Driving LEDs is a hot topic and one of the design techniques engineers are talking about is the use of constant current sources to drive LED fixtures.
Used to be we could use resistors to limit the LED drive current. Now, the buzz is about using constant current because it helps maintain a steady forward current through the LED as the voltage drop across the LED junction varies. What are your thoughts? What are the gotchas when designing for LED circuits?
If you don't mind the expense for a constant current diode, try a CLD20B ($1.06).
Be sure to connect the LED and diode in the correct polarity. The CLD20B provides
a constant 20 ma provided the voltage across it is greater than about 5v and less than 45v.
A typical LED will drop 1.2v max, so if your supply voltage is high enough you can run
several LEDs in series.
I am trying to run LED taillights on my 69 Cougar. I am creating a Lekan holder, first test to run 16 Leds per display. If they drop 1.2 volts per bulb, would it be safe to say, that running about 14 bulbs per string? Perhaps to run in normal mode, lower lighting for parking lights I could run 2 bulbs more.
The CLD20B provides 20 ma constant current. You have to check that the leds you are using
operate ok on 20 ma. Then you need to know the voltage drop across the led with 20 ma
through it, call that value Vled. If you are using a 12 volt battery (typical automotive use),
you need a minimum of 5 v across the CLD20B for it to operate, leaving 7 volts for the
leds. The maximum number of leds in a string is then 7 volts divided by Vled. Typical
Vled is 1.2 volts, but you have to check this. If Vled=1.2 volts, you can run up to 7/1.2=5
(or probably 6, since you usually have a little more than 12 volts) leds in one string. You can
run fewer leds with no problem, the CLD20B handles the additional voltage across it. More
generally, if you have a power supply other than 12 volts, call it Vsupply, the number of leds
in a string can be up to (Vsupply-5volts)/Vled. The only restriction is that Vsupply cannot
exceed the voltage rating of the CLD20B, which is 45 volts.
Great reply. I will need 6 CLD20B units minimal, perhaps many more. I am thinking of a moving sign board for information about my car at car show using my tail lights. I also like the idea of running much less current than 6 multi filamented bulbs. Anyway, thanks for the informatiom.
Instead of buying a chip, plus all the cost of postage and packing, you can create a constant current source with 2 transistors and a few other components.Here is a circuit:
and here is the eBook with all the other circuits:
I did this for a flashing third brakelight on my pickup truck and my Harley Davidson motorcycle. LEDs are current devices and are best driven with a current source. If you pay attention to detail, you can also drive them with a voltage source using series resistors. In my design, I created 7 parallel strings of 3 LEDs in series for a total of 21 LEDs. With the resistors and LEDs the power dissipation is about 4.5 watts, a fraction of the incandescent bulbs it replaced at 24W each. The same design has incorporated additional components to work as a tail light. Each string is driven with 50mA for brake and about 3mA for tail light.
The biggest challenge is to protect the LEDs from over voltage, voltage surges and voltage spikes. The 12V source (nominal 14V) in an automobile can be hit with voltage spikes as high as 60 Volts which will blow out your LEDS. Even the automobile manufacturers don't get this right and you will find many cars on the road with missing segments and brightness variations in the LED lights.
My oscillator/driver circuit disconnects the outputs to the LEDs when the input voltge exceeds 16V and reconnects when the voltage drops back into a safe range which guarantees that the LEDs see no overvoltage. Output rise/fall times are on the order of a few milliseconds which prevents any voltage overshoot or undershoot at the LEDs which can also damage them. The driver output uses Power MOSFETS and the whole thing can handle about 3 amps non pulsed which can drive about 8 LED brakelight/tail light fixtures.
I chose to use a voltage source for the driver in the event I wanted to use the existing lights on a vehicle. The tradeoff is that I cannot use my box on any vehicle LED lamp arrays which are driven by a current source.
I purchased all of my components at Digikey. I did the electrical and mechanical design. The circuit board for the oscillator/driver is approx 2"x2" 4 layer board manufactured by Sunstone. My boards were assembeld by Screaming Circuits. My yield was 100%
Sunstone and Sceaming Circuits are partnered with Digikey which can make a one-man-band operate as a fairly large company. Works for me!
Here's the overall wiring diagram used in a 1935 Ford.
Message was edited by: Peabody
Yeah me as well, Thought I understood how to deal with LED's back when you just regulated the current/voltage to get the brightness appropriate without burning them up and you were good. Then one day I started hearing about strobing them, (anybody know details about that) I kinda threw up my hands and ignored it hoping it would go away. It seems to have done so. Now were back the other way to regulated constant current from a clean DC source. I think that is more like the way they should be treated. Seemed to me that the strobing thing was like adding mileage real fast, but then ? I'm listening.
