Led Down Light

i want to power a 3.2 V 20mA led light. if i use a 3.2V dc power supply with 400 mA rating what happens?
will i burn the LED up with 400mA? or do i need to bring it down to 20mA before i connect it to the LED
LEDs always need a current limited supply. There are many ways to achieve that, but a voltage supply with a resistor in series with the LED is the simplest. The resistor is calculated by ohms law. Use the Supply voltage - the LED forward voltage (Vf) for the voltage. Use the desired LED current for the current. A figure like 10 or 20mA may suit.
Use R = V / I to calculate the resistor.
As you can see, the 3.2V supply leaves no room for the resistor to limit the current. You need a few more volts than the LED itself needs, so that V isn't zero or negative.
Yes you do need to limit the current, and the simple resistor method requires there is more than 3.2V. Perhaps 5V to 12V is fine. The answer is not obvious, if you connect the LED directly. The voltage is the "same". It may not light, it may light brilliantly and very briefly before it fails, it may even work for a while with indeterminate brightness then fail. Depends just how the power supply matches the LED. No good.
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