# Modeling project (ilumination part)

#### florian.martins20

Joined Jan 13, 2017
2
Hi!!
I'm a little stuck right now... I'm creating a model of a simple construction a do for my internship (eletrical technican). I already make all the count and I need to connect about 120 leds in 9 series. So far alright. But my problema is all the series dont have the same number of leds.. So I'm affraid it would burn leds in the series I have less leds.The series is something like that:
1º>19 leds
2º>17leds
3º>16leds
4º>11leds
5º>20leds
6º>18leds
7º>19leds
8º>20leds
9º>18leds
Could anyone help for the following> wich source should I buy? How much volts do I need? and wich lm hould i mount?

Thanks anyways for any feedback.

#### ericgibbs

Joined Jan 29, 2010
15,544
hi 20,
We need to know, the LED colour or Forward voltage drop of the LED and the required LED operating current.
You must have a series current limiting resistor in each series.
E

#### florian.martins20

Joined Jan 13, 2017
2
hi 20,
We need to know, the LED colour or Forward voltage drop of the LED and the required LED operating current.
You must have a series current limiting resistor in each series.
E
I already don t choose the LEDs yet. But I'm gonna choose the most simple ones...White light and about 1 to 4 volt each one.

#### ericgibbs

Joined Jan 29, 2010
15,544
hi,
The method is to add all the LED forward voltage drops together.
Say 10 Red LED's at 1.8Vfwd, thats 18V total.
If the supply voltage was 24Vdc then you would have to drop 6V across a series resistor.
Say the operating current for the LED is 20mA, that would require a series resistor of: [24v - 18v]/0.02 = 300 Ohms.

Say the operating current for the LED is 10mA, that would require a series resistor of: [24v - 18v]/0.01 =600 Ohms.
OK.?
E

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#### Alec_t

Joined Sep 17, 2013
12,822
White light and about 1 to 4 volt each one.
No. A white LED has a Vfwd of about 3V, so if you supplied it with much less than 3V you'd get very little light and if you supplied it with 4V you'd destroy it.
LEDs should be driven with controlled current; not a fixed voltage (though the supply has to have enough voltage to provide that current).