0.5Watt LED Driver - Electronic Circuits Projects Diagrams Free

yngndrw · 26th August 2007, 12:38 AM

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 · 26th August 2007, 01:52 AM

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 · 26th August 2007, 02:02 AM

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 · 26th August 2007, 02:15 AM

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 · 26th August 2007, 02:34 AM

Quote:

Originally Posted by 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 ?

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 · 26th August 2007, 02:54 AM

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 · 26th August 2007, 11:20 AM

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 · 26th August 2007, 02:27 PM

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 · 26th August 2007, 02:49 PM

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

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26th August 2007, 12:38 AM	(permalink)
yngndrw	0.5Watt LED Driver 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.

26th August 2007, 01:52 AM	(permalink)
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. __________________ Uncle $crooge

26th August 2007, 02:02 AM	(permalink)
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 ?

26th August 2007, 02:15 AM	(permalink)
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. __________________ Uncle $crooge

26th August 2007, 02:54 AM	(permalink)
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.

26th August 2007, 11:20 AM	(permalink)
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. __________________ I also post at the following sites: http://www.stop-microsoft.org http://www.heated-debates.com Screen name: Aloone_Jonez And http://www.silicontronics.com, same screen name as here.

26th August 2007, 02:27 PM	(permalink)
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.

26th August 2007, 02:49 PM	(permalink)
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. __________________ If you don't have a planet, what good are gold bars? want to contact me directly? gmail gordonthree check out my project website: http://projects.dimension-x.net Favorite numbers: 09 F9 11 02 9D 74 E3 5B D8 41 56 C5 63 56 88 C0