Wiring Leds

[Toleman]

Hello there,

Just a few questions on wiring leds. Im wanting to make a led board for a bit of experimentation. I grow chilli peppers and want to see how well a few seedlings in vegatative stage grow under leds.

I am looking to purchase a bulk pack of 50 leds
The *typical* spec is as follows:

10mm in size
Blue 465 - 475nm perfect for photosynthisis
min -3v max - 3.8v
120,000cdm intensity (@3v) brightest i could find.

Questions:

1. Would i need a resistor if i wired 4 leds in series being 3v each as i was thinking 12v/4leds =3v

2 Could I use a 12v halogen downlight transformer or does it contain a ballast/starter that would damage the leds? its 4.9A so would support current side of it

Thanks

[Toleman]

this may help for spec of leds. If i do need a resistor what would i need thanks, Chris

http://cgi.ebay.co.uk/50-PCS-10mm-40-5-Chips-B lue-LED-100mA-130-000mcd-NEW_W0QQitemZ22029680 3589QQcmdZViewItem?hash=item220296803589&_trkp arms=72%3A1301%7C39%3A1%7C66%3A2%7C65%3A12%7C2 40%3A1318&_trksid=p3286.c0.m14

[Hymie]

From your post, I presume your proposed LED array will consist of 12 parallel sets of 4 LEDs in series.

Answering your second question first, if you used a 12V halogen light transformer (as the power source), you would need to add a rectified circuit. The halogen transformer output is an AC voltage, whereas the LEDs need a DC supply voltage. The rectified voltage from the 12Vac transformer would be around 17Vdc.

You would need a resistor in each of the 12 paralleled LED chains. The reason for this is that each LED will have its own forward switch on voltage, which will vary slightly from one LED to another. This would lead to some of the 4 LED chains having a lower operating voltage than other parallel chains of 4 LEDs. Applying the same voltage across the array will result in the chain of 4 LEDs with the lower forward switch on voltage, drawing more current than others. This chain will be brighter than the others and the LEDs will have a shorter life, due to excessive current. By selecting the correct resistor value, the extra current draw will result in a greater voltage drop across the resistor � thereby resulting in an equal current to each chain of 4 LEDs.

At 20mA per LED chain, you would only require a total supply current of around 0.24A.

[Hymie]

If you were to rectify the transformer output to give 17Vdc, each LED has a specified nominal voltage of 3.4V x 4 = 13.6V. Therefore the resistor would drop 17V � 13.6V = 3.4V. At a nominal current of 20mA, the resistor value in each chain should be 3.4/0.02=170Ω (or nearest preferred value).

[Toleman]

You got it 14 sets of 4.

Thanks. Your exampes have been a great help especially if I choose to use a different power supply as I will be able to work the answer out for myself.

Final thing i've thought.

Im after as much light as possible. I will run them at the 3.4v but 20mA seems quite a drop down from 100mA max

Whats the highest but a safe maximum current for running these without damaging?

Thanks, Chris

[Toleman]

sorry 12* sets of 4

[beso]

A better ballasting system than a resistor is a constant current circuit. This keeps the current stable even if the input voltage fluctuates. Also the LEDs have a negative temperature coefficient meaning the hotter they get the lower the voltage so you can get some thermal runaway effects when just using a resistor. This limits the minimum resistor value because the voltage drop in the resistor is proportional to the stability of the circuit.

One very simple constant current circuit uses an LM317 voltage regulator IC and one resistor. The circuit will do up to an Amp depending on the thermal considerations of the heatsink and the input voltage.

These regulators maintain an output voltage of 1.2 Volts above the adjust pin. Connect a suitable resistor between the output and adjust pins that provides a 1.2 volt drop at the desired current. Supply the LEDs from the end of the resistor on the adjust pin.

A small equal resistance in each string of parallelled LEDs will balance the current in each string.

However the LM317 has a maximum input to output voltage of 35 volts so it can work in quite a high voltage circuit in this way since it has no ground pin.

It would be much more efficient to put all the LEDs in series and run them from a higher voltage. This reduces the current and the losses in the IC and the resistor. You would no longer need the balancing resistors in each string.

[Hymie]

There are many possible circuits to do what you want � but the simplest is to limit the LED current via resistance.

Based on a maximum circuit current draw of 0.4A, (without going into the maths), a 4,700�F reservoir capacitor will give your rectified dc voltage ripple of around 1V, which should be fine for this application.

To calculate your required resistance value, you need to measure the rectified dc supply voltage. This may well be over 20Vdc, due to the transformer regulation at full load. The required resistance is calculated from the supply voltage minus the LED voltage (x4) divided by the required current.

e.g. at a supply voltage of 20Vdc and an LED voltage of 13.6Vdc, gives a voltage drop of 6.4V. At 100mA gives a resistance value of 64Ω. At 20mA the required resistance value is 320Ω. By having 4 LEDs connected in series with a 64Ω fixed resistor and a 270Ω variable resistor will enable you to adjust the LED current from 20mA up to 100mA.

Once you have adjusted the LED brightness to the required level � you can determine the current flow by measuring the voltage drop across the two resistors, power down the circuit and measured the value of the resistance. The current flow is simply the measured voltage divided by the resistance.

One final thought, 100mA is the absolute maximum rating of the LEDs, at 100mA their expected life is likely to be significantly less than that at 20mA. You may find yourself buying another 50 LEDs.

[Toleman]

Thanks, You've made things easier for me,

Chris

[beso]

Indeed the 100mA will be the maximum instantaneous current. It will die in milliseconds at this current. Stick to 20mA. Also don't pack them too close together as the heat is their worst enemy.

I strongly recommend the LM317 constant current circuit. It is so cheap, simple, robust and stable. It is immune to the ripple as well.

Two components. The LM317 only has three leads and costs like a dollar.

The resistor value is not dependent on the input voltage or the forward drop in the LEDs. The input voltage only needs to be about three volts above the LED voltage.

Resistor (Ohms) = 1.2 / current (Amps)