Luxeon Rebel 145 lumens output

[bascerballer4]

I found this website on LM317t. This is just a reference for myself. Anyone who is curious about this issue also may feel free to use this link.

http://www.reuk.co.uk/Using-The-LM317T-With-LED-Lighting.htm

 

[AZ Sky]

OK, the following circuit will do what you want:

The current through the led's is set by the LM317 regulator and resistor setup as a current regulator instead of voltage regulator. The current that the regulator/resistor combination supplies is determined by the formula I=1.25/R where I is the regulated current, 1.25V is the fixed regulator reference, and R is the resistance from the LM317 out terminal to the adjust terminal.

I recommend that you use a separate current regulator for each string of leds for the following reasons.

(1) Spreading out the power dissipation. What you want to do isn't the most efficient circuit design. Each resistor will dissipate almost 1 watt each and whatever "extra" supply voltage you use will be dissipated in the regulator - for example if you use a 18V supply voltage, you will have 18V minus the 1.25 volts across the resistor minus the 14.4 volts across the string of leds, this leaves 2.35 volts times 700ma = 1.64 watts in the regulator. If you did this with just one regulator you would be dissipating over 5.5 watts in that regulator.

(2) Balanced led output. There is no way to be sure that each four led string will have a balanced current draw and will draw exactly 700ma. If you hook all four strings in parallel to a single regulated current source it is quite possible for one string of leds to draw more current that the others leaving the other strings not as bright.

As far as the required power supply voltage, you will need one that supplies at least enough voltage to cover the forward voltage of the led string plus the 1.25 volts across the resistors - at bare minimum I would say a 16V supply but 18V would give you a little headroom. You will be supplying 4 strings of leds at 700ma each, that is 2.8 Amps total so make sure the power supply you use will handle at least 3 Amps, I would recommend 5 Amps for a little overhead.

 

[Luminescene]

That link looks right, you would need 1.78ohms for 700ma and at least 1watt, 1.8 ohm 2.5w is made by Vishay, part number RWM 4X10 1R8 5%, and would give you 694ma, close enough.

 

[bascerballer4]

Couldn't say it any better AZ Sky. Thanks for the whole circuit, this helps so much more. Thanks a whole bunch. I will post pictures a few weeks from now when I'm done with my set up. :rock: :rock: :rock: :rock:

Thanks for the part # Luminescene.

After reading the link, this is what I came up with also. This is similar to AZ Sky's. His is way much better

 

[bascerballer4]

Lumiescene, the part number you gave me says that the resistor is a 3 watt. Will 3 watts make a big difference in my LED output?

 

[bascerballer4]

I found a company that makes the resistors I need.
http://www.goldmine-elec-products.com/prodinfo.asp?number=G14549

 

[bascerballer4]

Found a company that has 16V 3.5amp adapter
http://stores.channeladvisor.com/Digitalet/items/item.aspx?itemid=76296

Can someone verify if these items I listed will work on this circuit?

 

[Luminescene]

Lumiescene, the part number you gave me says that the resistor is a 3 watt. Will 3 watts make a big difference in my LED output?

No, so long as the wattage is more than needed its fine.
Good luck

 

[TorchBoy]

After reading the link, this is what I came up with also. This is similar to AZ Sky's. His is way much better

Better in that it's actually right? It's perhaps not as cute as your diagram, but do be careful where you put that current set resistor. (Like, on the other side of the ADJ connection.)

Found a company that has 16V 3.5amp adapter
http://stores.channeladvisor.com/Digitalet/items/item.aspx?itemid=76296

Can someone verify if these items I listed will work on this circuit?

Will it supply comfortably over 15.65V with a 2.8 amp load? Sounds like you're trying to cut it fine again. Don't forget heatsinks for the LM317Ts.

Edit: When they say it's 16V, that's a nominal value. The page wouldn't load for me so I couldn't see if it's switchmode (probably is if it's for a laptop) but even so there is some variance in its output voltage.

 

[bascerballer4]

 

[bascerballer4]

 

[TorchBoy]

When I mentioned variance in the power supply I meant as a load was applied the voltage will drop.

From figure 8 in the Motorola LM317T datasheet it looks like there's almost a 2 volt drop between Vin and Vout at room temperature. That's in addition to the 1.25V drop across the current setting resistor. (I knew there was some reason why I don't use an LM317T for battery powered stuff.) So IMHO you should go with at least the 18V power supply.

