Amperes of the pins of a PSU.

[dency45]

I like to use my power supply for experimenting some electronic stuff in my breadboard. I have a question about the pins of the power supply, like the yellow it would produce 12v., in the description located in the power supply, the 12v will produce 15A. so if use the 12v, it will always produce 15A? or it will produce the right amount of ampere needed by the load connected to 12V?

 

[noidea_77]

It will produce the right amount of ampere and one cable can not carry the 15A for more then two seconds. A computer PSU is probably not the right one for electronic experimenting, because it's a switching power supply.

 

[InvalidError]

A 12V power supply with 15A rating means the output will be held at (close to) 12V as long as you draw less than 15A from it and do not change the load current faster than the PSU can cope with.

Since outputs will have some switching noise, additional filtering may be required depending on how noise-sensitive your experiments may be.

 

[djscribbles]

I would agree, you would probably be better off getting a wall transformer (like a cell-phone charger) for your needs, they're pretty inexpensive and should deliver about the same level of quality power (it will have some ripple, turn on transients, etc compared to a lab power supply).

However, I think you are asking if the PSU will force 15A to your load; electrically that is impossible, voltage=current*resistance, if you have a fixed voltage and a fixed load, then the current will vary based on the two. 12V/ 20kohm = 0.6mA; conversely 12V/.1ohm = 120A = smoke machine.

The 15A simply means that the PSU can source up to that amount before it starts having problems (sagging voltage, overheating, or damaging components).

As noidea_77 said, don't try to source to much current over one wire. Too much current in a wire will generate a lot of heat and melt the insulation on the wire.

And seriously, be careful, a computer PSU handles a LOT of energy, if you aren't careful you could seriously hurt yourself (also don't open the PSU for any reason, the caps inside are large enough to kill you).

 

[noidea_77]

I would agree, you would probably be better off getting a wall transformer (like a cell-phone charger) for your needs, they're pretty inexpensive and should deliver about the same level of quality power (it will have some ripple, turn on transients, etc compared to a lab power supply).

However, I think you are asking if the PSU will force 15A to your load; electrically that is impossible, voltage=current*resistance, if you have a fixed voltage and a fixed load, then the current will vary based on the two. 12V/ 20kohm = 0.6mA; conversely 12V/.1ohm = 120A = smoke machine.

The 15A simply means that the PSU can source up to that amount before it starts having problems (sagging voltage, overheating, or damaging components).

As noidea_77 said, don't try to source to much current over one wire. Too much current in a wire will generate a lot of heat and melt the insulation on the wire.

And seriously, be careful, a computer PSU handles a LOT of energy, if you aren't careful you could seriously hurt yourself (also don't open the PSU for any reason, the caps inside are large enough to kill you).

Many thanks for the details! I was to lazy for the full story.

 

[InvalidError]

(also don't open the PSU for any reason, the caps inside are large enough to kill you).

The size of caps is irrelevant, you will not die or even feel anything from licking a 2.7V 3200F high-current ultracapacitor which is about the size of a Guiness beer can. You also won't die from the discharge of a 100pF cap charged to 100kV (Graaff generator), though it may hurt a little.

What can kill is current and capacitors are not necessary for that. A metal fork in the outlet easily does the job since outlet voltage is high enough to draw potentially lethal current through skin.

 

[dency45]

Thank you guys.

 

[dency45]

Best answer selected by dency45.

 

[djscribbles]

The size of caps is irrelevant, you will not die or even feel anything from licking a 2.7V 3200F high-current ultracapacitor which is about the size of a Guiness beer can. You also won't die from the discharge of a 100pF cap charged to 100kV (Graaff generator), though it may hurt a little.

What can kill is current and capacitors are not necessary for that. A metal fork in the outlet easily does the job since outlet voltage is high enough to draw potentially lethal current through skin.

I suppose I should have clarified that one (and perhaps I've got some good old misinformation), but my understanding is that the caps hold enough charge for a long enough period of time to be dangerous even when the PSU is unplugged. I've heard a similar explanation for the dangers of the cathode on CRT monitors.

I haven't really ever questioned these warnings (from a high school A+ certification course a long long time ago), or perhaps the hardware has changed enough in 10 years that this is no longer true (though I would be surprised by that). I'm certainly curious to know if I've got one of those common misconceptions

 

[InvalidError]

I suppose I should have clarified that one (and perhaps I've got some good old misinformation), but my understanding is that the caps hold enough charge for a long enough period of time to be dangerous even when the PSU is unplugged.

The primary-side capacitors get charged to 350-400V in modern APFC PSUs and can hold significant voltage for 15-30 seconds after unplugging, which can indeed be hazardous if you manage to disassemble a PSU within 30 seconds of unplugging it or run the PSU PCB outside its enclosure which is definitely not recommended unless absolutely necessary.

As long as OP leaves the PSU inside its metal box, the only capacitors he is exposed to are the low-voltage output caps which are harmless to humans unless you poke needles through your skin to bypass skin resistance.

I've heard a similar explanation for the dangers of the cathode on CRT monitors.

The biggest danger with CRTs is not so much the 20-30kV cathode voltage. It is the reaction people might have when they get zapped that may cause far greater damages and injuries. The zap will definitely be painful but is harmless in itself. However, the reflex of yanking your hand away might cause your hand or clothes to snag the CRT, pull it off the bench, drop it on your foot/floor and send shrapnel flying across the workshop which may injure someone else, damage other equipment or induce someone else into damaging other equipment and/or injure himself.

 

[djscribbles]

Thanks for the info. While I don't intend to put any of it to the test, it is nice to know