Parallel lighting main.

[stockers]

This years project (amongst others) is to build the main station and village behind it. I am thinking of putting in a main supply using redundant mains wiring cable(I have the old red/black twin & earth available).
I entend to use LEDs for lighting powered from this main in parallel. I will put in 12V or a little more with the capacity of a couple of amps.
Simple question - what do I need in the way of resistors - or better still, can I put 3 or 4 LEDs in series fed from the parallel main and do away with the resistors - advice sought please.

 

[nicebutdim]

I found this very helpful: http://www.hebeiltd.com.cn/?p=zz.led.resistor.calculator
There are lots of sites out there that help with this, you will need to know the current draw of the led's you're intending to use though. Parallel is the easiest to do though.
I also used this one: http://ledcalculator.net/

 

[mike]

i ask the nice man at maplins.. buy the leds with the rersisters, its not to hard to soilder them on, nice evening job..

 

[stockers]

True on the resitor front, but I seem to remember someone on here suggesting that several leds in series dont need resistors.

 

[ntpntpntp]

If you can arrange them in strings of series-wired LED's (still with a resistor in each string if necessary) you'll use less current than if each is wired in parallel and with its own resistor.

As has been said, there are several online LED/resistor calculators around. I quite like this one:

http://www.hobby-hour.com/electronics/ledcalc.php

 

[dutchelm]

An LED is a current operated device and cannot control its own current. To use an LED without its series resistor is installing a problem.

If you have 12 v available why not wire 5 LEDs in series with a small series resistor of about 100ohms.

 

[Gizzy]

Let's do the maths!

V
R = __

I

We know V is 12 volts so you need to know the current draw of each LED and total them as I.

Then you can work out your resistor value R.

As a certain Meerkat might say, 'even son of Mongoose do that'....

 

[stockers]

Cheers guys. I like the Hobby Hour calculator the best. Not only does it tell you what to use but gives and explaination as well - helps to understand things. and I sometimes need a bit of help there!

 

[MRail]

My system is by Hozelock. Two sets of 4 lights each run from an indoor transformer.
Originally 12V AC, later replaced by new version at 24V AC.

Station lamps are from a string of "30 Garden Lights" separated and each fitted with a 1K resistor, then planted in the platforms with the wiring liked in the Hoselock "main". Various buildings are also linked into the "main". The whole kaboodle is on a timer switch, morning and evening.

 

[Gizzy]

Station lamps are from a string of "30 Garden Lights" separated and each fitted with a 1K resistor, then planted in the platforms with the wiring liked in the Hoselock "main". Various buildings are also linked into the "main". The whole kaboodle is on a timer switch, morning and evening.

I also have a set of these 'Garden Lights' which look like the old time gas lamps. I brought them from Miamigo. They seem hard to come by these days?

I also have some 'Pole Lights' which burnt out the lamps every month or so. I found that Mondeo side light lamps fitted these and I used to buy spares by the bucket load, until I fitted Mondeo LED lamps as a replacement. More expensive at first at £15, but I haven't had to replace them since!

Mine come on at dusk as they are plugged into a socket with a PIR sensor, something my mate Bill at the MRC gave me....

 

[Madman]

Check out Ebay, and other sources of electrical or electronic supplies.. Look for LED Drivers. They are inexpensive and put out a constant 12 VDC, with and incoming voltage of 120VAC. I'm not sure what voltage you guys use for your house current over there, but these drivers are probably available in a 220VAC to 12VDC configuration.

Now, also on Ebay I have purchased many 12 Volt ready LEDs. They are simply LEDs with a pre-wired resistor and 6" or 8" wire leads. They are very inexpensive. Look up 12 volt 5 MM LED.

 

[stockers]

Right then. I think I am getting my head round this now. Please feel free to correct me - this is a fast learning curve.
1. Putting three or four leds in series needs a smaller resistor than one led on its own - as resistors use current(power) a group in series will use quite a bit less power than the same amount of individual leds with there own resistor.
2. I can run these series groups off of my parallel main feed.
3. Due to the smaller resistor in series groups the power consumption for a group of (say) five leds is the same as for one! Seeing as I intend to use dozens this is important.

