# Lighting buildings with LEDs Advice



## jimben (Jun 27, 2018)

If you have a power pack that produces 12-16 volts DC, LEDs are very efficient vs incandescent bulbs. A 10ma current draw 12 volt LED makes a brighter light vs a 100ma incandescent bulb plus LEDs never burns out. You can use ten 10ma LEDS to the single 100ma incandescent bulb for same current draw. You would run out of transformer controller power (capacity) in short order with incandescent bulbs.

LEDs do have one issue and it is more directional light plus the little LED chip inside is not a soft light. Just a tip I use is to place a upside down V shape paper over the LED. It removes the bright look and provides more uniform lighting. It is a little dimmer, but still more than bright enough. To simulate an incandescent bulb typical in most pre 2000 year layouts, use warm white LEDs.

If you have 12 volts or higher DC output from your transformer controller use a resistor in series to the LEDs to drop to about 10 volts DC for 12 volt DC rated LEDs. At 16 volts DC output 100 ohms works for 120ma current draw. 50 ohms for 240ma. 200 ohms for 60ma current draw. 9-10 volts provides lower peak voltage than 12 volts voltage and will protect any 12 volt DC LEDs that do not have a filtered DC such as provides by the newer filtered wall warts. Some LEDs will fail at 12 volts unfiltered DC. Brightness at 10 volts is close to 12 volts.

To change 17-18 volts AC to DC, use a single silicon diode to change to DC. 1 to 3 amp diodes at 100 volts or higher voltage rating (called PIV) work well. Simply place the diode in series with one side of the AC output. The resistors values, if needed, are different values vs a controller producing DC voltage to get the 10 volts DC from 17-18 volts AC. Most 17 volt AC accessory voltage provides 9-10 volts DC without a resistor. I used a power supply software program and result was 17 volts AC produced about 10 volts DC with 200ma current load using one diode. One diode called half-wave rectification produces 60Hz pulsating DC. Most cannot see a 60Hz light flicker.

Diode specs
1N4002 - 1 amp at 200 volts
1N5402 - 3 amps at 200 volts


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## DonR (Oct 18, 2012)

The easiest way to light buildings is to use
the LED strip lights that come on a reel. You
can cut them off in 3 LED strips that take
a 12 V DC input. There are so many no longer
needed wall warts that put out that current. Just
one could power a whole city. As to color, the
warm white emulates florescent tubes.

Probably the most work to light a building is to
'black out' the roof and walls. I use a combination
of thick black paint, black electricians tape, and
cardboard strips. A nice touch is to black out
this or that window. At night you usually don't
see every room lighted.

Don


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## Stan D (Feb 1, 2019)

My experience is limited to 5mm LEDs (so far). I've found a little paint can soften the harshness. A little white does wonders. Yellow makes a good "bug light" for a porch. Yes, I could just use a yellow LED, but the paint also softens it.


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## jimben (Jun 27, 2018)

DonR said:


> The easiest way to light buildings is to use
> the LED strip lights that come on a reel. You
> can cut them off in 3 LED strips that take
> a 12 V DC input. There are so many no longer
> ...


If using a wall wart, get the newer switching type so the LEDs get filtered DC to last many years. The older and larger DC type are transformer type without filtering. I only seen switching types on ebay now. Good deal is the under $6 new wall wart rated at 2 amp @ 12 DC. Two amps will operate 200 10ma LEDs.


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## gregc (Apr 25, 2015)

jimben said:


> If using a wall wart, get the newer switching type so the LEDs get filtered DC to last many years.


i don't see why filtered DC to extend the life of LEDs. A common way to control the brightness of LEDs is with PWM which is repeatedly turning the LED on and off.


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## jimben (Jun 27, 2018)

gregc said:


> i don't see why filtered DC to extend the life of LEDs. A common way to control the brightness of LEDs is with PWM which is repeatedly turning the LED on and off.


It is not filtered DC extends the life of LEDs. It is unfiltered DC with using a lower than LED specified voltage that extends the life of LEDs. Explanation is below.

LEDs are very sensitive to reverse high peak to average voltage. Non filtered 12 volts DC has a peak of 1.41 to average DC voltage or a peak of 16.92 volts. Most transformer controllers are 14-15 volts average DC output. The peak reverse voltage of a 15 volt DC transformer controller is as high as 21.15 volts. The 12 volt LED can burn out. Operating around 10 volts unfiltered DC is about 14 volts reverse peak to average and does not affect the LED life. 

Some of my 12 volts LEDs did burn out in a few hours at that 16.92 volts reverse peak voltage when using a lower value resistor to reduce the 14 volts average voltage to 12 volts average voltage. None have burned out using a higher resistance series resistor at an average of 10 volts DC output or about 14 volts reverse DC peak. The 12 volt DC LEDs are about 90% brightness at 10 volts compared to 12 volts DC.

