Forums » General Electronics Discussion

I'm trying to power a 7 Watt LED with a 3.6 Volt battery supply and current from a generator that produces at 17mA. Can someone help me figure out the correct resistors and components needed for this circuit? Thanks.

Here is the circuit: https://www.circuitlab.com/circuit/884b3j/led-crank-circuit/

Power = Volts * Amps.

3.6V * 0.017A = 0.0612W

I don't think you will get much light out of a 7W led with only that much power available.

A high power LED will have a forward voltage of around 3.5V - 4V. If your battery is 3.6V you will have very little, if any, excess forward voltage with which to drive current through a resistor into the LED.

You need a battery that can source much more than 17mA and a step-up (Boost) switching power supply set up to regulate the current through the LED.

See:

and

for more about LEDs and driving them from a voltage source close to the LED forward voltage through a resistor.

okay so I updated the circuit. I increased the capacitor to 600uF and R3 is now 10e-3 Ohms. Right before the LED I'm getting 3.299 V and 606 mA with the 3.6 volt battery. btw I'm using AA NicCad batteries. But when I increase the battery to 4.8 Volts (4 batteries) the voltage right before the LED is 4.138V and the current is 1.315A which means that the wattage is 5.44Watts. Which should be enough to power the 7 Watt LED. but are there any problems with this? And how long will this power the LED for?

Sorry but you can't expect a 10mR resistor to have any effect in controlling the current through your LED.

10mR is almost certainly much less than the sum of the switch contact and the battery source resistance. So the current through the LED is limited by those resistances not the 10mR resistor.

Any LED needs to be driven essentially by a constant current source. A voltage source with a fixed series resistor will do as a rough approximation to this only if the voltage source is reasonably stable and the drop across the resistor swamps the change in forward voltage vs. current of the LED.

Try sweeping your battery voltage by even just a few 100mV and see how the LED current changes widely.

As to how long you can power the LED, that depends on the capacity of the batteries and how well the load current is controlled.

I think you would do well to look at LED driver circuits and chips from the usual vendors such as TI, OnSemi, Diodes Inc., Linear Tech., etc.

You will find that they are very different from just a battery, a resistor and a LED.