LED Strips as Grow Lights

You rarely see any large grows using LED strips for grow lights. That is not an accident.

LED strips as grow lights

LED Basics

LEDs have highly nonlinear voltage vs current characteristics. Let’s take a look at a graph of a typical LED (in this case, a green indicator LED)

Typical LED I/V Curve on a

What you can see is that the LED will not conduct up until a voltage threshold is reached, then it will conduct from almost no light to max power within the span of about 1V. After that, the you exceed that maximum power output of the LED and it will burn out.

Because the voltage has very little tolerance, designers always create current controllers for LEDs.

How do those cheap LED strips limit current?

Those LED strips you see for sale on eBay, Alibaba, or Amazon are rated at a specific voltage. Does this mean that they have no driver and that they are being controlled strictly by voltage? Of course not. They are using the simplest of the LED drivers: The current limiting resistor.
LED current limiting resistor circuit
Current limiting resistor circuit. (CC BY-SA 2.5, https://commons.wikimedia.org/w/index.php?curid=866109)

The current limiting resistor is a simple circuit that prevents too much current from flowing through the LED. Every indicator light on the planet uses this circuit. It’s simple and requires only 2 parts. The second part (the resistor) costs fractions of a cent, making it incredibly cost effective.

Does the cheapest of LED drivers work?

In a typical use case for us, like as side lighting in a space bucket, these strips are setup for use with 12V and are using single white LED emitters. For example, look at the Cree PLCC4 White LED. If we look at the datasheet, we can see that the LED will drop between 2.8V and 4.2V depending on manufacturing tolerances. They can sell them for you in various bins that have only a narrow range of forward voltages.

Chart of forward voltage bins on our sample LED showing that the LED can drop between 2.8 and 4.2 volts at 30 milliamps

Additionally, we can also see that the max forward current is 35mA:

So,if we are shooting for the worst case scenario with a 2.8V drop and 12V input, that we wisely choose to bump to 14.5 (because cars run at 14.5 to charge the battery), then we math the resistor to be:

I_{led}=\frac{V_{in} - V_F}{R_{Limit}} \\[10pt] 30ma=\frac{14.5V - 2.8V}{R_{Limit}} \\[10pt] R_{Limit}=\frac{11.7V}{.03A} \\[10pt] R_{Limit}=390\Omega

And 390 is a standard value so we’re good. If everything fails and we get one of those 4.2V LEDs, then our forward current is:

26.4ma=\frac{14.5V - 4.2V}{390\Omega}

So, a 12% loss of current and corresponding loss of light output under the absolute worst conditions. If we could keep to a bin of LEDs that didn’t vary by more than .4V on the output, the variance would be completely unnoticeable. This makes for a perfect manufacturing trade off for a product designed to light the ground around your car.

Why not use LED strips as grow lights?

We don’t drive horticultural lighting like this because of efficiency. The resistor work by burning off power that we don’t want going into the light.

Power is the voltage multiplied by the current, so in our 14.5V circuit pushing 30mA, that’s .435W. The current is the same for both components, so the power is distributed proportionally to the voltage across each component, so…

P_{LED}=2.8V*.03A = .084W\\[10pt] P_R=(14.5-V_{LED)*.03A}=11.7V*.03A=.351W\\[10pt] Efficiency = \frac{.084W}{.084W+.351W}=\mathbf{18.5\%}\\[20pt] \mbox{\huge 18.5 fucking \%}\\


That’s right, your best, possible case, not factoring in power supply losses, LED inefficiency, transmission losses, manufacturing issues, or dust, is 18.5% on this circuit.

Conclusion

While I do like the LED strips for accent lighting and some of my other projects, because they optimize for price instead of efficiency, you should consider LED strips as completely unsuited for grow lights. Yes, if you need a little extra side lighting on a tiny grow, they won’t break the bank. However, if you are wondering why we don’t use LED strips as grow lights, wonder no more: You would need 5x as many lights, and 5x as much power.

Take a look at our articles on proper LED lighting of your grow space to get a better idea on what to get.

1 year ago

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