Increased efficiency regulations are the reason that LED's and constant current sources are being talked about. LED's have a better luminous efficacy (are able to output more lumens per watt) than incandescent and flourescent lighting. This has caused a desire to switch over to LED's. Because of their non-linear nature LED's cannot be plugged into the same outlet as an incandescent. The ac needs to be converted to dc. New ac/dc current sources, such as the VLED series from CUI inc, offer an easy and efficienct way of replacing incandescents with LED's. This series converts the ac into a constant current dc and is also designed to work with industry standard triac dimmers. This significantly simplifies the move from incandescents to LED's.
And if you are into LLC LED driver design here is an interesting article from Texas Instruments, on "How PWM LED dimming allows the dc/dc transfer function to remain constant while the LED load is varied with dimming", read at http://www2.electronicproducts.com/LLC_LED_driver_design_simplified-article-facn_texas_sep2011-html.aspx
Yes, Mike gives important information about the question."The CLD20B provides 20 ma constant current. You have to check that the leds you are using operate ok on 20 ma. Then you need to know the voltage drop across the led with 20 mathrough it, call that value Vled. If you are using a 12 volt battery (typical automotive use), you need a minimum of 5 v across the CLD20B for it to operate, leaving 7 volts for the leds. "
If the CLD20B requires a minimum voltage drop of 5v out of a car battery at 12v, its power efficiency is less than 60%.
Often, a car battery provides higher than 12v, so the power efficiency will often be even less. Thus, the CLD20B makes sense only for much higher supply voltages.
You pay much higher prices for LED's than for other automotive light sources in order to buy much higher power efficiencies. Thus, LED automotive lighting should use much more efficienct (> 90%) Switch-Mode drivers. Fortunately, plenty of them are available (if not from Digikey?)
Granted, the CLD20B is inefficient at low voltages, but efficiency was not an issue in the original question.
If you want better efficiency, or a LED current other than 20 ma, you can use a programmable current
source IC like the LT3092. The minimum voltage across this chip is 1.2V, so the amount of "wasted" power
can be as small as 1.2V times the LED current.
Good enough is the enemy of better.
For a handful of LEDs in a tail light lamp, a series current limit resistor may be entirely adequate.
For space craft, efficiency is an important consideration.
'Improvements' need always be evaluated in terms of cost vs benefit.
There are a lot of very good ready to use led drivers around that can handle numbers of leds and situations with out all the trouble of diodes, resistors and transistor or chip combinations that will drive the leds at a constant current safely such as the Mornsun KC24W series that have a number of specific led drivers that are easy to use - either 4 wire simple ones that only require voltage in and voltage out or 6 wire ones that can be used in the same way but with analog or PWM dimming.
Most of the mothods I've seen in this forum only relate to simple 20ma types of leds, most of the more usefull leds are the powerful types used for proper lighting & other applications, these require the use of constant currents such as 350ma - 700ma to deliver the required output and life span and these led drivers can be used in a number of series/parralell conections to acheive multiple led application for high brightness such as tail lights for example for auarium / coral lighting etc.
I am using small solar panels to charge sealed gel cell batteries. The batteries are used for low level LED lighting using moderately inexpensive 1 watt LED's in series. What makes the lighting circuitry easy is the ON Semiconductor NSI45090 constant current regulator, set to 160mA. Using the LED’s at half the rated power keeps them cool enough that I did not have to design for heat dissipation.
The lighting stays even while the battery voltage drops until the voltage is just the forward voltage of the LED’s. The added benefit is that when the battery is first charged, the current is not higher the way a resistor would allow. When the voltage drops below the forward voltage of the LED’s, the constant current regulator will no longer impede the current so that the LED dims less, unlike a resistor which would still reduce the current.
For the LED’s that I run at the full 350mA, I use a small constant current switching regulator. The results are impressive.
Using a constant current device means that what would have been a small circuit is now a simple component, much the way a logic gate is used instead of all the transistors needed to build the gate.
What I would like to see now is an array of constant current devices in a package the way an array of resistors is packaged.
The proper contant current devices such as the Mornsun KC24W series are high efficiency switching regulated devices that are small and dont get hot with lots of dissipation, they have wide in put and out put voltage capabilities allowing either single led or multiple led configurations, the devices are simple to use with mostly just voltage in and voltage out connections however dimmable units are available.The only considerations are mainly what configuration and voltage and what current you need these are available in 300ma - 700ma and are leaded so you dont even need a pcb.
Your design is clever, except for your assumption that
"Using the LED’s at half the rated power keeps them cool enough that I did not have to design for heat dissipation."
I agree that by using the LED's at half the rated power, you don't have to worry about heat transfer from the LED's to the circuit board, IF the circuit board is already maintained at a sufficiently low temperarure.
But you'll still have to design for heat dissipation from the circuit board to ambient, to assure that the circuit board is maintained at a sufficiently low temperarure at all times.
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