Actually IMHO you'd be just as well off using a single resistor for each series of LEDs. It's cheaper, easier to put together, smaller, and just as efficient, since you want to drop just as many volts, one way or another, and your source voltage isn't going to steadily drop over time because it's not a battery.

FWIW the only way to get better efficiency would be to use pulse width modulation, but even then, if you were pulsing them at a current at which the LEDs don't make light efficiently, it's a lot of hassle and cost for no good return.

How much area of heat sink would I need for each regulator?

How long is a piece of string? How cool do you want them to run? How much do you want to spend on them? How big do you want them to be? I strongly endorse the KISS principle, hence using resistors.

 

[bascerballer4]

Torchboy, you probably have some experience with luxdrive. Would you recommend me to use Luxdrive's PowerPuck 700mA?

"This drive uses filtered switching technologies and LEDdynamics' proprietary drive and sense electronics to provide constant current to LED arrays," ledsupply claims.

 

[bascerballer4]

When I mentioned variance in the power supply I meant as a load was applied the voltage will drop.

From figure 8 in the Motorola LM317T datasheet it looks like there's almost a 2 volt drop between Vin and Vout at room temperature. That's in addition to the 1.25V drop across the current setting resistor. (I knew there was some reason why I don't use an LM317T for battery powered stuff.) So IMHO you should go with at least the 18V power supply.

Actually IMHO you'd be just as well off using a single resistor for each series of LEDs. It's cheaper, easier to put together, smaller, and just as efficient, since you want to drop just as many volts, one way or another, and your source voltage isn't going to steadily drop over time because it's not a battery.

FWIW the only way to get better efficiency would be to use pulse width modulation, but even then, if you were pulsing them at a current at which the LEDs don't make light efficiently, it's a lot of hassle and cost for no good return.



How long is a piece of string? How cool do you want them to run? How much do you want to spend on them? How big do you want them to be? I strongly endorse the KISS principle, hence using resistors.

Can you draw a diagram like AZ Sky's diagram to show way of putting 16 leds together with an 18 volt adapter? Would the adapter need to be atleast 3 amps also?

 

[TorchBoy]

Torchboy, you probably have some experience with luxdrive. Would you recommend me to use Luxdrive's PowerPuck 700mA?

"This drive uses filtered switching technologies and LEDdynamics' proprietary drive and sense electronics to provide constant current to LED arrays," ledsupply claims.

I've never used one, and only seriously considered their MicroPuck, since my needs are a bit different to yours. However, I couldn't quite convince myself of its value for what I wanted to do. I like the way it's so versatile though.

Some thoughts for you.

The PowerPuck needs an input margin of at least 2V, so you'd probably need the 18V power supply, where it would offer about 86% efficiency (from figure 1 in its datasheet). Using a plain old resistor would be "only" about 80% efficient and a lot cheaper, since you'd want a 700mA unit for each string of LEDs, four in total.

If you used the 16V power supply the PowerPuck might drop out of regulation or it might run the LED strings at a bit less than 700mA, at roughly 90% efficiency. What current does 3.4-3.5V correspond to?

Can you draw a diagram like AZ Sky's diagram to show way of putting 16 leds together with an 18 volt adapter? Would the adapter need to be atleast 3 amps also?

Look familiar? I like this diagram. R1 = 5.6ohm (as you calculated), 3 watt. Simple and cost effective. I'd personally try it that way first, find that I was happy with the way it worked, and instead of worrying about getting the extra 6% efficiency I'd spend the money on something else (like more LEDs, petrol for the next caving trip, etc). YMMV (no pun intended).

4 series of LEDs each at 700mA = 4 x 0.7 = 2.8A, so yes, it would need to supply at least that much.

I hope those thoughts are of some help. Can anyone think of other reasons to use a PowerPuck or LM317T?

 

[bascerballer4]

Your thoughts of the diagram is how I would have made my first circuit. I thought of making it but I've read a few bad things about making a circuit for LEDs without a current regulator.

One reason is because the current passing through the LEDs need to be at a constant current of its capability. Whether your LEDs are kept at 350mA or 700mA, if somehow the current is too high out of the LED's maximum current, the LED can burn out.

