What electrical components should i USE?

I am making my own power regulator by using a Computer power Supply...The output its 3.3V (30.0 A), 5V (30.0A) and 12V(15 A) .

1. I wanted to put LED as an indicator for all the 3 output, but i know that the max Voltage for a LED is 5V, and if i use a resistor for 5V and 12V to change to voltage suitable for the LED, it will cut down the output and i cannot power up my 5V or 12V things.( I never combine all the power supply cable output to, but they are separate.)( Around 12 output so have to use 12 switch)



2. I wanted to put a variable resistor for the 12V(15A) output to control the voltage such as fan speed or motor speed, but when i test run by using a 50K惟 variable resistant to make a test run, the variable resistor come out smoke and burns.



Please teach me and tell how to put the fixed resistor for the LED and the correct variable resistor to control the voltage? And the safety precaution.

THANKSWhat electrical components should i USE?
Both your LED and fan speed questions point to a common problem, you are completely ignoring both the current and power dissipated. Trying to do this by thinking about voltage alone is pointless.



Starting with the LED situation:



%26gt;teach me and tell how to put the fixed resistor for the LED and the correct variable resistor to control the voltage?



Rule one: Always design LEDs based on current, not on voltage!



%26gt;but i know that the max Voltage for a LED is 5V,



Unless you bought some oddball LED with a resistor built in, I don't think so.



A common 5mm LED is usually driven at about 20 ma (0.02 amps). The forward voltage depends on the color, for a green LED it will be about 2.2 volts.



There are plenty of LED calculators on the Internet; if you connect it between the 5 volt output and ground, you will get a resistor of about 150 ohms (2.2 volt LED @ 20ma). If the LED is too bright, increase the value of the resistor.



The 0.02 amps that the LED uses is negligible compared to the 30 amps available.



%26gt;, it will cut down the output and i cannot power up my 5V or 12V things



Not sure what you mean, but computer PS are not designed to run with zero load, you probably need to put a load ( ~10 ohm 10 watt) on the 5 volt output for it to work properly.



%26gt;I never combine all the power supply cable output to, but they are separate.)( Around 12 output so have to use 12 switch)



No idea why you would do this. The outputs are not independent. There are plenty of examples on the Internet of DIY projects similar to this. Make sure you understand that the capacitors in these supplies can store enough power to do serious damage to you even if the power supply is unplugged.



On to the fan situation:



You need to know how much current the fan draws at 12 volts. It might be labeled on the fan, but check it with a multimeter to be sure.



For example, if the fan uses 200ma and you want to be able to drop the voltage down to 7 volts, you need a pot/rheostat of 25 ohms or a bit more that can dissipate at least 1 watt. Because of the way pots/rheostats are rated you probably want it rated at more than one watt.



12 volts - 7 volts = 5 volts max drop needed



P(watts) = volts X current(amps) = 5 X 0.2 = 1 watt dissipated in pot/rheostatWhat electrical components should i USE?
You can add a resistor for the LED, just take the output(s) from the point before the resistor is connected.

You need to work out the resistance - google for led resistor calculator



You need to know the power consumed by the motor or fan and chose a variable to suit. Computer fan controllers are easily available that will do this for you.