# Driving 8x1W & 4x2,5W LEDs (LM317???)

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#### misiacik7

##### New Member
Hi there,

I want to drive LEDs
8x1W LED (350mA, 3,5 - 4 V)
4x2,5W LED (700mA, 3,5 - 4 V)

I dont have bought the power supply for it yet, cause I dont know which one would be best cause of the method of construction...

Would be best to put them into three rails?
1.Rail 4x1W
2.Rail 4x1W
3.Rail 4x2,5W

Would be best to drive them via LM317 via constant curent?
If yes, how would i do that? And the limitations of it? I looked in datasheet, dropoff 3V, 1,5A. What does it mean? how many LEDs can I drive via single LM317? only 1,5A/,35A? or depends on power supply voltage???

for example if I had 18V, 1,7A, 30W power supply, how many 1W LEDs and how many 2,5W LEDs can I drive via single LM317...? how is the formula for calculate that?
((18V-3V)/3,75V) / per rail = 4LEDs consume 350mA (1rail)
1,5A/,35 = 4 rails of 4 LEDs?
i am sure i am wrong HELP Or to buy 15V, 1,6A, 25W power supply. Will the voltage split and power all four LEDs (350mA) equally per rail if I put them in rail without any resistor, or LM317...? What will be their Voltages? 3,75 each? and the curent flow them 4/3,75*350mA?

All the power supplies I mentioned are stabilized

Thanks in advice Last edited:
Your circuit diagram looks to be correct.
You need about 2v5 across the regulator and a small voltage across the current limiting resistor. Try the circuit and see if it delivers the voltage you need.

In this case I'll ignore LEDs required current & only concentrate on what's its required voltage (A safer value).I'd use a 7805 regulator & apply a high current pass transistor with a huge heatsink.

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and what about if I bought 15V stabilized PSU 1,6A?

what would be values of each led if I put them in those 3 rails?

D1 - D4 (350mA)
V=15V/4V = 3,75V I=3,75/4*,35=328mA?

D5 - D8 (350mA)
V=15V/4V = 3,75V I=3,75/4*,35=328mA?

D9 - D12 (700mA)
V=15V/4V = 3,75V I=3,75/4*,70=656mA?

Am I correct? Would they "shine" equally? I mean the same LEDs... And which way is better? regulate V or I... I heard I, cause of the difference of the same LEDs in luminicity...

A 15v supply will not give you enough voltage to illuminate the LEDs in the circuit you want to use.

why? doesnt the Input voltage split into 3,75V each?

I tried that LEDs on modified PSU from PC on +3,3V and it shined I only ask if the voltage split into 15/4 = 3,75V each?

If yes, than the best would be this one, no need of any other components...

I just explained before that you need 2v5 across the regulator and a small voltage for the constant current resistor.

The resulting output voltage is not sufficient to guarantee illuminating 4 LEDs in series.

sorry, but I meant it without the regulators and power resistors...

only PSU and 3x4LEDs without any other compontents...

Providing you can add a resistor to each leg that provides the necessary current, you can use 3 resistors.

for input voltage higher than 4x4V, no?

but if I had 15V PSU, would the voltage across each LED split or not?

LEDs work with current, not voltage. You don't know what is their actual voltage which might be 3.5V or 4.0V or in between. so you must limit the current with a circuit or a resistor.

If the LEDs are actually 3.5V then four need 14.0V. A 15.0V power supply voltage will cause an extremely high current which will burn them out.

If the LEDs are actually 4.0V then four need 16V and will be very dim with a 15.0V power supply.

Use a 20V power supply and an LM317 current regulator circuit set for 600mA to 700mA.

But I still dont get it, how much LEDs can I add to a "leg"/rail if I use the current regulation circuit via LM317

For example I have 20V 1,5A.

20V-3V(dropoff) = 17V
max 4V / LED => 17V/4 = aprox. 4LEDs per "leg"/rail ? or can I add more per "leg"/rail ???

1.rail of 350mA LEDs = consumption 350mA to the PSU?
2.rail of 350mA LEDs = consumption 350mA to the PSU?
3.rail of 700mA LEDs = consumption 700mA to the PSU?
(if I set each LM317 to max. value of LED)

total consumption will be 1,4A?

thanks

The LM317 needs 2.5V across it plus 1.25V for the current-sensing resistor. If your LEDs are actually 4.0V then your power supply must be at least 16V + 2.5V + 1.25V= 19.75V.

The LM317 is guaranteed to have 1.5A when it has 15V or less across it.

The currents will not split equally. The "leg-rail" with the lowest total voltage will draw 2.2A and instantly burn out the LEDs. Each "leg-rail" needs its own LM317 current regulator or each "leg-rail" can have its own series current-limiting resistor instead of using LM317 regulators.

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I still dont get the "If your LEDs are actually 4.0V"...

In datasheet, there is a chart (TEST CONDITION IF = 350mA)
FORWARD VOLTAGE: TYP: 3.5V
MAX: 4.0V.
LUMINOUS FLUX: MIN: 30 lm
MAX: 50 lm

Does it mean, if i drive them via constant current each of LED can vary by Voltage? They are not the same, if I compare one to another? If yes, if a put a resistor before the leg, wouldnt they vary by light output?

