Messed up with LED driver calc, can't think, brain to mush mode....

[Rockingit]

Hi Troops,

not so much an electrical issue as an electronics one, just realised that I've dropped a hairy one with making some crimbo led display lights (for me, for home). LED driver I got isn't up to the job, and my brain has ceased to function (mixture of cider and soldering fumes).... 10 x varied 105-130mW 5mm leds @ 3.7vf - 4.2vf in series. What size driver/s both current and voltage?

R.

 

[spark 68]

You need to provide a bit more information, ie. what is the current requirements for the LED's ?

see this: LED series parallel array wizard

 

[topquark]

If we say average of 120mW (x10) @4V, that's 1.2W @ 4V = total current required of 0.3A. You'll need to adjust to fit the driver voltage but LEDs aren't too fussy.

 

[spark 68]

Hi TQ,

If he is putting them in series though, he will have to allow for the volts drop for each one, ie. if they have a VD of 3.6V each he will need to push at least 36 Volts through the string. (he has 10)

 

[topquark]

Hi TQ,

If he is putting them in series though, he will have to allow for the volts drop for each one, ie. if they have a VD of 3.6V each he will need to push at least 36 Volts through the string. (he has 10)

Quite right spark 68 (I've had a few beers ). As they're in series it's the VD that's important, the current will be the same through them all.

 

[topquark]

How about one of these guys: Buy External Power Supplies Power Supply,enclosed,IP67,36V,1A,36W RS LPV-35-36 online from RS for next day delivery. (suitable for outside use as well).

 

[spark 68]

Hi again,

he will need a current limiting resistor still, and some of his LED's have a VD of 4.2 Volts.

The formula for this is: R = (supply Voltage - Total VD)/ the required current.

Eg. for one LED on a 5v Supply, assuming a VD of 1.7V and requiring a 100mA is
R = (5-1.7)/0.01A = 3.3/0.01 = 330 so current limiting resistor = 330 ohms in this case.

He would have to add all of the VD's together and workout what his max mA is (this would be for the lowest rated LED) and then find a PSU which exceeds this margin by a couple of volts (at least) and then calculate the resistor (to the nearest preferred value), not forgetting it's wattage.

I hope this helps.

 

[Rockingit]

Cheers gents, mucho appreciated. I did use to know this stuff, a loong time ago, scary how you forget it if you don't use it.

The driver I got (maplins) is an auto sensing constant current driver, 3 - 36Vdc, output power 1-8W and output current of 350mA. The data sheet does show an exampe wiring set of both 3 and 8 leds wired directly in series, no resistors needed. But, is this making an assumption that you're using 350mW leds??? Have attached it.View attachment plc35xx_And_plc70xx_-03.pdf

Have I either grossly under calculated the demand, or do I just need to wire them as 'series - parallel pairs/triples' to make up my o/p current?

I've a suspicion that I may have accidentally fried a couple!

I've four leds which are:

DC forward voltage:	3.6V typ (4.0Vmax)
DC forward current	30mA
DC reverse voltage:	5V(max)
DC reverse current:	50 uA(max)
Power dissipation:	120mW
Optical output power:	6.0 mW typ.

two are:

Forward voltage at If=20mA:	4.0V
Forward current max.:	30mA
Reverse voltage max.:	5V
Power dissipation PT:	105W

four are:

If max = 100mA
Vf max = 3.6Vdc
Power dis = 100mW

 

[topquark]

Hi again,

he will need a current limiting resistor still, and some of his LED's have a VD of 4.2 Volts.

The formula for this is: R = (supply Voltage - Total VD)/ the required current.

Eg. for one LED on a 5v Supply, assuming a VD of 1.7V and requiring a 100mA is
R = (5-1.7)/0.01A = 3.3/0.01 = 330 so current limiting resistor = 330 ohms in this case.

He would have to add all of the VD's together and workout what his max mA is (this would be for the lowest rated LED) and then find a PSU which exceeds this margin by a couple of volts (at least) and then calculate the resistor (to the nearest preferred value), not forgetting it's wattage.

I hope this helps.

Hehe, thanks for the informative post. I did realise that.

 

[topquark]

Hi rockingit. Yes, that'll be making that assumption.

You'll first need to work out how you are going to wire them before you can work out what to drive it with (I missed the series specification in my drunken stupor). Are you happy to do the calcs for the limit resistor then or do you want it done (it's fairly trivial)?