My experience with white LEDs is they behave much like a incandescent bulb. A white LED starts to illuminate at about 2.1 volts and gets brighter as the voltage increases.I keep a current meter in series as I increase the voltage and stop before the current reaches 18ma or so.If you use a voltmeter directly on the LED you can see the voltage change with resistor value.The LEDs only draw micro amps with the voltage at about 2.4 volts.Mel My Model Railroad http://melvineperry.blogspot.com/ Bakersfield, California Turned 84 in July, aging is definitely not for wimps.
CSX RobertIf you set the voltage to the LED's rated voltage drop you should be fine. They are rated for maximum current, but they also have a current vs voltage curve. For example, the top right graph on page 7 of this LED's data sheet.
it looks like there's about a ~0.1+V difference between 20 and 30 ma. how finely and how carefully do you need to set the voltage to avoid blowing out a bunch of LEDs w/o resistors.
i agree with Mel about setting votlage while measuring current, but how well does that work with a bunch of LEDs
it's interesting that these particular LEDs have back-to-back 5.2V zener diodes to protect them. i can understand reverse voltage protection, but not sure about forward voltage.
greg - Philadelphia & Reading / Reading
CSX Robert If you set the voltage to the LED's rated voltage drop you should be fine. They are rated for maximum current, but they also have a current vs voltage curve. For example, the top right graph on page 7 of this LED's data sheet.
If you set the voltage to the LED's rated voltage drop you should be fine. They are rated for maximum current, but they also have a current vs voltage curve. For example, the top right graph on page 7 of this LED's data sheet.
The table below will help you figure out what the operating voltage is. If you don't know for sure, always guestimate on the low voltage side. It all depends on the mfg process and which chemicals were used to create the LED in question.
Forward Voltages for common LED colors
Don - Specializing in layout DC->DCC conversions
Modeling C&O transition era and steel industries There's Nothing Like Big Steam!
gregcit's rated at a max continuous current of 30 ma and max forward voltage drop of 3.6V. it looks like there's about a ~0.1+V difference between 20 and 30 ma. how finely and how carefully do you need to set the voltage to avoid blowing out a bunch of LEDs w/o resistors.
Most LEDs are plenty bright enough for model railroad applications at 5-10 ma, so if I was using this method (I haven't, I've always used resistors) I wouldn' t be getting very cloase to the LED's limit. It this particular case I probably wouldn't go above 3.25 volts and may not even go above 3 if bright enough.
I've never seen that circuit in an LED before, but it looks like kit's not really there to protect the LED, but to allow other LEDs wired in series to work if one is bad.
GentlemenI set up my breadboard with 20 Warm White LEDs in parallel without any resistors.The LEDs are from three different batches, 2mm, 3mm & 5mm.All of the LEDs started at 2.1 volts and the brightness increased just like an incandescent bulb to 3.5 volts which is brighter than I would use on my layout.I checked several individually for current at 3.5 volts all were within .05ma at 13.5ma.I’m sold on parallel LEDs on my model railroad.The Meter was a Fluke 179.Mel My Model Railroad http://melvineperry.blogspot.com/ Bakersfield, California Turned 84 in July, aging is definitely not for wimps.
RR_Mel GentlemenI set up my breadboard with 20 Warm White LEDs in parallel without any resistors.The LEDs are from three different batches, 2mm, 3mm & 5mm.All of the LEDs started at 2.1 volts and the brightness increased just like an incandescent bulb to 3.5 volts which is brighter than I would use on my layout.I checked several individually for current at 3.5 volts all were within .05ma at 13.5ma.I’m sold on parallel LEDs on my model railroad.The Meter was a Fluke 179.Mel My Model Railroad http://melvineperry.blogspot.com/ Bakersfield, California Turned 84 in July, aging is definitely not for wimps.
CSX RobertMost LEDs are plenty bright enough for model railroad applications at 5-10 ma, so if I was using this method (I haven't, I've always used resistors) I wouldn' t be getting very cloase to the LED's limit
CSX RobertI've never seen that circuit in an LED before, but it looks like kit's not really there to protect the LED, but to allow other LEDs wired in series to work if one is bad.
After a lot of thought I have made a decision on voltage vs current LEDs. While a fixed LED voltage will work for some applications it won’t work for all.I’ll use my passenger car interiors as an example, almost every passenger car has different lighting situations. My lounge cars use four different levels of brightness and will require resistors. My supply voltage for my passenger cars is 4 volts and has worked very nicely for several years. I set the brightness level as needed with resistors for each condition. Every car has a ten turn 2k pot in series with the LEDs to set the entire car lighting brightness.A string of street lights would work better as a voltage LED because all street lights would be equal brightness.I will treat the LEDs as bulbs with some needing a resistor for dimming. Mel My Model Railroad http://melvineperry.blogspot.com/ Bakersfield, California Turned 84 in July, aging is definitely not for wimps.
RR_MelAfter a lot of thought I have made a decision on voltage vs current LEDs. While a fixed LED voltage will work for some applications it won’t work for all.
I haven’t as of yet used a buck converter for direct LED drive but plan to do so in the future. Most likely street lighting, at this time all my street lights are incandescent.I use 4 volts for my passenger cars from the lead car which is a baggage car with a track to 4 volt power supply. All of my passenger cars are daisy chained. I use a TP4056 charger board in my baggage cars to charge a AA size Lithium battery. The TP4056 has a USB input powered by a Mel track to 5 volt regulator.As I operate dual mode DC or DCC I use a DCC function decoder to turn on and off the 4 volts to the passenger cars. The decoder trips a SPST micro relay and the relay has a magnetic latching switch paralleling the relay contacts for DC operation.