Filtered DC such as in a switching type wall wart is 12 volts DC without a reverse peak voltage so you can use the full 12 volts DC on 12 volt LEDs. 

The PWM (pulse width modulated) transformer controllers likely does not have a high peak to average voltage output thus can use the full 12 volts on the LEDs if the accessory voltage is also PWM controlled. I am not sure if the PWM type transformer controller applies PWM to the accessory DC voltage. So, it is safe to use 10 volts average DC on a 12 volt DC rated LED.


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## gunrunnerjohn (Nov 10, 2010)

Actually, if you rectify the DC, you don't risk reverse voltage as it's all positive. The rectifier specifically doesn't allow any negative going voltage, and thus no reverse voltage on the LED. See the waveform below.

The only reason to filter it for a simple LED is some people are actually sensitive to the 60hz flicker that is emitted, however most people can't see it.


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## gregc (Apr 25, 2015)

jimben said:


> LEDs are very sensitive to reverse high peak to average voltage.


yes, reverse voltage



jimben said:


> Non filtered 12 volts DC has a peak of 1.41 to average DC voltage or a peak of 16.92 volts.


by non-filtered, you mean, Not half, full-wave rectified. I believe the average voltage for a half-cycle is 0.707 of peak. so a peak voltage of 16.9 and a peak-to-average ratio of 1.4. Zero reverse voltage.




jimben said:


> Most transformer controllers are 14-15 volts average DC output. The peak reverse voltage of a 15 volt DC transformer controller is as high as 21.15 volts.


i don't understand where this reverse voltage is coming from?

you certainly don't want to use AC, although the LEDs may light


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## jimben (Jun 27, 2018)

gregc said:


> yes, reverse voltage
> 
> 
> 
> ...


Download PSU Designer II power supply simulator from Duncan amps . Half or full wave rectification has same 1.41 peak voltage.

You assume diodes have no leakage current. Although in the micro amp range, they do produce a reverse voltage. 
If you want to rid the reverse voltage statement and simply use peak forward voltage in my response, it still will burn out the LED when using the full 12 volts of unfiltered DC. My 12 volt LEDs did not burn out at 12 volts average unfiltered DC for no reason.


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## gunrunnerjohn (Nov 10, 2010)

I have no idea what you're experiencing, so I think we'll agree to disagree on this point. As long as it works for you, keep doing it. Since a simple diode has worked for me since LED's were invented, I'll keep doing it my way, that works for me.


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## jimben (Jun 27, 2018)

gunrunnerjohn said:


> I have no idea what you're experiencing, so I think we'll agree to disagree on this point. As long as it works for you, keep doing it. Since a simple diode has worked for me since LED's were invented, I'll keep doing it my way, that works for me.


For the rest of you out there concerning disagreements from poster above. Follow my advice and your LEDs will last. Software based Power supply simulators are not wrong.


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## MichaelE (Mar 7, 2018)

gunrunnerjohn said:


> I have no idea what you're experiencing, so I think we'll agree to disagree on this point. As long as it works for you, keep doing it. Since a simple diode has worked for me since LED's were invented, I'll keep doing it my way, that works for me.


They've been working for Viessmann for decades too. LED's work just fine on AC with a diode.


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## gunrunnerjohn (Nov 10, 2010)

Since all your Lionel Legacy and PS/3 LED lighting is running on PWM waveforms without filtering, I'd say the odds for them lasting are pretty good.


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## T-Man (May 16, 2008)

I make sure the other diode is a LED soldered together at the opposing fields. Then by stringing the pairs and adding resistance for any voltage needed.


I use a pair for HO to determine track polarity.

Explained in this link


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## jimben (Jun 27, 2018)

LEDs vary in manufacture. Some will fail at high peak voltages and apparently some will not. I would play it safe and use 10 volts DC using a resistor unless using a filtered or PWM power. My LEDs did fail at unfiltered 12 volts DC using two different brands (10ma & 20ma) until I lowered the voltage to 10 volts.


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## T-Man (May 16, 2008)

I have noticed that engine mounted LEDs need more than a diode due to peak voltages on the AC motor changes speeds.


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## gregc (Apr 25, 2015)

jimben said:


> So, it is safe to use 10 volts average DC on a 12 volt DC rated LED.


you're discussing LED strips, not individual LEDs which require a resistors to limit current?


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## gunrunnerjohn (Nov 10, 2010)

LED strips would be even less likely to fail with reverse voltage. The 12V strips have three LED's in series with a 120 or 150 ohm resistor. So, the reverse voltage specification would be tripled as you have three in series.


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## jimben (Jun 27, 2018)

gregc said:


> you're discussing LED strips, not individual LEDs which require a resistors to limit current?


I am discussing all LEDs, strip or individual type or other types as well if exists.


