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I'm building a grow light using 100 rgb leds. I know that I need resistors but I don't know how to calculate which ones to buy.

Also before I buy the leds I have the choice between common cathode and common anode, which would work best for my project?
I've tried online calculators but they only work for normal leds. Here's the technical information:

Forward Voltage / Current: R:2V-2.2V G:3V-3.2V B:3V-3.2V | 20mA (each color)

I intend on using a 6v lantern battery as my power supply.

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    \$\begingroup\$ A lantern battery isn't going to cut it for 100 RGB LEDs. \$\endgroup\$ – Ignacio Vazquez-Abrams Aug 11 '17 at 1:37
  • \$\begingroup\$ An RGB LED is just three "normal" LEDs. With that knowledge, you can use an online calculator. However, with 100 LEDs, you probably shouldn't use constant voltage (and therefore you won't have resistors). Research constant current drivers. \$\endgroup\$ – uint128_t Aug 11 '17 at 1:42
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    \$\begingroup\$ You wouldn't use the green LED ever, So why not just get the individual Red & Blue LED's and save money and ignore the common cathode/common anode problem. \$\endgroup\$ – Harry Svensson Aug 11 '17 at 2:59
  • \$\begingroup\$ So let me get this straight, if an RGB LED is just 3 LEDs, requiring 20 mA each in this case and I'm using 100 LEDs then I'll need a 6000 mA constant current power supply. If the output voltage was 3-3.2 volts then how would I lower the voltage to 2-2.2 for the red LEDs? Also NASA has shown that green light helps plants as long as red and blue light are provided. \$\endgroup\$ – Maas Morin Aug 11 '17 at 3:01
  • \$\begingroup\$ 6 amps yes. Half of the power will be wasted as heat in the resistor. And your battery will last a little under 2 hours in ideal conditions. \$\endgroup\$ – Passerby Aug 11 '17 at 4:14
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Ideally, you should have a current source for each color so you can control their intensity separately. Be aware that a 6 A (6000 mA) current source is not that easy to build and, more importantly, you need to make sure the battery you are planning on using can provide that much current (it should say on its datasheet).

Just to clarify a bit, different colors of LEDs will intrinsically have different forward voltage drop, like you mentioned, and they will emit different amount of light for the same amount of current you put through them. That is why it is very important to have control over each channel (color); then you can fine tune how much of each color you want and likely optimize the growth of your plants.

If you want to have your plant exposed to it for long periods of time, you better consider using a power supply instead of batteries to power your LEDs.

Finally, the common cathode or anode issue will depend on what kind of driver you will use. Some of the RGB LED controllers out there will work with common anode and some with common cathode. My suggestion is for you to decide on how you will drive these LEDs first, and then choose the type of LEDs you want.

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You seem to have a few wrong ideas:

1) RGB LEDs are a good idea here.

Nope. Sorry. If you use RGBs, you will be stuck with providing 3 current limiting resistors per LED. That will be 300 for your setup. If you'll just take by word for it, common cathode and common anode is not a useful question. So let's assume constant anode. Then your 2 LEDs will look like

schematic

simulate this circuit – Schematic created using CircuitLab

Note that you cannot connect them in series, since this will short all three cathodes to the preceding common anode. Since the three colors have different operating voltages, all will be clamped by the red, and the green and blue will never turn on. If you try to connect the anode of one to a single cathode of another, the entire current from the three will be forced through the single.

2) You think you can use a single power supply.

Well, you can, but you need to think this through.

schematic

simulate this circuit

In this case a reasonable value for V1 might be about 5 volts - at 6 amps, and you'll need 300 resistors. Not only that, the resistors for each color will have to be different if you want the same current through each LED.

If you go to single LEDs, you can connect them in series

schematic

simulate this circuit

and now V1 can be something like 8 or 9 volts at 3 amps. Again, the resistor values for each color will need to be different to get the same current in each.

Of course, if you want to reduce the number of resistor values, you can mix your colors in series, with each string being a multiple of 3 long.

schematic

simulate this circuit

Since each string has one of each color, the voltage across each string will be the same for the same current, and you can use a common resistor value.

And you can now string your LEDs into longer strings. The longer the string, the more voltage and less current you need. For a single supply, the maximum voltage needed will be about (n x 3.2) + 2 or 3 volts, where n is the number of LEDs in series. In the extreme case, you could have 3 strings of 100 LEDs, with a V1 of somewhere over 320 volts, but only needing 60 mA. But I wouldn't recommend that.

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