Need (lots of) help on DIY light

[bradsab]

Yes most linear drivers will hold the current constant until the batteries reach some particular voltage, then will start to drop. The driver will always need to have some voltage across it, and mine is designed to keep up to 10A constant current down to 4.0V input, 3.6V output. It would continue to provide output as the voltage drops, down to at least 3.0V. After that I wouldn't be comfortable promising what would happen, but it would probably work down to 2.0-2.5V. At lower power settings, the minimum voltage is a little lower.

D

One thing I'm confused about: your driver will put out constant current at whatever amps you make it to be. But regardless of the amp level you choose, it will always have 3.6V output? All the charts I've seen on the SST-90 show higher amps = higher voltage. Looking again at this thread White LED lumen testing - Page 11 - CandlePowerForums (I attached the chart I'm referring to) can you hold the V to 3.6 and push it to 9A and still get 2136 lumens? Or hold 3.6V and drop to 1A and get 307 lumens? If lumen output is totally current driven regardless of voltage, could you make the output a constant 4.0V and raise the driver's efficiency?

 

[DIWdiver]

One thing I'm confused about: your driver will put out constant current at whatever amps you make it to be. But regardless of the amp level you choose, it will always have 3.6V output? All the charts I've seen on the SST-90 show higher amps = higher voltage. Looking again at this thread White LED lumen testing - Page 11 - CandlePowerForums (I attached the chart I'm referring to) can you hold the V to 3.6 and push it to 9A and still get 2136 lumens? Or hold 3.6V and drop to 1A and get 307 lumens? If lumen output is totally current driven regardless of voltage, could you make the output a constant 4.0V and raise the driver's efficiency?

No, the driver determines the current, then the LED determines the voltage based on that current. The reason that I've been saying 3.6V is that when you push 10A through the SST-90, you end up with about 3.6V (according to the I vs. V plot in the data sheet). That will actually vary a little from unit to unit, and will also vary with die temperature.

D

 

[outdoor1]

If anyone needs a DIY canister just let me know. I have a lathe and have made some cans and heads from delrin,acrylic,aluminum,pvc,abs.It started as a hobby now its an obsession.

 

[bradsab]

No, the driver determines the current, then the LED determines the voltage based on that current. The reason that I've been saying 3.6V is that when you push 10A through the SST-90, you end up with about 3.6V (according to the I vs. V plot in the data sheet). That will actually vary a little from unit to unit, and will also vary with die temperature.

D

Can you easily explain what I vs. V plot means? Just for my education

 

[Packhorse]

Ohms law

I= current
V= Voltage
R=resistance.

I=V/R
V=IxR
R=V/I

So if the Vf =4volt (this is what I have found with a SST-90) and current= 9 amp then the resistance of the LED is 0.44 ohm, This resistance is not fix but changes with temperature. It gets lower when its hot meaning that either it will draw more current when direct driven ( or resistor) or the CC driver will put out less voltage in an attempt to keep the current constant.

 

[DIWdiver]

Can you easily explain what I vs. V plot means? Just for my education

Actually, what I meant is that in the data sheet there's a graph showing drive current vs. forward voltage. At 10A it shows about 3.6V.

It's interesting that Packhorse observes 4V at 9A. That's clearly way off what the data sheet shows. It wouldnt' be the first time a data sheet didn't tell the whole story.

D

 

[bradsab]

Actually, what I meant is that in the data sheet there's a graph showing drive current vs. forward voltage. At 10A it shows about 3.6V.

It's interesting that Packhorse observes 4V at 9A. That's clearly way off what the data sheet shows. It wouldnt' be the first time a data sheet didn't tell the whole story.

D

That's why I've been looking at the test data on the CPF link. I don't know about your industry, but in mine, I rarely find something that performs per manufacturer specs.

 

[Packhorse]

Vf does change from one LED to the next.
I have also heard reports of Vf lowering after a burn in period.

 

[bradsab]

Well...

Necessity being inventions' mother, the design has changed. Being blessed with a CAD program means I am also cursed with the ability to quickly create parts that are really hard to produce. Too hard to produce myself. To have them done professionally was going to be about $900, which effectively killed the project. Back to drawing. Actually, DIWdiver's drivers saved the light. They're enough smaller, and by using mini switches, I redesigned the head to be much easier to machine. The critical stuff still had to be done CNC, but the rest I can do myself. It's coming along nicely, but there remains a problem...

There is now VERY little room for wiring. I'm going to use 16 ga. cable from the canister to the head, but once the wires get inside the head, I have to use something less bulky. Maximum wire length inside the head will be about 4". 18 ga. is still pretty tight; will I lose much going with 20 ga.? I need to find something really flexible. How small can I go for the switching wires?

 

[DIWdiver]

Well...

There is now VERY little room for wiring. I'm going to use 16 ga. cable from the canister to the head, but once the wires get inside the head, I have to use something less bulky. Maximum wire length inside the head will be about 4". 18 ga. is still pretty tight; will I lose much going with 20 ga.? I need to find something really flexible. How small can I go for the switching wires?

20 ga will handle 7A okay. At 7A you will loose about 0.07V per foot. I would avoid using a single strand of it for 14A, but you can double it up wherever it handles both leds.

D