 

[dutchelm]

A typical LED drops 2 volts & needs 20mA to light up.

5 LEDs in series drop 2v x 5 = 10 volts. this means that if the supply is 12 v we need to drop 2 volts across our resistor. Resistance is 2v divided by 20mA = 100ohms. This assumes the supply is DC. If it is AC we must allow for the voltage drop across the rectifier.

The easiest way is to put an ammeter in series with the circuit & change resistors if the current isn't correct (or the LEDs are too bright or too dim).

If you have groups of LEDs in parallel the voltage drop should be calculated using the total current or use a separate resistor for each group.

 

[don9GLC]

Remember the old saying about the cost of a Rolls Royce 'If you have to ask, then you can't afford it'!

Its similar with leds and series resistors. If you have to ask if you need them, then you do!

Its quite a complicated mathematical model but an led does not have a 'resistance'. A 2V 20mA led does not have a resistance of 100 Ohms. It varies with the Voltage and current. In certain conditions this can lead to the current increasing causing burnout of the led. The usual way of avoiding this is a series resistor. This reduces the Voltage across the led when the current increases and prevents 'thermal runaway'. The usual rule of thumb I have encountered is that the resistor should have a Voltage at the design conditions of at least 50% of the supply Voltage.

It is possible with certain leds in certain conditions with the appropriate power supply that you may be able to operate with smaller resistors or parallel arrangements, but there is some risk in that. Hence the Rolls Royce comment.

Even with 100 leds you are not talking about a lot of power; however if its a concern why not use a 6V supply?

 

[Neil Robinson]

Right then. I think I am getting my head round this now. Please feel free to correct me - this is a fast learning curve.
1. Putting three or four leds in series needs a smaller resistor than one led on its own - as resistors use current(power) a group in series will use quite a bit less power than the same amount of individual leds with there own resistor.
2. I can run these series groups off of my parallel main feed.
3. Due to the smaller resistor in series groups the power consumption for a group of (say) five leds is the same as for one! Seeing as I intend to use dozens this is important.

Basically yes.

In all cases the power taken by each similar led is the same. When you only have the one led you'll need a bigger resistor which will 'waste' a fair bit of power relatively speaking. With more leds in a chain then less power is 'wasted' in the smaller resistor and more, in total, is used by the leds.

 

[Dtsteam]

I drive my incandescent lights from the DCC feed. What would I need to do to make it suitable to drive some LEDs ?

Thanks to Nick for the Hobby hour site - its the best LED calculator I've seen.

 

[stockers]

Well - with my new found knowledge - whats the voltage of your DCC system

 

[Madman]

Something else I failed to mention about LEDs. I light my passenger cars with 5 mm LEDs powered by two AA 1.5 volt batteries. I use NO resistors as the 3 volts produced by the two batteries is within the safe limit if operation. Before I converted my cars to LED lighting, I experimented with the circuit. I powered a half dozen LEDs with two AA cells for over a week. They were still going strong when I decided to stop the experiment and start converting cars over to LEDs.

 

[MRail]

I drive my incandescent lights from the DCC feed. What would I need to do to make it suitable to drive some LEDs ?

I believe if you connect a suitable resistor to your LED, it can then be susbtituted for the incandescent bulb.
If using a DC supply, the LED will need to be polarised correctly.

 

[Dtsteam]

I drive my incandescent lights from the DCC feed. What would I need to do to make it suitable to drive some LEDs ?

I believe if you connect a suitable resistor to your LED, it can then be susbtituted for the incandescent bulb.
If using a DC supply, the LED will need to be polarised correctly.

Thanks Rob & Stockers (its 18v, and yes, there will be an exam )

My question is more around the DCC signal, which I understand is an AC signal of roughly 8Khz. Can I just connect an led and resistor or do I need something else ?