LED power source

[CSX Robert]

Ok, the breakman that plugs into the caboose steps is really neat.

[RR_Mel]

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, California

Growing old is a bummer, aging is definitely not for wimps.

[CSX Robert]

RR_Mel

Yes, the voltage and current regulation remain extremely good within the input voltage ratings.

Input voltage: 4.75V-24V
Output voltage: 0.92V-15V
Output Current: 2A (maximum)
Voltage Regulation: ± 2.5%
Load Regulation: ± 0.5%

https://www.ebay.com/itm/133210562518

There 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, California

Growing 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.

[gregc]

mvlandsw

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.

putting a buck-converter ($3) in each passenger car is overkill, besides being expensive and space consuming

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-24V
Output voltage: 0.92V-15V
Output Current: 2A (maximum)
Voltage Regulation: ± 2.5%
Load Regulation: ± 0.5%

https://www.ebay.com/itm/133210562518

There 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, California

Growing old is a bummer, aging is definitely not for wimps.

[mvlandsw]

   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.

[speedybee]

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)

[RR_Mel]

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.

[gregc]

RR_Mel

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.

so while you use the buck-regulators to accurately regulate the power supply voltages, you don't rely on it to drive the LEDs without resistors?    you use different value resistors to control the intensity or deal with different LED type/manuacturers

[RR_Mel]

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.

[gregc]

CSX Robert

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

that seems a sensible approach

CSX Robert

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.

thanks.   i hadn't thought of that

[DigitalGriffin]

RR_Mel

Gentlemen

I 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.

 

That's surprising.  13ma is on the low side for that high a voltage.  *scratches head*  Is that a PWM voltage source?   or AC transformer power source?

But hey if it works... :D  Just remember anything over 20ma will dramatically shorten it's life span.

[RR_Mel]

Gentlemen

I 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 Robert]

gregc

it'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.

[DigitalGriffin]

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.

 

Robert is correct.  You can hook a WHITE LED to a 120V source or a 3V source, it will light the same way.  On the 3V source, the ground leg will = 0 V.  On the 120V source, the ground leg will measure 116.5V.  (Provided you put in a proper resistor to limit current)

LEDs become auto current limiting if you stay below their operating voltage.  (Or V_forward voltage)

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

[gregc]

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.

it'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.

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.

[RR_Mel]

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 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.

[gregc]

trainnut1250

The adjustment pots take lots of turns to make the adjustment, so they are easy to set accurately.

yes, but how do you know what voltage to set them to? 

if they are ~3.4V LEDs, is that what you set them to, 3.40V and no more?

i thought LEDs were spec'd at a max current, not max voltage?   and of course that current can be exceeded with care.   i've determined resistance values of IR emitters by measuring the current since you can't see their intensity

[DigitalGriffin]

As others have alluded, you need parrallel wiring over series.

For example, a white LED has a forward voltage of ~3.5V

If you wire two of them in series, you will need at least 7V.  Three of them 10.5V (3*3.5V)

Anything above 12V, I do not recommend using for an amateur.  It gets into "pee your pants" to "Exploding hearts" terratory.

Many people have used White LED lights off christmas tree sets with great success.  But you'll have to cut off each one, and add a resistor to each one, and tap into a 12V bus for each one.  It's very time consuming.  But such is the nature of MRR.

[RR_Mel]

I normally buy my LEDs in batches of a 100 so coloring should be close.

I use glass stain for tail lights and marker lights so all LEDs would be the same voltage.  Done it that way for 30 or 40 years with micro bulbs.

I have experimented with SMD LEDs using red and amber so those will still need resistors but those are in larger vehicles (18 Wheelers). Easier to use ⅛ watt or chip resistors.

In this Kenworth I used .5mm fiber optics from the amber LED to the roof.

Right now I’m using 5 volts for my LED vehicles and resistors but I think I will see how 2.8 to 3 volts work for headlights using resistors for red and amber or a combo in series.

Sure sounds a lot easier if it works.

I had made the connectors from brass rods and tubing for my bulb equipped vehicles fed by 1.35 volts for many years.  For my LED vehicles I went with a Arduino style header type connector hopping to prevent voltage problems.



Mel


 
My Model Railroad   
http://melvineperry.blogspot.com/
 
Bakersfield, California
 
Turned 84 in July, aging is definitely not for wimps.

[CSX Robert]

RR_Mel

After thinking on this I beleive I will go with another converter set to 2.9 volts or so, much easier than trying to cram a resistor in a tight place.   I haven't been able to find any micro bulbs in some time so I've been forced to go with LEDs for my vehicles.  I've been fighting to squeeze ⅛ watt resistors in my vehicles, with a fixed voltage no resistors needed.

 

The resistors don't have to go inside the vehicle, they can go anywhere between the power soure and the LED.  One problem with driving LEDs without a resistor is different colors (and even the same color often times) have different voltage drops.  If you have white headlights, red taillights, and amber marker lights, that's three different voltages.  Red and amber voltage drop are pretty close, so you might be able to power them off the same voltage (though you would have no way of individually adjusting the brightness and there's a good chance they wouldn't match), but you would almost certainly have to use a separate voltage for the white.

