L.E.D. circuit question.

[Jamiel]

I have 6 L.E.Ds to put into a building and wanted to know if either of the below circuit diagrams would be be OK, or if there is any advantage to either circuit, or if both are wrong and how I should approach this.

12v power will come directly from a transformer from the mains.

In series.

In parallel.

 

Regards.

Jamie

[terrysoham]

Depending on the voltage of each led (most are between two to three volts), it is doubtful that you could power 6 leds in series from a 12v supply. So I'd go with the parallel circuit. However, do find a program that will calculate what value resistor you need.

 

Regards

[Il Grifone]

I would concur with the series/parallel solution. Most LEDs require about 10-30 mA and 1kΩ is suitable for a 12v supply. For 3 in series I would use 330Ω. This could be varied depending on the brightness obtained, though this does not vary a great deal with current.

 

Ideally the LEDs should be fed with DC, so a bridge rectifier should be included in the supply. It may need the addition of a capacitor to smooth the rectifier output to avoid 'flicker'. (A 1000μF/25V (or higher) should do* - This will be an electrolytic so watch the polarity - reverse polarity will cause it to explode.)

 

* It's not critical, a lower value will probably be OK.

[meil]

You say the 12v will come direct from a transformer yet you show 12v DC on your diagram. Which is correct? Also your diagram shows you are putting 24v across the LEDs (+12 to -12 implies 24v).

 

So are you using 12v or 24v?.

[Jamiel]

Thank you all for the replies.

Meil, I was unaware that my labeling was wrong, I am just using a 12volt feed, I should just have put the + and - signs on.

As for the transformer, I thought that a standard 12 volt transformer would chance (rectify?) the output, but I will check as I know how LEDs are affected by reversing the current, a rectifier and capacitor is a very helpful suggestion.

I will try two 3 LED series each with a resistor. I have 1000ohm resistors to add in.

I suspect there will be a little trial and error in the learning of circuits, but I am sure I will get there in the end.

It is funny how model railways draw on so many skills, and the knowledge shared on the forum is so helpful. I haven't touched making a circuit since my teenage years.

Thank you again for the replies.

Jamie

[DCB]

I guess it should be 12v + amd 12v o for 12V DC.  However you do need to know what voltage you are actually getting to work out the resistance, many 12 volt model railway power supplies are  20 volt + off load.  Computer PSUs are usually 12 volt + or - about 1% and also 5 volt, but lack overload cut outs.

Drawing 1 Series will rely on absolutely identical LEDs to get identical light outputs. If they are not identical, as in different colours, it may not work at all or have some very bright and some very dull LEDs.  If drawing 2 has 6 individual resistors it should be fine as you can adjust the resistor values but 80% of the power will be dissipated by the resistors.  I much prefer to start with with a 3 volt supply for yellow/red/green LEDs and 5 volt for clear ones so less heat is created in the resistors and less power consumed..

[Vecchio]

if you use LEDs in parallel you should use 1 resistor for each LED. Otherwise the light intensity may be different between the LEDS.

If you want to use 2 times 3 in series use one resistor per 3 LEDS. Series circuit has the advantage that the current through the LEDs is absolutely the same, so the light intensity is the same.  

Sorry David - I do not agree - series circuit - same current through all LEDS, parallel circuit - current divides between the LEDs depending on their inner resistance. This can be cured by one resistor for each LED as then the resistor will define what the current through the LED is.

There was a German guy called Kirchhoff who looked into that some time ago....

[terrysoham]

Jamie,

As you see several answers to your question! The nice thing about low voltage electronics is that you can try out all the proposed answers at very little cost to yourself - even destroying an LED is not expensive. So my advice is experiment (on your workbench first) before installing them in your buildings - you learn so much that way.

Regards

[Il Grifone]

The LED light intensity will not vary a great deal between LEDs from the same batch and for building illumination does not matter in any case. Of course, different colours should not be mixed in a parallel arrangement as the voltage drop per LED varies with the type (colour) and the type with the highest drop (white and blue are higher than red for instance) will take most of the current. The LED voltage is almost constant, so Kirchoff's law is not 100% applicable in this case.

 

The power lost in the resistors is measured in mW and not really an issue. The poor regulation of cheap transformers is also not really going to make alot of difference as the LED brightness does not vary a lot with current which will be steady anyway, unless individual LEDs are turned on and off which does not seem to be the case here.

 

In Grifone tradition, my LEDs are all salvaged. (Xmas tree lights are a good source or direct from China - some suppliers tend to be expensive*). Warm white is better for representing incandescent lighting. Fluorescent lighting has slightly green tinge but I would ignore this - our eyes do anyway.

 

*Expensive LEDs could well be high specification first grade components, but use of these would really be overkill for model railway illumination.

[Jamiel]

Thank you for all the input.

I am going to use the LEDs as lights in my station building, so if there is a different brightness from some of them it won't matter. To be safe I will not mix different types of LED on the same branch, I have plenty of resistors to put in if needed.

 

They are mounted in way that is easy to replace, apart from one, which I have already blown - my daughter asked me to show her the light again, and I forgot I had removed the resistor so with a 9v battery it instantly blew. That is back now after hacking back some plastic to get the old one out.

The power supply will probably be an old 12v DC power from a computer add on or phone charger, I have a box full of them I have kept as phones and disk drives were replaced.

I am really grateful for the replies, and hope the topic will be of benefit to others looking to add lighting to their models.

Jamie

[Jamiel]

A few photos to show the advice given above put into action.

3 LEDs from a Digitrains pack in series, below and above, powered by a 9 volt battery for the moment.

 

The wiring after other series have been added. I kept each type of LED on a separate series, and those in the porch and staircase building separate. Most of the additional LEDs are from Maplin, but I have found these to be quite dim, all the information the packs give is S067, LED yellow 3mm, N23GL. I have added an additional LED, from a RM Electronics pack to one room as the Maplin ones cast very little light.

 

I know these next two photos really need to be shot on a tripod, but just to give an idea of how the lighting looks when the room is darkened.

 

 

I am sure that as I build further buildings and rolling stock in to which I add lights then I get get a much better idea of how to approach using LEDs, but the advice I have been given here was invaluable and really helped me get underway lighting my station building.

Now back to modelling the building and adding lamp shades made from pill blister packs.

Many thanks.

Jamie