So tell me last time... what would be best way to drive the LEDs?

and which one of these PSU to buy... 15V, 1.60A, 24W
24V, 1.00A, 24W
12V, 2.50A, 30W
18V, 1.70A, 30W
24V, 1.25A, 30W
12V, 2.00A, 24W
18V, 1.33A, 24W
24V, 1.00A, 24W

thanks I still dont get the "If your LEDs are actually 4.0V"...

In datasheet, there is a chart (TEST CONDITION IF = 350mA)
FORWARD VOLTAGE: TYP: 3.5V
MAX: 4.0V.
LUMINOUS FLUX: MIN: 30 lm
MAX: 50 lm

Does it mean, if i drive them via constant current each of LED can vary by Voltage?
Their voltage could be less than 3.5V to 4.0V and you will not know.

They are not the same, if I compare one to another? If yes, if a put a resistor before the leg, wouldnt they vary by light output?
If the voltage is high enough for LEDs in series then they all have exactly the same current that is limited with a current-limiting circuit.

So tell me last time... what would be best way to drive the LEDs?
Each "leg" is a series string of LEDs with a current-limiting circuit.
you can use current-limiting resistors for each string if you measure the voltage of each string then calculate the resistance needed.

and which one of these PSU to buy... 15V, 1.60A, 24W
24V, 1.00A, 24W
12V, 2.50A, 30W
18V, 1.70A, 30W
24V, 1.25A, 30W12V, 2.00A, 24W
18V, 1.33A, 24W
24V, 1.00A, 24W
Four 4.0V LEDs in series need 16V plus 3.25V for a current-limiting circuit. Then you need at least 18.25V.
I can't remember how much current you want. Don't operate the LEDs at their absolute max allowed current.

3legs:

1.leg - 350mA (4LEDs)
2.leg - 350mA (4LEDs)
3.leg - 700mA (4LEDs)

By this "you can use current-limiting resistors for each string if you measure the voltage of each string then calculate the resistance needed." you mean to put a resitor before each leg to drain some "Volts"... ??? (Uin - 4 * 3.75V)/,35??

this is the best way to do my problem?

Your LEDs are not 3.75V each! They might be anywhere from 3.2V to 4.0V each.
So you need a power supply voltage high enough for four 4.0V LEDs plus a few volts for the current-limiting resistor. The resistor limits the current due to the voltage across it as said by Ohm's Law. You can measure the voltage across the resistor then replace its value for whatever current you want.

You have two legs with 350mA and one leg with 700mA so the total current is 1400mA.
The LEDs will probably burn up because you are using them at their absolute max allowed current without considering how you will cool them. You must also calculate the power dissipated in each resistor then select a suitable size.

I dont want to drive them to maximum of their values...

But I rather do things safe, than to be sorry...

Therefor I want if it is possible by my available choices of PSU to do it via Constant Current Regulator, rather than via single resistor per series...

But if you say it is unnecessary, or by my choices of lower cost PSU it is not possible cause of dropoff Voltage, I would believe you, and do it via that resistor thanks for your patience I don't know why you do not understand this extremely simple circuit. Ohm's Law tells you. An 8 years old kid can learn Ohm's Law in one minute.

Your LEDs have a minimum voltage of maybe 3.2V (the manufacturer does not say), or a typical voltage of 3.5V for most of them, or a max voltage of 4.0V for some of them. You don't know the voltage of your LEDs unless you calculate a resistance that will not burn them out if they are the minimum voltage and with your measured loaded power supply voltage, then power them then measure their voltages.
Then you can use Ohm's Law to calculate suitable resistor values.

Your original circuit would work if your power supply is 20V or more. That's 4*4V (for the diodes max) plus about 4V for the regulator.

Since the 700mA string's regulator is dissipating between (4V*0.7A) = 2.8W to (7.2V*0.7A) = 5W it will need a heat sink. (4V is 20V minus 4V for each diode, and 7.2V is 20V minus 3.2V for each diode.) Note the 1.8 ohm current setting resistor will dissipate 940mW. The 350mA string's regulators will be dissipating half that much power and need a smaller heat sink.

---------second option--------

If you want to reduce the power dissipation of the regulators, you can take some risk. The 'typical' LM317 needs 1.7V plus 1.2V (=2.9) for the current setting resistor at or above 25deg C. The 'typical' of your LEDs is 3.75V. It will be lower when hot. So, if you reduce the input to 18V, the circuit is (IMO) 99% likely to work as you desire. Note I did not say 100%.

Others will disagree with my 99% estimate, and still others will say 99% is unacceptable.
But it is, after all, your project.

A resistor on a 4 LED string needs to be ((Vreg-(4*Vled))/Iled).

Let's try a 15V supply with no resistor.
If 4*Vled turned out to be 14V the current would be unlimited and if 4*Vled was 16V, the LEDs would not light. If 4*Vled was 15.0V the current is anybody's guess.

Let's try a 20V supply with a 7.5 ohm resistor. 3.75V LEDS would get appx. 700mA. The 7.5 ohm resistor would get 5W so it should be rated at 10W. (A 15 ohm resistor serving the 350mA string would have similar performance.)
BUT! A string of 3.5V LEDs receives 860mA. If they are 4.0V each they receive 570mA.

If we choose 18V with 4 ohms in series, the currents would be 1000mA and 500mA.

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