Absolute beginner - LED power supply

[taj1987]

Hello,

I am at a dead end and could really do with some help.

I need to power Luxeon LEDs (forward current 700 mA, forward voltage 6.84 V) for an experiment but have absolutely no idea how to power them. I have acces to universal power sources, can I use one of these and attach it with crocodile clips?

The LED can be found at https://uk.farnell.com/lumileds/lxhl-...?Ntt=lxhl-pm02

I have found LED power supplies such as **broken link removed** but have no idea whatsoever if this would do the job.

Am I always going to require a resistor with an LED, I was trying to avoid this because I cannot find anyone to help me in person and am completely clueless. I have other LED options 3.0 mW, T-1 3/4 or 7 mW, TO-1 3/4, would these still require resistors?


I apologise for my lack of knowledge but have contacted all of the suppliers of the LED and power supply that I could find but have had no help

 

[hannobisschoff@gmail.com]

Can you please tell what supply voltage you're using. Then it's just a current limiting resistor, and you're good to go.

 

[hannobisschoff@gmail.com]

Are the LEDs intended for lighting purposes?

 

[taj1987]

The LEDs are to excite certain tracers. I am going to suspend them, somehow, not quite figured that bit out yet. I am not sure what you mean by supply voltage. I have access to regular power sources or I can purchase another one if required (or if it's easier). Do all LEDs need resistors? For example, are mounted LEDs exactly the same?

 

[hannobisschoff@gmail.com]

Hi, what I mean by the power source, is:are you going to use a battery, a wall plug, etc. And no, not all led's need current limiting resistors, but the thing with led's are, they "have VERY little internal resistance. So, according to Ohms law (just ask about this if you don't know) the greater the voltage, the greater the current. Now, current is the "harmful" one, so we need a resistor to slow the flow of current down. This is only needed if your source current (the electricity out of the battery) is higher than your LED current maximum. In your case 700mA. Do you see why the power source you are going to use is so important. So in order to decide the size of the resistor that one must use, you need to know the supply voltage. (what you use to power the circuit). What are you going to use to power your LEDs?

 

[Reloadron]

Take it easy, this will not be difficult. I can't seem to get your links to work in your first post. This may be because I am at work but you may want to check your links.

As to the resistor thing. All you need to know is the voltage you plan to run the LED at. For example if I have a 12 volt power supply and a forward current 700 mA, forward voltage 6.84 V LED we get something like this.

Vsupply 12 - Vled 6.84 / Iled .7 amp = 7.37 Ohms and since there is no 7.37 ohm resistor you would use a 10 ohm resistor rated for 5 or 10 watts. Actually 10 ohm 10 watt are real common.

Anyway, this is why people want to know what your supply voltage will be. Rest assured you can toss this together.

Ron

 

[hannobisschoff@gmail.com]

Take it easy, this will not be difficult. I can't seem to get your links to work in your first post. This may be because I am at work but you may want to check your links.

As to the resistor thing. All you need to know is the voltage you plan to run the LED at. For example if I have a 12 volt power supply and a forward current 700 mA, forward voltage 6.84 V LED we get something like this.

Vsupply 12 - Vled 6.84 / Iled .7 amp = 7.37 Ohms and since there is no 7.37 ohm resistor you would use a 10 ohm resistor rated for 5 or 10 watts. Actually 10 ohm 10 watt are real common.

Anyway, this is why people want to know what your supply voltage will be. Rest assured you can toss this together.

Ron

Couldn't have said it better. And there are tons of LED calculators on the internet if you are lazy.
Google is your best friend, Blah blah blah. just search for "LED calculator"

 

[hannobisschoff@gmail.com]

Let us know if you figured it out

 

[crutschow]

Do all LEDs need resistors? For example, are mounted LEDs exactly the same?

To answer this directly, yes, all LEDs need some method of limiting current through them. This can either be a resistor in series (infrequently done as part of the LED package) or by an active constant-current type circuit.

The resistor is fine for most uses. A constant-current circuit is better if you have a significant variation in the supply voltage (such as an automobile battery) and you want the LED light output to stay constant independent of supply voltage.

 

[hannobisschoff@gmail.com]

To answer this directly, yes, all LEDs need some method of limiting current through them. This can either be a resistor in series (infrequently done as part of the LED package) or by an active constant-current type circuit.

The resistor is fine for most uses. A constant-current circuit is better if you have a significant variation in the supply voltage (such as an automobile battery) and you want the LED light output to stay constant independent of supply voltage.

What, like a voltage regulator?

 

[crutschow]

What, like a voltage regulator?

No. But you can use a regulator, such as an LM317, to make a constant-current source.

 

[taj1987]

Ah yes, my links have (...) in the middle. Hopefully the one below will work.

https://uk.farnell.com/lumileds/lxhl-pm02/led-luxeon-v-green/dp/1106605?Ntt=lxhl-pm02

Thanks everyone, your responses are just what I needed.

In response to the power supply, it would be a regular wall plug socket, but I'm not entirely sure why this is relevant, would I not adjust a programmable power supply (I think this is what they're called,sorry last time I used one was in school) Or is this the type of power source that you were referring to?

 

[hannobisschoff@gmail.com]

But the 317 is a voltage regulator??

 

[Reloadron]

Taken from the LM117 / LM317 data sheet.

Also, it makes an especially simple adjustable switching regulator,
a programmable output regulator, or by connecting a
fixed resistor between the adjustment pin and output, the
LM117 can be used as a precision current regulator.

See page 18 of the linked to data sheet for a few examples of current regulation configurations.

Ron

 

[Noggin]

I haven't seen this mentioned so I wanted to say it. Those LED's are going to be very hot! You'll want them mounted on a good heat sink or they may die within seconds.