i could adjust the voltage up and down. so i set it to 10v added a load( in this case a fan) with the DMM still attached and the multimeter went from a 10v reading to all over the place

better?

 

[Things]

A fan probably isn't a big enough load, and you'll also probably need to use the "dump resistor on the 5V line" trick to get it stable.

Even a very basic computer draws a few hundred watts.

 

[bennett326]

A fan probably isn't a big enough load, and you'll also probably need to use the "dump resistor on the 5V line" trick to get it stable.

Even a very basic computer draws a few hundred watts.

thank you :beer: how big does my dump resister need to be the only high W resisters i have is 1 ohm 10 W i'm guessing that's not enough resistance

EDIT: also are there any other lm317 variants like the lm338 that go above 5A

 

[LaZeRz]

I used a LD1084 because it can handle 5 amps of draw and has lower voltage dropout than the LM series.

I also used a ceramic plate resistor (10ohm, >10watt) thermally glue'd to a heatsink.