LED Lowervoltage resistor

[jayjay011]

I have a 5.4 volt battery and need some help on how to lower the voltage to 3.7-4 volts, which resistor do I use or any ideas?

thanks

JB

 

[audioguru]

An LED sets its own voltage. Your circuit must limit the current usually with a series resistor.
Ohm's Law is used to calculate the value of the resistor.

Assuming that your 3.8V LED works well with a current of 20mA then the resistor is (5.4V - 3.8V)/20mA= 80 ohms. An 82 ohm resistor is the closest standard value.

 

[jayjay011]

it is a ULTRAFIRE T6 5Mode CREE 900 Lumens XM-L LED Lamp Bulb
Operating Voltage: 3.7-4.2V and it is only one LED I am trying to use in my torch so I will only need one 82 ohm resistor?

 

[audioguru]

I have never heard of a 5.4V battery. It is not lead-acid, Ni-Cad, Ni-MH, Alkaline, carbon-zinc nor Lithium. What is it?

I could not find a datasheet for the LED you are using so I don't know its maximum allowed current. For as torch you probably want a current a lot more than only 20mA so the resistor value must be much less and maybe the current should be regulated.

 

[colin55]

You need a buck circuit:
**broken link removed**

BUCK CONVERTER for HIGH-POWER LED 250mA - 1watt LED
This circuit will drive 1watt white LED from a 5v4 supply and is capable of delivering 300mA.
The driver transistor is BD 327 and the inductor is 70 turns of 0.25mm wire wound on the core of a 10mH inductor.
The voltage across the LED is approx 3.3v - 3.5v
The 1R is used to measure the mV across it. 300mV equals 300mA LED current.
The diode MUST be high speed. A non-high-speed diode increases current 50mA!
This circuit is the best design as it does not put peaks of current though the LED. Reduce 390R slightly to increase max. current.

 

[carbonzit]

You need a buck circuit:

This circuit will drive 1watt white LED from a 7v supply and is capable of delivering 300mA.

But Colin, the O.P. said they have a 5.4V battery. (Type unknown so far.) So that ain't gonna work.