| Electronics > Projects, Designs, and Technical Stuff |
| Power Supply Conversion |
| (1/4) > >> |
| fsleeman:
I am in the process of converting an unused PC power supply to a bench type supply as mentioned in many places online. Most of the time I will be using the LM317T circuit I am adding to run off the +12V line but will also be using the 3.3/5/12 lines directly. My only concern is short circuit protection for the three voltage lines. The variable output should shut down if shorted but I am not sure what will happen if, say the 5V line, is shorted, and not sure I want to find out. Does anybody have experience doing this or ideas how to make this a little safer? What dose a commercial bench power supply do when it is shorted? |
| armandas:
--- Quote from: fsleeman on March 12, 2010, 09:35:46 pm ---What dose a commercial bench power supply do when it is shorted? --- End quote --- Shuts off I suppose. It's called current limiting. |
| desolatordan:
They usually use an opamp to compare the voltage across a sense resistor to a setpoint, then use the output of the opamp to mess with the feedback in order to lower the output voltage. There is an example of an adjustable constant current/constant voltage circuit in the LM317 datasheet. |
| mkissin:
I urge people to be very careful when using an old computer power supply as a lab bench supply. It's perfect as long as everything is fine, but when something goes wrong, it tends to go horribly wrong. Take the 300W supply that I have lying around in my workshop. It's rated to supply 20A on the 3.3V, 30A on the 5V and 15A on the 12V rail. Now say you hook something up incorrectly and your +5V rail gets shorted to ground through one of your components. That PC power supply is just going to pump 30A through the fault, happy as Larry, until something on your board dies catastrophically (most components tend to die as short circuits, not open, so whatever dies will probably actually be blown apart) or one of your PCB tracks vaporises. Im not saying don't do it, because it's obviously the cheapest and easiest way to get a bench supply, but be very careful when powering a new circuit with it! You should probably add fuses inline with each of the high current voltage rails. They won't save semiconductors from death, but they will generally prevent more extreme failures. Also note that often computer power supplies require a specific output rail to be loaded to a certain point before any of the other rails will come into regulation. It's probably either the +12V or +5V rail, and you can just add a fixed resistor inside the supply to draw a constant few hundred milliamps. That tends to be enough. |
| fsleeman:
--- Quote from: mkissin on March 13, 2010, 02:58:19 am ---Now say you hook something up incorrectly and your +5V rail gets shorted to ground through one of your components. That PC power supply is just going to pump 30A through the fault, happy as Larry, until something on your board dies catastrophically (most components tend to die as short circuits, not open, so whatever dies will probably actually be blown apart) or one of your PCB tracks vaporises. --- End quote --- Yep, that is what I am concerned about and fuses seems like the simplest solution. I have never added fuses to anything I have designed before, is there anything special I should know about? --- Quote from: mkissin on March 13, 2010, 02:58:19 am ---Also note that often computer power supplies require a specific output rail to be loaded to a certain point before any of the other rails will come into regulation. It's probably either the +12V or +5V rail, and you can just add a fixed resistor inside the supply to draw a constant few hundred milliamps. That tends to be enough. --- End quote --- From what I have read, most people suggest a 10W 10 Ohm resistor on the 5V line. |
| Navigation |
| Message Index |
| Next page |