0.5Watt LED Driver

[yngndrw]

Hi,

Most high power LEDs are run off constant current drivers, but is this really needed ?

Can a 0.5Watt LED be run using a normal safety resistor as long as the resistor is rated high enough and the LED has a heatsink ?

Are the only reasons these LEDs are run off constant current drivers because of varying input voltages and for efficiency ?

Thanks.

 

[audioguru]

LEDs are rated for current. Their forward voltage is a wide range of voltages so you must measure the forward voltage to calculate a current-limiting resistor for it.

 

[yngndrw]

Well the LED spec says:
Forward Voltage : 3.0v-4.0v
Forward Current : 150mA

Should I assume the lower or upper end of the forward voltage scale ?

 

[audioguru]

How can you assume a voltage? Assume the lowest voltage then most LEDs will be too dim but none will burn out.

You either measure the forward voltage and calculate a current-limiting resistor for an LED or you make a constant current source for it.

 

[pc88]

Well the LED spec says:
Forward Voltage : 3.0v-4.0v
Forward Current : 150mA

Should I assume the lower or upper end of the forward voltage scale ?

Assuming the lower forward voltage will result in a conservative (higher) estimate for the resistor. However, if your supply voltage is not close to 3V (like >6 V), using either value is not going to make much of a difference.

That said, it wouldn't hurt to actually measure it.

 

[yngndrw]

Okay then I'll measure it, is a constant current source required to measure the forward voltage drop or can I just use a safety resistor which is in a reasonable range ? It's a little embarrassing to have to ask such questions about an LED, but I never really looked into them that much.

Thank you for your help.

 

[Hero999]

Use a proper current source rather than a simple series resistor, then you won't have to worry about the forward voltage providing your regulator can provide the voltage at the lowest supply voltage.

 

[yngndrw]

True, I was thinking of just testing how bright it was untill I can get a proper regulator setup, but you're right I may aswell just get a regulator sorted out now - It would be a different brightness anyway.

Thanks everyone.

 

[justDIY]

LM317 and a cheap 9-12vdc ac-dc wall wart can be your constant current source. Its a great setup for bench testing things.