I am new to hardware.

My breadboard just arrived and I am trying to light up my LED. I used computer power supply for power (red wire for + black for -) and used strands of LAN cable for connect LED and power supply. It works!

But the both of two stands of power supply side cable jacket melted and computer slowly shutted down.

I just use old computer power supply's FDD power cable's strands (red for plus and black for minus).

Could somebody let me know why this happened and how to use power supply safely?

  • 1
    \$\begingroup\$ You should read up on the basics (voltage, current, power, use of resistors to limit current through a LED) before you continue. Otherwise you'll destroy a lot of stuff and maybe set the house on fire. \$\endgroup\$
    – starblue
    Apr 9, 2011 at 8:25
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    \$\begingroup\$ what book or document do you suggest? \$\endgroup\$
    – kim taeyun
    Apr 9, 2011 at 8:49
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    \$\begingroup\$ allaboutcircuits.com/vol_3/index.html \$\endgroup\$
    – rwong
    Apr 9, 2011 at 19:27

2 Answers 2


seems like somewhere in your setup, there is a short circuit. Computer power supplies deliver a very high current easily capable of heating up wires. Can You try to draw a circuit diagram or upload a picture that shows the details of how your setup is wired? Are you using a resistor in series with the LED?

You might want to try your first experiments with a power supply not as strong as a PC power supply (old wall wart with something 5V / 1A, ...).

Also, I recommend not using the power of your running PC for experimentation. You risk damaging your entire PC, not just the power supply.

  • \$\begingroup\$ there's nothing special, PC Power supply(red,black)=> two strands of lan cable => breadboard => wire => LED. could you tell me what do you suggest for my experiments? 9V or 5V battery charger would be good choice? \$\endgroup\$
    – kim taeyun
    Apr 9, 2011 at 8:44
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    \$\begingroup\$ Battery chargers do not necessarily output a regulated DC voltage. They may be current regulated or even give you a pulsed output. Are you using a resistor in series with the LED? \$\endgroup\$
    – zebonaut
    Apr 9, 2011 at 9:25
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    \$\begingroup\$ If it's not out of your budget, you can find pretty cheap laboratory power supplies on eBay and craigslist. The advantage here is that you can limit the output current and not risk destroying anything. Once you're comfortable that your circuit is generally correct, then you can slowly increase the output current to the amount you need. \$\endgroup\$
    – Dave
    Apr 10, 2011 at 5:10
  • \$\begingroup\$ If you must, save the power supply brick from a broken external HD enclosure's power supply and cut off the connector. Usually these are +5 and +12V. Bring them out to a small box with binding posts. Or just alligator clips (insulated!). You'll have something you can use for many different projects. \$\endgroup\$
    – dmoisan
    Jun 5, 2011 at 15:21

An LED acts like a diode, and in the forward bias (when the voltage is applied in the correct direction), the diode will have a small voltage drop. The voltage drop for LED can be found in Wikipedia.

Notice that diode does not behave like a resistor (does not obey Ohm's law): once a diode is in forward bias, its voltage drop will remain fairly constant for a large range of currents. That is, until the diode melts or overheats due to overcurrent. Other electrical elements may also melt.

If your voltage source is much larger than 0.7V, this will lead to a very large electrical current flowing, and the heat caused by the electrical current will be dissipated along various places in the circuit (because the remaining voltage drop will have to be distributed along the rest of the circuit, which consists of mostly good conductors and therefore has a very low resistance).

To prevent this from happening, please connect a resistor in series with the LED, or use a voltage source that is about the same as the LED's voltage drop.


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