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### Topic: Looking for advice on driving 3w and 1w LED's (Read 6452 times)previous topic - next topic

#### Boffin1

#15
##### Nov 07, 2012, 01:36 pm
I think I missed the bit when we went from 10v battery to 5 v supply,    but you could put the red and green in series  ( 6v ) and run them from a pwm constant current current device from the main battery?

45 years of editing projects with a knife and soldering iron, then I found Arduino !

#### MatCat

#16
##### Nov 07, 2012, 01:46 pm
Actually the primary system battery is roughly 15v, that would be a lot of wasted heat to linearly lose in voltage drop!  So instead I am going to power the lighting system with a switching regulator that can provide 4A at about 5.2v.  Series would be preferred but the voltage isn't quite enough, I don't know if the UBEC's I have coming are switchable to 6v, if it is I will just run those two in series, there is a .5v drop on the current sink too to keep thought of, that puts the theoretical need at 6.5v, however the green LED is spec'd down to 3.4v, so 3.4v + 2.2v + 0.5v = 6.1, and if the UBEC has 6v selector realistic output will be 6.2, I actually have another 7A UBEC that puts out 6.2, I might just use that one.

#### MatCat

#17
##### Nov 07, 2012, 01:51 pm
Oh another thought on these red and green LED's... The actual LED's will be each on about 1.2 meters of 22 AWG wire, which means in series from + to - there is a travel of about 4.8 meters of 22AWG wire, I wonder what that does to resistance and capacitance, and if it is enough to merit bumping up the total constant current sink to overcome the loss, plus it will all be on PWM so technically it should be able to be a bit higher because of that too, any thoughts?

#### Grumpy_Mike

#18
##### Nov 09, 2012, 01:13 pm
I would not bother about the length it is short enough.
Mind you it will radiate like stink but then you are not going for FCC approval are you.

#### fungus

#19
##### Nov 09, 2012, 01:30 pm

there is a travel of about 4.8 meters of 22AWG wire, I wonder what that does to resistance and capacitance

At these power levels it might drop half a volt along the wire because of resistance. No biggie, but make sure you have enough overhead in the power supply.

and if it is enough to merit bumping up the total constant current sink to overcome the loss

All you need to worry about is that is has enough volts to play with.

No, I don't answer questions sent in private messages (but I do accept thank-you notes...)

#### MatCat

#20
##### Nov 09, 2012, 06:44 pm

I would not bother about the length it is short enough.
Mind you it will radiate like stink but then you are not going for FCC approval are you.

I hadn't thought of that...  like a giant antenna...  hrm...  any ideas on preventing that?

#### fungus

#21
##### Nov 09, 2012, 07:05 pmLast Edit: Nov 09, 2012, 07:10 pm by fungus Reason: 1

I hadn't thought of that...  like a giant antenna...  hrm...  any ideas on preventing that?

Shielded wire...?
No, I don't answer questions sent in private messages (but I do accept thank-you notes...)

#### MatCat

#22
##### Nov 10, 2012, 04:05 am
I will use my SDR and experiment and see if I pick up any odd frequencies from using the lights as is... Mainly RC airplanes have lights so I am not thinking it's going to be a big issues, especially as long as they aren't messing with 433MHz or 1.3GHz hehe.

#### MatCat

#23
##### Nov 10, 2012, 08:56 am
A question... I am using these cat4101 constant current sink drivers, and I am wondering... SINCE it is constant current, does that mean if I parallel multiple LED's to the driver that each LED will just get LED Count / Current Sink ma of current?  Technically having the LED paralleled means that each LED will see 5v, and the sink will only sink 700ma total as that is how it is set, so if I parallel 2 white LED's that at full power consume 700ma each with a forward voltage of 3.5v onto a 700ma sink, then it should in reality split equally the current allowing only 350ma through each, and dissipating 1.25v x2 to heat (.5v is used by current driver), and continue the pattern regardless of the number of LED's... am I correct in my understanding, and or is there more to understand exactly how I would figure it out properly, thanks!

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