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LEDs Question


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#1 Astech

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Posted 13 March 2017 - 04:58 PM

I'm looking to put LED lighting in two clear Vulcans as part of my overhaul. Given the size of the blaster and the number of LEDs involved I'm thinking of using several parallel circuits to help alleviate voltage drop. I've got two questions about this:

 

1. Is it actually worth usng parallel circuits, or is the voltage drop between LEDs so small as to make it trivial? I'm expecting to use around 25 LEDs all up with its own power source.

 

2. If I do use, say, 5 parallel circuits of 5 LEDs conected in series to each other - with each one being a 5mm red LED - what's the recommended input voltage (I'll be using lithium ion batteries)

 

Thanks in advance.


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#2 Draconis

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Posted 13 March 2017 - 06:14 PM

1)  All diodes, including LEDs, have a specific voltage drop based on their composition.  Most normal rectifier diodes are 0.7v, but LEDs are higher and vary with the color.  You will need to check the specifications of the units you are using to calculate what your drops will be.  This chart will give you some vague idea, though.

LED-Colour.gif

 

2) Five red LEDs is going to require an input of something in the 8-10V.  I would recommend 3S, with a current limiting resistor.


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#3 Duke Wintermaul

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Posted 13 March 2017 - 06:23 PM

Assuming the common forward voltage of 2v, and again assuming the common current rating of 30mA. If these aren't the values of the diodes you bought nothing I say will be accurate.

A 5x5 array, if you're set on using Lithium, would require a 3s pack at 11.1volts.

Each 'tree' of the parallel circuit would need a ~39ohm resistor. 

LiPo's are i'll suited for LED array use, I'd recommend a 12volt NiMH pack. This will stabilize your circuit and avoid the problems caused by the Lithiums variable voltage. You'll also need to rework the array, a 5 'branch' array with 6 LED's on 4 branches (1ohm resistor per branch) and the last diode on it's own 'branch' with a 360ohm resistor.


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#4 SirBrass

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Posted 13 March 2017 - 06:36 PM

If you're going to run them in parallel you need a resistor value which equals (V1-V2)/I. Where V1 is your power source voltage, V2 is the voltage drop across the LED, and I is the amount of current your LED is rated to draw (usually around 20mA IIRC). So you wire the correct resistor (use the closest higher resistor value to the calculated resistance) in series with the led.

If you wire multiple leds in series, your V2 is the sum of their voltage drops & I is the current draw of the leds (don't sum it, as current drawn is equal to the highest demand from the devices in series). When you DO sum currents is when finding out how much current a parallel circuit will demand and you take the sum of the current demands of each circuit loop. You won't have to worry about that as a lion battery pack will have more than sufficient discharge capability to drive a bank of LEDs for a nerf blaster. Where lion batteries fail at delivering is in high current applications like flywheel & pusher motors. Just make sure the resistors you use are the right values according to (V1-V2)/I.

We're getting into intro to linear circuit theory here so I'll stop.

Important edit: Agreed with Duke here. For Pete's sake, don't drive these LEDs with a lipo! That's only okay if you're using a voltage regulator! A lipo is massive overkill and only appropriate with circuit protections and because it's being used to drive upgraded flywheels & pushers.

Edited by SirBrass, 13 March 2017 - 06:38 PM.

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#5 shardbearer

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Posted 15 March 2017 - 04:25 AM

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Wiring LEDs in parallel is the standard way of doing it. With higher voltage batteries it's better to wire several LEDs in series with one resistor to make a string, and then several strings in parallel. Each string needs its own resistor though.

If you already are putting a lipo in your blaster, it's totally fine to run them off of that battery. You don't need a voltage regulator. Note that the LEDs will dim when the battery is under heavy load.


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