[trainnut1250]

RR_Mel

 

 

trainnut1250

I'm a convert to the buck voltage regulators after Mel posted about them a few years ago. I have about ten in use on the layout currently set at various voltages - mainly 1.3V for mini bulbs and 2.9 volts for LEDs - I'm a big fan

Guy

 

 

 

 

Thanks for the post!!!!  So simple, stupid of me to not think of that.  But then I only have 70 or so years in electronics.

Mel


 
My Model Railroad   
http://melvineperry.blogspot.com/
 
Bakersfield, California
 
Turned 84 in July, aging is definitely not for wimps.

 

 

Mel,

 

It wasn't my idea - one of my train buddies is a retired EE and he suggested it after I was complaining about having to fit resistors in structures....

I do agree these converters are kind of addicitve...now several of my train buddies are using them. I have bought a whole bunch and I will just give one to modelers visiting the layout who ask about them and want to try em out. "the first one is free...." LOL!

 

Guy

[trainnut1250]

gregc

 

 

trainnut1250

I am very careful about setting voltages.....I have thought about using two regulators in series in case of an overvoltage failure....

 

 

i wasn't suggesting a failure.   i was asking how the voltage is adjusted.

 

i would think the regulator needs to first be set to a low voltage before connecting any LEDs.   after the LEDs are connected the voltage is increased.   at first, the voltage may be too low for any of the LEDs to light.   then as the voltage is increased after the LED light, they get brighter.

but if the voltage is increased too far they start blowing out

so when do you know when to stop increasing the voltage?

 

Greg,

 

The converters have a read out for the ouput voltage so I crank them down to well below my target and then adjust back up to desired voltage before hooking up any circuits. The adjustment pots take lots of turns to make the adjustment, so they are easy to set accurately.

Guy

[RR_Mel]

trainnut1250

I'm a convert to the buck voltage regulators after Mel posted about them a few years ago. I have about ten in use on the layout currently set at various voltages - mainly 1.3V for mini bulbs and 2.9 volts for LEDs - I'm a big fan

Guy

 

Thanks for the post!!!!  So simple, stupid of me to not think of that.  But then I only have 70 or so years in electronics.

Mel


 
My Model Railroad   
http://melvineperry.blogspot.com/
 
Bakersfield, California
 
Turned 84 in July, aging is definitely not for wimps.

[RR_Mel]

 

The setup on the converters is very easy, the 20 turn pots work great.  I put a meter on the out and tweak them to the wanted voltage, the out put voltage only drops .1 volt from min to max load at 1.3 to my highest voltage of 8.5 volts.  The only time I mess with the current is when I’m charging batteries.

 

When you think about it its not much different from setting the voltage to 1.3 volts for the 1½ volt micro bulbs.  I do have a couple of 10 amp diodes in series across my 1.3 volt converter incase something craps out in the converter, I have 300 micro bulbs.

 

I’ve been operating my 8 amp converter now for 3 years set to 1.3 volts (4.9 amps) and its very solid.  For a $4 buck converter its done better than more expensive stuff.

 

I really got caught up in the cheapo buck converters and now use them for all kinds of goodies.  I have 16 of them and no failures, some running very low current and others running very high current.

 

EDIT:

 

I glued a couple of the 8 amp converters to my bench power supply for working on my Arduino goodies.

 

 

EDIT:

 

After thinking on this I beleive I will go with another converter set to 2.9 volts or so, much easier than trying to cram a resistor in a tight place.

 

I haven't been able to find any micro bulbs in some time so I've been forced to go with LEDs for my vehicles.  I've been fighting to squeeze ⅛ watt resistors in my vehicles, with a fixed voltage no resistors needed.

 

 

Mel


 
My Model Railroad   
http://melvineperry.blogspot.com/
 
Bakersfield, California
 
Turned 84 in July, aging is definitely not for wimps.

[gregc]

trainnut1250

I am very careful about setting voltages.....I have thought about using two regulators in series in case of an overvoltage failure....

i wasn't suggesting a failure.   i was asking how the voltage is adjusted.

i would think the regulator needs to first be set to a low voltage before connecting any LEDs.   after the LEDs are connected the voltage is increased.   at first, the voltage may be too low for any of the LEDs to light.   then as the voltage is increased after the LED light, they get brighter.

but if the voltage is increased too far they start blowing out

so when do you know when to stop increasing the voltage?

[trainnut1250]

gregc

 

 

trainnut1250

I'm a convert to the buck voltage regulators after Mel posted about them a few years ago. I have about ten in use on the layout currently set at various voltages - mainly 1.3V for mini bulbs and 2.9 volts for LEDs - I'm a big fan

 

 

when you 2.9V, do you have series resistor for each LED?

 

if not, wouldn't mistakenly setting the voltage too high blow out all the LEDs?

 

Greg,

No I don't use resistors - that is one of the reasons for having the voltage regulators.

I am very careful about setting voltages.....I have thought about using two regulators in series in case of an overvoltage failure....

 

Guy

[gregc]

trainnut1250

I'm a convert to the buck voltage regulators after Mel posted about them a few years ago. I have about ten in use on the layout currently set at various voltages - mainly 1.3V for mini bulbs and 2.9 volts for LEDs - I'm a big fan

when you 2.9V, do you have series resistor for each LED?

if not, wouldn't mistakenly setting the voltage too high blow out all the LEDs?