Help needed on street lighting

[Country joe]

On this subject I know nothing, I'm wanting to fit 10 led streetlights, I don't wish to use my controller.

Thinking of using a 12 volt power supply adaptor charger for led screen, would this be good and what else would I need to purchase, the lights are ready made up.

[Chrissaf]

I totally agree with you about not wasting valuable power from your controller to operate layout lighting.

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Any general purpose 12 volt DC supply will be OK. You didn't say what the current rating is for your LCD screen charger. Look at the label on it, what does it say. It may give a value in mA (milli-amperes) or it may give a value in W (Watts).

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If the lights are "ready made up" then I would expect them to include a resistor in one wire leg of the lamp. If the resistors have been supplied by your lamp retailer separately in the packaging, then each lamp must have one resistor in series with one leg of the lamp. LEDs are polarity sensitive and will have a positive and negative wire, they will not light up if connected to your 12 volt DC supply the wrong way round. If you do not include the resistor (if not already wired in by the seller) then the LEDs will burnout when connected to a 12 volt supply.

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The most common resistor value included in ready made LED lamps is 1,000 ohms (1K). This gives a current requirement of 10mA per LED on a 12 volt DC supply. Therefore you need a supply of 100mA minimum for 10 street lamps. It is best to not run a power supply at 100% capacity, so I recommend a 12 volt power supply with at least double that current capacity (200mA) and preferably 500mA (0.5A) or higher. In Watts terms, 0.5A on 12 volts is 6W [and 200mA = 2.4W].

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So, in conclusion, look at the label on your proposed 12 volt supply, does it give an output rating that exceeds 200mA or 2.4W. If it does, then you are good to go. If not, buy a 12 volt supply with a 2 Amp or higher rating and this will future proof you against further lighting expansion at a later date. Once you see the effect of LED lighting on your layout, you will want to add more, I am sure.

[Country joe]

Thanks my control is the hornby elite, I was looking at a video on YouTube, and the guy says if you use a 12 volt regulated charger you can set it for 3 amp you don't need resistors as the 3 amp charge will not blow them.

[trainlover23]

 Look at the Woodland scenics just plug range it very good

[Howbi]

@Country joe..........Assuming your street lights are connected in paralell....... You can't push 12volts into a 3volt LED without destroying it no matter what amperage the power supply is.........that's why you have to have a resistor on each LED.........so I'm LED to believe........ 🤔...........HB

[Chrissaf]

Don't believe all what you see on YouTube. It winds me up just how much disinformation about electronic circuits there is on YouTube. I've been working in the electronics field of technology for more than 40 years.

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Firstly your YouTube video was talking about power supplies limited to  3 VOLT outputs not 3 Amps. LEDs will vaporise if you pass 3 Amps through them. They are current sensitive devices designed to operate on currents in the less than 30mA range (30mA is 0.03 Amps).

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I won't bore you (or the others on here) with two pages of theory as to why using 3 Volt power supplies is not the best solution. I've done it before already on this forum. By all means use a 3 volt power supply, but if you do, you will be driving the LED at its maximum power rating and potentially shorten its life. The LED should always include a current limiting resistor in circuit to control the current even on a 3 volt supply. On a 3 volt supply, the resistor only needs to be a small value typically about 47 ohms. The other issue of using a 3 volt supply without any resistors is that if you mix and match different types of LEDs in parallel across this 3 volt supply without the resistors, some of the LEDs may be either dim or not come on at all. This is because the LED is a semi-conductor (just like a transistor or an IC) different types of LEDs have different forward bias voltages. Including the resistor, balances out these differences allowing all the different types of LEDs to light up equally when sharing a common supply.

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If I write any more I am in danger of doing a 'war and peace' LED tutorial again.

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So in conclusion, ignore the bad advice on YouTube (there will be members on here that will come back and say that they do it that way with a 3 volt supply and no resistors and everything is fine). I say that they have been lucky. Just because something can be done, does not make it 'best engineering practice'. Always include a resistor with an LED.

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EDIT: Personally I wouldn't use up valuable power of your Elite controller to operate lighting, but if you do, the Aux output of the Elite is 15 volts and not adjustable. The standard resistors included in pre-made layout lighting is 1,000 ohms [1K] (as I discussed in my earlier reply). This value is acceptable for use on a 15 volt supply, but the LEDs will be bright. Personally I would increase the resistor value to something higher. Say 2,000 ohms [2K]. If the sellers 1K resistor is already wired in and covered with heat shrink, then just add an additional 1K resistor in series to obtain 2K.