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## jimben (Jun 27, 2018)

gunrunnerjohn said:


> LED strips would be even less likely to fail with reverse voltage. The 12V strips have three LED's in series with a 120 or 150 ohm resistor. So, the reverse voltage specification would be tripled as you have three in series.


Likely better for peak voltages..


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## gunrunnerjohn (Nov 10, 2010)

Peak voltages are not a problem for LED's, only reverse voltage. LED's routinely run on PWM drive with peak voltages of anywhere from 12 to 20 volts and don't suffer any excessive failures. With LED's it's all about minimizing the reverse voltage and keeping the average current within the ratings of the specific part and not exceeding the peak current rating.

Here's a typical LED specification, this is for a CREE surface mount chip that I've used in several projects. The maximum peak forward current is 100ma, but the maximum average forward current is 25ma. The recommend operating current for this particular part is 20ma. If you were trying to light this at full brightness, you could use 100ma pulses at a 20% duty cycle and achieve the same brightness as a steady 20ma DC supply. You would also achieve the same intensity. PWM is very widely used as it makes it very easy to vary the intensity of the illumination using digital processing vs. the much more expensive analog current regulation. You can also have a large array of LED's all driven by a multiplexed PWM drive and individually control the intensity of each LED separately.


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## gregc (Apr 25, 2015)

trying to understand your experience ... i'd hate for people to get the wrong impression



jimben said:


> Some of my 12 volts LEDs did burn out in a few hours at that 16.92 volts reverse peak voltage when using a lower value resistor to reduce the 14 volts average voltage to 12 volts average voltage.


don't understand why you say "reverse peak voltage" and not just peak voltage

don't understand why you would use a "lower value resistor" when providing a higher voltage to a LED strip designed for 12V. You need a higher resistance to limit the current to the same value drawn with 12V.

LEDs are rated by the current they draw. The max is ~20ma.

If an LED strip uses three 3V LEDs (they draw 20 ma at 3V) and a 150 Ohm resistor when supplied with 12V, the current is 20 ma ((12V- 3*3V))/150)

If supplied with 14V, a 250 Ohm resistor should be used ((14V - (3*3V)) / 0.02




jimben said:


> None have burned out using a higher resistance series resistor at an average of 10 volts DC output or about 14 volts reverse DC peak.


not surprising that a higher resistance works with a lower voltage since both a higher resistance alone would reduce the current as well as reducing the voltage.



jimben said:


> The PWM (pulse width modulated) transformer controllers likely does not have a high peak to average voltage output ...


the peak to average ratio of pulse width modulated waveform is 100 / %on. If its on 10% of the time the the average voltage is 10% the peak and the peak to average is 10.



jimben said:


> I am not sure if the PWM type transformer controller applies PWM to the accessory DC voltage.


it doesn't. PWM only applies to the track voltage.


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## Stan D (Feb 1, 2019)

Voltage is what can kill an LED. Both too much and not enough. You can reduce the voltage with a resistor to dim it a little, but at a certain point it doesn't like it and will fail. That's why LEDs for your house aren't all dimmable. Those that are have a circuit in them that convert less voltage in to PWM.

Unlike voltage, there is no such thing as too much current (amps). A device will only draw as much current as it needs. It is important to have enough of a power supply to handle a device.


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## gregc (Apr 25, 2015)

Stan D said:


> A device will only draw as much current as it needs.


a Light Emitting Diode is a diode, a semiconductor. It is not like a resistive device like an incandescent lamp that operates at a specific voltage.

see the current-voltage characteristics of diodes. A diode only conducts (a current) when the voltage across it is positive and above a threshold where is then passes a large amount of current with very little increase in voltage. 

See curve for a diode below where the horizontal axis is voltage and the vertical axis is current. The current is zero for negative voltage less than the max reverse voltage and below the threshold which is ~0.7V for a conventional diode (1.4 - 3+V for LEDs). Large amounts of current pass for voltages above the threshold with very little increase if voltage across the diode.

exceeding the current limit on a diode will damage it. A common max current for an LED is ~20ma. 

it's easier to drive an LED with a current limiter such as the CL2 Constant-Current LED Driver than with a voltage regulator (used as a voltage regulator).


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## gunrunnerjohn (Nov 10, 2010)

Stan D said:


> Voltage is what can kill an LED. Both too much and not enough. You can reduce the voltage with a resistor to dim it a little, but at a certain point it doesn't like it and will fail. That's why LEDs for your house aren't all dimmable. Those that are have a circuit in them that convert less voltage in to PWM.
> 
> Unlike voltage, there is no such thing as too much current (amps). A device will only draw as much current as it needs. It is important to have enough of a power supply to handle a device.


Actually, you're incorrect on both points. There is no minimum voltage specification for an LED, they will simply not light if they have insufficient voltage, but it won't harm them. The reason that your 120V LED lamps aren't dimmable isn't the LEDs used, but rather the support circuitry that converts from 120V to LED currents.

