Simple LED Constant Current Lighting Circuit

There is a lot of effort going on to improve the technology of LED (Light Emitting Diode) lighting. The reason is that LEDs are much more efficient than standard incandescent or CFL light bulbs.  They also last longer than either of the alternatives and don’t have issues with disposal like CFL bulbs.  LED lighting has a lot going for it except one thing. It is still pretty expensive.  At any rate I have been doing a fair amount of reading in the engineering trade journals on the subject and have been wanting to play around with LED lighting.  Over the last year or two I have been collecting high output LEDs that I have been able to pick up inexpensively at hamfests and surplus places.  In some case I have gotten some nice LEDs for under $.05 each.

When you are using an LED as an indicator in a project you usually just drive it with a voltage or microcontroller and a current limiting resistor.  In these situations you really don’t worry too much about driving the LED for its maximum output or worry too much if the current changes a bit due to variations in the supply voltage or changes to the VI (voltage-current) curve as the LED heats up a bit.  You also don’t tend to drive it anywhere near the LED's limits so there is plenty of margin and the LED will last practically forever.

In a lighting application you want to get as much light as possible out of them so the LEDs should be driven with constant current. The same amount of current should always be going through the LEDs regardless of changes in the circuit over time, temperature or other reason. Needless to say, there is a lot of work going on developing integrated circuits for driving LEDs. Most are similar to switching power supplies. These are very efficient, often over 90%.  The downside is they are somewhat complex for playing around with and many come in fine pitch surface mount packages that make building a circuit without a circuit board a major exercise in frustration.  Many are kind of expensive in small quantities as well. My pile of inexpensive LEDs continued to grow. Finally I came across a neat circuit in the January 6, 2011 edition of EDN magazine.   The author, Eliot Johnston,   used a simple two transistor, two resistor analog constant current circuit to drive LEDs in an outdoor illumination system. Brilliantly simple! Sorry about the pun.

http://www.edn.com/design/led/4363924/Transistors-drive-LEDs-to-light-the-path

Figure 1 shows the schematic I used. It was derived from the article.  Basically R1 provides base current to drive Q1.  This turns Q1 on, and current flows through the diodes, through Q1 and R2.  As the current through R2 increases, the voltage across R2 increases. It raises the voltage at the junction of the emitter of Q1 and the base of Q2.  This partially turns on Q2.   That lowers the voltage seen by the base of Q1 which combined with the increase of the voltage at the Q1 emitter reduces Q1’s base current which in turn reduces the Q1 collector current, and therefore the current through the LEDs. Anything that disturbs the LED current level changes the transistors operation to move the current back to the steady state value.

The amount of current through the LED string is set by the value of R2.  This simple equation sets the current.

I = .67/R  or  R = .67/I

The data sheet for high brightness LEDs will specify what current the device should be run at.  Usually the data sheet refers to that as Forward Current, or IF. From there you can select the proper resistor to provide that current.  The next step is deciding what system voltage to use for the LEDs you want to use.  For the circuit to work, you need to have about 1.8V minimum between ground and the collector of Q1.  The difference between the supply voltage and the voltage drop across the transistors is the voltage available for the LEDs.

VL = Vs – 1.8

VL is the voltage available to drive the LEDs. Vs is the supply voltage (Vcc on the schematic).

Figure 1. LED Driver Circuit

LEDs have a voltage drop across them. The data sheets usually call this Forward Voltage, or VF. Red LEDs have the lowest drop.  Blue and white LEDs have a larger VF.  This will be specified in the data sheet.  The voltage VL will determine how many LEDs can be used. 

# of LEDs = VL / VF  = (Vs -1.8)/VF

If you want to use different LED types, just add up the voltage drop for each LED used and be sure it does not exceed VL above.  If you use different LED types, you need to set the current for the lowest rated LED. This means the other LEDs will not be operating at their maximum brightness. If you plan to use different LED colors try to pick ones that have the same recommended operating currents.

The other consideration is the supply voltage.  If you want to use a lot of diodes you will need a higher voltage of course. You don’t want it too high though. The difference between the total LED drop and the supply voltage will be dropped across Q1.  That voltage multiplied by the current will be the amount of power wasted in watts. If this power is too high you could destroy the transistors as well as wasting power. Part of this exercise is to come up with energy efficient lighting.  The supply voltage should be matched to the number LEDs needed.

Figure 2. LED Driver Prototype

You can also look at it another way, the cost of the support circuitry (the two transistors and resistors) per LED used. That is another reason for using a higher voltage.  The applications I had in mind for these would need to generate a lot of light. I would need to use a number of the circuits so maximizing the number of LEDs per constant current circuit makes economic sense. In the end I decided to use 24V.  I had a few 24VDC supplies left over from some equipment no longer used.  Figure 2 shows some test boards I built up on bare fiberglass circuit board material.  The picture shows the boards powered down. The white LEDs are too bright to get a good picture showing the construction details.

One of the applications I had in mind was to light some of my aquariums. I have about 40 aquariums. The types of fish I mostly keep like it relatively dark.  I have lights on some tanks, but I would like to add a bit of light to some of the other tanks without major increases in the monthly electric bill.  The plan was to build up some of these circuits with white LEDs for the fish tanks. 

Another one of my hobbies is gardening, especially vegetable gardening. I start a lot of my vegetables from seed in early spring and raise them with a light system until it gets warm enough to move the plants outside.

Figure 3. Circuit boards with various LED types.

I came across an article about how NASA was experimenting with LED lights for growing plants. If they ever do long deep space manned missions they will need to find a way to recycle wastes and grow food.  Plants can convert waste into food. Plants need light to grow, and the system will have to operate with as little energy as possible.  LEDs are the obvious choice for lighting if they can be made to work.  Their system uses arrays of red and blue LEDs. Plants use these wavelengths most effectively.  Green light is between the red and blue wave lengths, but plants reflect green light (that’s why they look green).  Green light is wasted as far as the plants are concerned.  It turns out red light is more important for plants blooming and blue light is better for plant growth. Since my goal is to start great vegetable seedlings, I planned to add more blue LED arrays than red ones to augment the CFLs I currently use in my plant lighting system. 

The prototypes in Figure 2 worked well so I decided to proceed with the aquarium and plant light project. Building up a large number of them would be extremely time consuming using the prototype construction method. I decided to do a circuit board layout and have some blank boards fabricated. For maximum flexibility I put pads for both through hole and surface mount LEDs.  The board has space for 10 LEDs although most combinations of LED types and supply voltages will use less. Figure 3 shows some built up circuit boards with different LED types.

Playing around with this circuit was a lot of fun.  The plant LED lights have been added to the current system running with the current CF lights.  I don’t want to take a chance with this season’s seedlings, so later this year I will run some tests comparing plant growth with and without the blue and red LED augmentation.  Maybe some day the entire system will be LED lighting. I have a few aquariums with the LED lights. The white LEDs have a bit of a blue hue to them.  I will see if I can find some LEDs with more natural color before doing more tanks. At any rate each board consumes about ¼ watt each.  The lights are on about 16 hours per day.  Not counting the inefficiency of the power supply, I am lighting a small aquarium for about 15 cents for an entire year!