Another reason why LEDs need a current regulator is because the different fluctuations in the current will shorten the lifetime of the LEDs and often even kill the LEDs. LEDs need to be kept at a constant current.

These are the main reasons why I wanted a circuit with an LM317T is because it help regulate the current passing through the LEDs which allow me to have healthy LEDs that will last to their optimal lifetime hours.

But since I also like the idea of conserving some extra cash on the side, I might take your advice of just using resistors without the LM317T, if there is enough evidence that resistors alone will keep LEDs working properly.

Anyhow, do you know where I can find such an adapter with 18V and atleast 2.8amps? I've looked all over town today for an 18V 2.8amp+ adapter but did not find any.

I found one adapter online, IBM 16V 4.5amps adapter, but I am not quite sure how I can connect the positive and negative onto my circuit when the end-part of the cord from the adapter seems to be just one wire. Any thoughts?

 

[TorchBoy]

One reason is because the current passing through the LEDs need to be at a constant current of its capability. Whether your LEDs are kept at 350mA or 700mA, if somehow the current is too high out of the LED's maximum current, the LED can burn out.

Leading causes that come to mind for any of the above-mentioned setups are dodgy soldering, placement of components that invites crossing the leads, and sabotage. Resistors are very unlikely to go short-circuit.

Another reason why LEDs need a current regulator is because the different fluctuations in the current will shorten the lifetime of the LEDs and often even kill the LEDs. LEDs need to be kept at a constant current.

Actually, LED current can vary hugely and not be significantly detrimental as long as you're not exceeding specified maxima. 700mA is very comfortably under the Rebel's maximum rating of 1A, and even with a resistor, the current in your setup would be constant... with one possible exception.

Heat is very bad for LEDs. Run those Rebels too hot and you will shorten their life, whatever you're using to drive them. (Also, without adequate cooling you risk thermal runaway.) How are you planning on heatsinking these Rebels?

Dare I suggest it, but if you're unsure about this stuff, perhaps you should be using star LEDs instead?

But since I also like the idea of conserving some extra cash on the side, I might take your advice of just using resistors without the LM317T, if there is enough evidence that resistors alone will keep LEDs working properly.

My vote goes for spending the money on fancy heatsinks instead of fancy power supplies. Keep any LED cool and you maximise its life.

Anyhow, do you know where I can find such an adapter with 18V and atleast 2.8amps? I've looked all over town today for an 18V 2.8amp+ adapter but did not find any.

I'm sorry, I must have misinterpreted post 39 as saying you had found one.

Sounds like you need to go back to the drawing board and do a big rethink on this project.

 

[bascerballer4]

Nah, I think I've done enough HW to get this project up and running. Like you said, its simple to run these LEDs on resistors, but I'm not a risk taker. I would rather spend another $20 on a constant current device that will save $40-50 from purchasing the LEDs, rather than running them just on resistors have problems and burn my LEDs.

I am not going to run these LEDs just on resistors alone, that is one problem most people make. Resistors are going to help reduce current that is coming through the LEDs, but a little power voltage change from the power supply and the current fluctuates. Next thing you know, your have just burnt your LEDs.

The reason why they make LM317T is so there can be a constant current flowing through the LEDs that is why AZ Sky suggest using LM317T on this setup. This helps regulate the current consistently even if there is change of current or voltage from the power supply. If the current shoots up and little from the power supply, the regulator will use that extra current and dissipate it out to its regulator, releasing heat. This is the reason why LM317T will need a heat sink.

The reason why LuxDrive makes BuckPucks, MicroDriver, PowerPuck, and Xitanium is because they know from experience that resistors alone will not help regulate the current and they know it is the common thing to do. They make these products so LEDs can be driven at a constant current, and no heat sinking is required.

If I run my setup on LM317T on each series of LEDs, I will need heatsinking for these LEDs, but not as much as I will need if I was to run them on resistors alone because the LM317T will dissipate most of the heat for me.

Depending if I'm running these leds on LM317T or a PowerPuck, I will need to determine the heat sinking needed. Heatsinking, I will worry about later after I figure out if I should run them on LM317T or PowerPuck.

 

[bascerballer4]

Anything else I need to know before I get this project started? Just making sure

 

[Russell52]

So I guess you have to actually call them to place a order for the new rebel,Cause I couldnt find a buy link:/???