Also, there is INDEED too much current for an LED! An LED is a current mode device, and they have a fairly sharp knee where current spikes to way over their specifications. Here's a graph of forward voltage vs forward current for a typical 20ma spec LED of various colors. Note that as the emitted light wavelength gets shorter, the operating voltage goes up. Take the white LED as an example, at 3.0V it's running at it's rated 20ma. However, at 3.5V it's already up to 40ma, twice it's rated current. Before it gets to 4.5V, it's already at 60ma, three times it's rated current. Contrast that to an incandescent bulb, and the different between 3V and 3.5V would be a very small current change.


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## Stan D (Feb 1, 2019)

I currently (no pun intended) have a structure lit with three 5mm 3 volt LEDs. They are rated at 20 ma. They are powered by two AA alkaline batteries in series capable of over 2000 mAh. How is it that basically 2 amps isn't overdriving 20 mA LEDs? It didn't burn out when I only had one in the circuit.


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## gunrunnerjohn (Nov 10, 2010)

Stan, how much voltage do two AA batteries in series present to the LED's?



Hint: It's about 3 volts. A quick check of my graph shows that 3V on a white LED will allow around 20ma to flow, just right.


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## Stan D (Feb 1, 2019)

3 volts. But your position is that current is the killer here. My position is that voltage kills the LED. The graph you posted shows that as the voltage increases, the LED draws more current, and I'd not argue that. As the voltage 4 or 5 volts, the LED will draw more, but then burns out. Taking the "W" line in the graph, at just over 4.5 volts, the LED is drawing 100 mA (1/10th of an amp). My "Brewery" (that's the structure) circuit is capable of 20 times that current, but at the correct voltage.


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## Dennis461 (Jan 5, 2018)

It's not current or voltage that kills an LED. It' letting the smoke out!


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## wvgca (Jan 21, 2013)

dunno ... I use whatever voltage power supply is handy, depends on whether or not I want to series them, mostly five to seven volt ...
and current, well it's been four to eight millamp, rather than the usual twenty ...I don't like leds very bright, just enough to notice them ..


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## gregc (Apr 25, 2015)

Stan D said:


> My position is that voltage kills the LED. ... but then burns out.


heat damages an electronic device, power in watts = Voltage * current. As you said, it "burns out".


a typical LED datasheet specifies max current, not max voltage nor provide a current-voltage curves.

when designing an LED circuit in an electronic device, it typically uses a regulated voltage available on the device. The designer doesn't add a 3.36V regulator to drive an LED. The designer uses the nominal operating voltage for the LED to determine the size of a current limiting resistor in series with the LED using the available regulated voltage.

after testing, the resistor size might be changed to adjust the brightness of the LED.


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## Stan D (Feb 1, 2019)

gregc said:


> a typical LED datasheet specifies max current, not max voltage nor provide a current-voltage curves.


Yes, but this is where I think some are confused. It shows the max current an LED _will_ draw, not the maximum it will handle. LEDs, like anything else, will only draw as much current as it needs. There is no "too much" current an LED can handle at the proper voltage.


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## gregc (Apr 25, 2015)

Stan D said:


> It shows the max current an LED _will_ draw, not the maximum it will handle. don;t LEDs, like anything else, will only draw as much current as it needs. There is no "too much" current an LED can handle at the proper voltage.


don't know what you background or experience is, but obviously an LED will draw more current than it can handle, otherwise it wouldn't burn-out.

i don't see a max nor proper voltage on a spec sheet, and the current spec is typically the max value .

an LED is not a resistive device following Ohms law, it's a semiconductor like a transistor.


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## Stan D (Feb 1, 2019)

Like anything in this world, nothing lasts forever. So when it is about to fail, an LED will draw more current. This can happen if too much voltage is applied. The circuit I described a few posts back proves my point. 2 full amps of AA battery won't "burn out" a 20 mA LED. Another example would be flashlights (another mild obsession of mine). I try and swap out the bulb with an LED. the batteries last way longer than with an incandescent bulb. 

My background in electronics is that while I'm not an electronic technician, I've always worked around electronics. I am also an Amateur Radio licensee, which does require some basic electronic knowledge. My early jobs were in the car stereo field, then in to commercial two-way radio(Motorola and the like), then to amateur radio. I was messing around with low voltage at single digits of age, about a decade before LEDs appeared in commercial products.


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## jimben (Jun 27, 2018)

Bottom line. 
If you use a transformer controller with DC output, it has a high peak to average voltage. Using a diode on transformer controller AC accessory voltage is about the same. Use 10 volts on 12 volt LEDs for long life. 
PWM controllers do not have a high peak to average voltage. Also switching type wall warts are filtered and do not have a high peak to average voltage. For these sources of DC power use 12 volts on 12 volt LEDs.


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