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Here is a trustworthy YouTube video resource relating to LEDs

[Lb101]

Ps as an amendment to this Ive probably made a glaring mistake, I’m sure peoples will correct me as appropriate!

Chrissaf is spot on as usual. I also work in microelectronics (digital and analogue) and it should be relatively straight forward.

Can I suggest you draw a simple circuit diagram before you start? It’s a straight forward circuit but always worth doing.

Check your power supply as Chissaf says which will help you work out how many led lights you can get. The amp rating is not a limiting factor for burnout, far from it , a higher rating one will give you more oomph for extra lights and other peripherals. It’s the voltage and correct resistor which are important assuming enough current.

Remember LEDs are diodes (clues in the name), so need to be wired the right way around on DC. You can tell by looking at them (look that one up as I’m not great at explaining).

wiring in paralell is the way forward. Like Christmas tree lights nowadays if one goes it wont effect the rest. 

Btw it doesn’t matter which side of the led the resistor sits as long as it’s in the parallel part of the circuit from the bus line (but does have a bearing on making sure you are getting the actual LED the correct way around (in DC). 

it can be difficult to explain with words. I’d lay out a ‘bus’ wire which are the wires from your power supply (extended if necessary with the correct wires) under your layout (these are the two polarities, positive and negative). 

Next run your street lights from one wire to the other across the circuit. Like side roads joint a main roasteach side

Big mistake Ive seen happen, make sure the last light is connected across the two bus wires, effectively acting as the closing connection of the circuit. If you Wire it all up in parallel and you simu connect the ‘ends’ of the bus/power line wires together, then you’ll get a short circuit no matter how many leds are connected (current will always follow this line of shortest load I.e non in this scenario, apart from the small bus wire resistance which won’t make a jot, at best your lights won’t work!). 

Conclusion: Importantly use resistors. its easy to by steet lights with the correct rating online. And easy to look up the colours of said resistors needed for each led as a double check.

[Lb101]

To late to edit. The last light in the ‘circuit‘ needs a resistor for that led too. 

And I emphasise you are using a suitable transformer as explained by Chrissaf. 

[mjb1961]

Hi ,,,I assume the talk about resistors is because the op is going to run 3v LEDs with a 12v power supply,,why doesn't he buy a power supply of the same voltage @ 3v ,or one of those switchable power supplies,3v ,6v 9v 12 v ,I've seen one on eBay for £6.99 ,I'm only suggesting this because I am going to get one to run some 6v led street lights ,from China but are very nice detailed and quite expensive at @5 for £16.99 ,,,so would I be ok with this theory of mine ,or am I destined to blow them ,,,,,mjb

[Chrissaf]

Not wishing to 'repeat myself' for the Nth time, my previous posts described why I am against using 3 volt supplies without additional resistors. It can be done, but it is not best engineering practice.

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Are you really sure the 6 volt ones from China are LEDs. 6 volts is a common voltage used for street lamps using 'Grain of Wheat' incandescent bulbs.

[mjb1961]

Not wishing to 'repeat myself' for the Nth time, my previous posts described why I am against using 3 volt supplies without additional resistors. It can be done, but it is not best engineering practice.

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Are you really sure the 6 volt ones from China are LEDs. 6 volts is a common voltage used for street lamps using 'Grain of Wheat' incandescent bulbs.

Hi Chrissaf,,,yes they are as you say bulbs ,not LEDs ,sorry for stating wrong info in my post ,,,,mjb

[Chrissaf]

In which case....no resistor is required if supplied by 6 volts......or put another way......a resistor is optional if you want to dull the brightness and make the GoW bulbs last longer OR run them on 12 volts. Since GoW bulbs draw significantly more current than an LED......expect them to run too hot to touch, and any resistor will probably need to have a significantly higher wattage. Typically about 1 watt. If you want to run them on a 12 volt supply assume a 56 or 68 Ohm 1W resistor (per street lamp) as a starting point. Note: the 1W resistors will get hot too. If you just want to dull the brightness on a 6 volt power supply, then try resistor values that are about half the values stated above i.e standard values of 27 or 33 Ohms @1W.

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PS - In sizing the 6 volt power supply current capability I would assume 100mA per bulb. In reality, the current drawn per bulb is likely to be in the region of 40mA to 80mA per bulb. But an allowance of 100mA ensures that the supply is not stressed to the extreme. So for 5 x street lamps you should allow a minimum 0.5 Amps at 6 volts for the specification of the supply.

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In my opinion, it is these bulb high currents and run hot bulb temperatures, plus the longevity of LEDs that makes it worthwhile paying the extra for LED based products.