I installed a resetable micro fuse on the output to the cars.All my passenger cars are daisy chained using micro connectors through the diaphragms.Every car has a 2k ten turn pot for adjusting the overall brightness.I run the brightness very low in my passenger cars for maximum realism. The average car draws about 3ma at 4 volts, 8 to 12 LEDs per car.My structure lighting is all incandescent, 12 volt GOW bulbs (about 350 bulbs) operating at 8½ volts from buck converters. I have 9 houses using Arduino Random Lighting Controllers driving 20 GOWs in each house, the Arduino 5 volts is also on a buck converter.My thing is tinkering around with electronics for my layout. I know my lighting draws a lot of current but it also looks more realistic for the 1950s. Each house with all 20 bulbs on draws 1 amp at 8½ volts. About another 150 GOWs not on Arduinos, 7 amps. My 98 vehicles draw 4.8 amps at 1.35 volts also on a buck converter. Mel My Model Railroad http://melvineperry.blogspot.com/ Bakersfield, California Turned 84 in July, aging is definitely not for wimps.
I am also chipping in a recommendation to not directly drive LEDs from a constant voltage source. As noted above, the current through an LED can change a lot with only small changes in voltage. And the voltage drop of a particular LED is variable based on current and temperature.
As the LED's temperature increases, its voltage drop decreases... Which means more current will flow through the LED, and so it gets hotter... Which means its forward voltage will drop further.... Etc.
Point is, using a constant voltage alone for an LED may give inconsistent results. LEDs are not incandescents. I also don't see the point in not using resistors, as they are super cheap and easy to attach, basically foolproof.
It's also good to remember that voltage and current are two sides of the same metaphorical coin. Sometimes it's best to design your circuit to obtain a targeted voltage (eg powering an IC) and sometimes it's better to design it to obtain a targeted current (eg LEDs)
Will the DC to DC Buck convertors maintain a constant output voltage when the input voltage varies?
I am thinking of using them to provide constant brightness LED lighting in American Flyer passenger cars with a rectifier and maybe a capacitor to prevent flickering.
Yes, the voltage and current regulation remain extremely good within the input voltage ratings.Input voltage: 4.75V-24VOutput voltage: 0.92V-15VOutput Current: 2A (maximum)Voltage Regulation: ± 2.5%Load Regulation: ± 0.5%https://www.ebay.com/itm/133210562518There is a 1.4 volt drop through a bridge rectifier so it will take 6.1 volts on the rails before the buck converter will work. That is why I use a battery. I keep the battery topped off at higher track voltages.I operate my LED supply voltage at 4 volts using a AA size Lithium battery charged from the rails. For individual LEDs a coin cell works great LIR2032.This works very good without a buck converter by charging the coin cell with track voltage through a resistor at about 40ma at full track voltage.Mel
My Model Railroad http://melvineperry.blogspot.com/ Bakersfield, CaliforniaGrowing old is a bummer, aging is definitely not for wimps.
mvlandswWill the DC to DC Buck convertors maintain a constant output voltage when the input voltage varies? I am thinking of using them to provide constant brightness LED lighting in American Flyer passenger cars with a rectifier and maybe a capacitor to prevent flickering.
even a less expensive adjustable voltage regulator (e.g. lm317, $0.44) won't start passing current until the input voltage is some amount > the desired output voltage. there are probably low drop-out regulators that minimize this voltage.
another approach is using an lm317 as a current regulator that avoids the need for series resisitor.
RR_Mel Yes, the voltage and current regulation remain extremely good within the input voltage ratings.Input voltage: 4.75V-24VOutput voltage: 0.92V-15VOutput Current: 2A (maximum)Voltage Regulation: ± 2.5%Load Regulation: ± 0.5%https://www.ebay.com/itm/133210562518There is a 1.4 volt drop through a bridge rectifier so it will take 6.1 volts on the rails before the buck converter will work. That is why I use a battery. I keep the battery topped off at higher track voltages.I operate my LED supply voltage at 4 volts using a AA size Lithium battery charged from the rails. For individual LEDs a coin cell works great LIR2032.This works very good without a buck converter by charging the coin cell with track voltage through a resistor at about 40ma at full track voltage.Mel My Model Railroad http://melvineperry.blogspot.com/ Bakersfield, CaliforniaGrowing old is a bummer, aging is definitely not for wimps.
So you're charging a li-ion coin cell from track voltage without a charge controller? That doesn't sound like a good idea.
CSX Robert So you're charging a li-ion coin cell from track voltage without a charge controller? That doesn't sound like a good idea.
It probability isn’t. I only have three cars done that way and so far so good. I went berserk with some cabooses.
The difference is I run the cabooses daily and rarely run the coaches.I had to change the charge resistor in the TP4056 to drop the charge current to 40ma, its been a long time but I think I used a 82KΩ chip resistor.
Mel My Model Railroad http://melvineperry.blogspot.com/ Bakersfield, CaliforniaGrowing old is a bummer, aging is definitely not for wimps.
Ok, the breakman that plugs into the caboose steps is really neat.