52 LEDs in parallel with one resistor

[Cecemel]

Hi,
Can someone first of all explain why you need a resistor in series with a led? (I think that is because a led is a diode and else you would just short out your battery or power supply but not sure)

I got 52 LEDs in parallel and don't want to use 52 resistors. PLEASE tell me there is a way using only 1 (5 is also fine;-)) resistor(s).

Regards Cecemel8-O

 

[andre_luis]

If the power supply already provides an average and peak current limitation there is no problem to drive directly without a resistor, assuming it is properly sized, otherwise once a LED is a device that behaves non linearly to temperature variation and to biasing current (particularly at the knee conduction curve region), a series resistor yield a current stability, but the price is the additional power consumption.

You can use just 1 resistor as you asked, but only if the power supply exceeds to total drop voltage required for the array.

 

[Cecemel]

If the power supply already provides an average and peak current limitation there is no problem to drive directly without a resistor, assuming it is properly sized, otherwise once a LED is a device that behaves non linearly to temperature variation and to biasing current (particularly at the knee conduction curve region), a series resistor yield a current stability, but the price is the additional power consumption.

You can use just 1 resistor as you asked, but only if the power supply exceeds to total drop voltage required for the array.

I'am using 2 AA battery's in series, do i need resistors for this?

Thx for reply,:grin:
Cecemel

I have an on-off-on switch and want to use it like this: on-off-dimmed. What resistor should i use?

 

[andre_luis]

Take in mind that visible light LEDs have different drop voltages, starting from 1,5v up to 2v for red and starting from 2,5v up to 3,5v for blue. In general requires 10mA to show a decent bright, so that paralleling all them would require a total current of 520mA, but would not work for 1.2v NiMH batteries.

 

[FvM]

The LED characteristic must be known. Operation without individual series resistors can cause thermal runaway, only in a higher current range the LED current will be self-balancing.

 

[andre_luis]

Another advantage resulted from the parallel arrangement employing just 1 series resistor, is that assembling with LEDs having the same design characteristics but from different manufacturing lots tends to present a small deviation on its non linear region, so that each one could exhibits different shine if biased near to that point of the curve.

 

[Audioguru]

I have a cheap Chinese flashlight (torch) with 24 white LEDs in parallel driven from three series AAA "super heavy duty" garbage (carbon zinc) battery cells.
Turned off, the three cells measure 4.15V. Turned on, they measure 3.30V and the voltage is dropping fast but I do not notice the light dimming.
There is a 1.2 ohm resistor in series with all 24 LEDs and the voltage across it measured 0.13V which calculates to 108mA.

With three series brand new Energizer AAA alkaline cells, the total OFF battery voltage is 4.8V, the ON voltage is 4.3V, the voltage across the series 1.2 ohms resistor is 0.55V and the current calculates to 458mA. It is extremely bright.

All 24 LEDs appear to have the same brightness so their forward voltages must be well matched.
Did you buy thousands of LEDs, measure them then sort them so that all 52 of your LEDs have the same forward voltage?


LEDs in parallel without separate current-limiting resistors:
One LED part number has a range of forward voltage so some will have extremely high current, be very bright and burn out soon but others will have low current and appear dim.
You forgot to say the color which indicates their average forward voltage.

AA battery cells are available in carbon zinc, alkaline, Lithium, Ni-Cad and Ni-MH. Their voltages vary so when the cells are new the current in the LEDs might be enough to burn them out.
You forgot to say which type of AA battery cells you have and if they are strong brand new name brand cells or weak used cheap cells.

I have graphs of Name Brand AA alkaline and Ni-MH cells at currents ranging up to 5000mA.
Please tell us the part number of your LEDs and which type of battery cells you have.