Wiring lights and LEDs (for signals)

[Andrew_au]

So I bought some LEDs and small bulbs yesterday to have a play with scratch building some signals for my garden railway. I have some questions on how to wire them up.

For reference, supply is a Roco z21 XL DCC system, putting roughly 20V across the tracks.

Bulbs​

Got a couple of 12V bulbs, 120mA current. ( 12V Lilliput Globes )

• I assume that these can handle the "AC" current from the track and don't need the source current to be rectified.
• How do I calculate the resistor value required to drop the power to something that won't burn out the globe?
  • Is a resistor the right way to go? Or should I run a separate power feed for the 12V lights?

LEDs​

Playing with 2 different products:

• Red/Green LEDs with their own mount ( Chrome Bezel with 5mm Green LED )
• Tri-colour LEDs ( Tricolour Red/Green/Orange 3mm LED 4.5/6mcd Round Diffused )

The most obvious issue is that LEDs require a DC supply, and my power run is "AC". Given that these could be anywhere on the layout, what's the best way to get a clean DC supply to the LED?

I'm using a mix of LGB 55525 (and equivalent) and Zimo MX820V to drive my turnouts. The MX820Y has signal outputs as well as turnout outputs - I could install a few of these and use their outputs to provide DC to the LEDs. I think I need a 1k resistor also (MX820E manual, diagram page 14).

Are there other good alternatives?

Once I have the DC problem sorted, I need to handle current. Is it OK to use a single current limiter (resistor or regulator) on the common anode? Or am I better to put a separate resistor on each LED? For small LEDs, is there any benefit to using a "driver" (as they seem to be called), or is an appropriately sized resistor a better option?

How do I calculate the right resistor? Both LEDs are specified as 2V and up to 30mA. Using LED Calculator with 20V / 30mA / 2V gives a 620 Ohm resistor. Does this sound correct?

Switching​

My initial plan was to simply switch the signals on/off using an LGB supplementary switch attached to the EPL drive. The supplementary switch is a DPDT, so I can use one pole as a SPDT which switches power between two lights (e.g. "red" or "green"). I realise this isn't necessarily prototypical, but it works for a first cut. Are there better ways to do it? At this stage, I'd rather tie the signals directly to the turnouts rather than manage them separately.

I deliberately got a 3 input LED for the red/green version so I could switch between the inputs using a single pole rather than requiring a full DPDT to reverse current for a 2 input LED.

 

[Greg Elmassian]

let's take the 12v bulbs running from 20v square wave AC.

to simplify, let's just pretend it is 20v dc pulsed at 50% duty cycle

you need to drop 8 volts at .12 amps. (Kirchoff's law)

wattage is volts times amps, so that's 0.96 watt... if this was DC, you would double it at least, so a 2 watt resistor. (rule of thumb for sizing actual wattage to resistor wattage)

If the duty cycle is 50%, then a 1 watt should do.

resistor value (Ohm's law) = voltage divided by current = 67 ohms

that is for EACH bulb

(hopefully at this point you are entertaining 12v bulbs, unless you want to go into the heating business)

we can take the LEDs next, and your "need" for "clean" DC....

Greg

 

[dunnyrail]

Sorry but I think you have failed to understand signals and signalling.

Firstly a signal is not predominantly to show that a route is set, yes that can be a secondary purpose but the main purpose is to show that the driver can enter the next section safely other trains not being present. With signal line this is of course more serious. But on double track you still do not want 2 trains in a section with one catching the other when travelling the same way.

So in your scenario the signals linked to a point show that the route from track 1 rather than track 2 is clear however that should not be possible with interlocking till the route has been proven clear by the box following your station. Referred to box in advance with British signalling. Then your signal can go from red to green once the point is set correct and the lever for the signal has been pulled. Of course in our models we rarely have such complex interlocking but it is fun to operate a station with proper use of point and signal with separate lever/switch for each.

Sorry if this is a bit techie but I feel it important to understand the basics of what signals are for.

 

[Andrew_au]

let's take the 12v bulbs running from 20v square wave AC.

to simplify, let's just pretend it is 20v dc pulsed at 50% duty cycle

50% duty cycle would imply a diode also, right? Otherwise I'm at 100% duty cycle with a square wave, just in different directions at different times.

So still 67 Ohms, but 2 watt rated? So Blue-Violet-Black, or similar.

(hopefully at this point you are entertaining 12v bulbs, unless you want to go into the heating business)

Joke is too subtle for me.

 

[Greg Elmassian]

Actually I made a mistake, you have 100% duty cycle with 12v bulb wired to track power. so you need the 2 watt resistor.

The joke is that EACH bulb is generating 1 watt of heat inside your loco... you will indeed be in the "heating business....

rule of thumb with power resistors is to at least double the wattage. A one watt resistor will burn your skin and can melt plastic if run at 1 watt.

Greg

 

[Andrew_au]

Sorry but I think you have failed to understand signals and signalling.

I've read (and mostly understood) the entire NSW and ARTC signalling guidelines. I'm just invoking rule 1.

My first goal is to get some indicators on the network to communicate the information that I care about.

 

[Andrew_au]

The joke is that EACH bulb is generating 1 watt of heat inside your loco... you will indeed be in the "heating business....

Well, the plan is to use them for fixed indicator signals so they will be generating heat trackside instead. I can use 6V / 0.2A bulbs in the same housing, which would imply 70 Ohm resistors dissipating 2.8W. I don't think that's a net win, however?

Does this imply that if I used a shared resistor for more than one 12V (0.12A) globe (globes wired in parallel), it would need to dissipate 1W per globe? So, if I go with light globes, each globe should have its own resistor and I should make sure that the resistors are over-rated for power and somewhere well ventilated?

 

[idlemarvel]

Hi Andrew,
One of the problems I found with outdoor light signals (LED or incandescent) is that you can't see them unless you are close and in line with them (driver's perpective). If you want to use them as indicators then I would recommend semaphore signals.

 

[PhilP]

Andrew,
One of the things Greg is inferring, but is perhaps not clear to you, is that using the 'wrong voltage' and dropper-resistors, for your bulbs is also inefficient..
As an example:
Ten-off, two-aspect signals (twenty bulbs). So 20 x 2W is 40W of power just adding to global warming.
Or to put it another way, 2 Amps of power from your system lost to you.

I think it would be better to run from a regulated supply. - You can derive this from the track if you wish.
A small regulator board, will be much more efficient, and give you the option of adjusting the output.

Bulbs outside, are not particularly visible, you can get high-intensity LEDs (which gives a lot more light, at a lot less current, than even standard LEDs.

PhilP

 

[AlanL]

Got a couple of 12V bulbs, 120mA current

Did you consider 24 volt bulbs? No issues with resistors but still power hungry.
LED's are the best.

Tri-colour LEDs ( Tricolour Red/Green/Orange 3mm LED 4.5/6mcd Round Diffused )

They would be extremely dim, I'm not certain but may not visible outdoors.
I used ultra bright LED's with light output of 1800/2000 mcd. They are visible in direct sunlight.

The most obvious issue is that LEDs require a DC supply, and my power run is "AC". Given that these could be anywhere on the layout, what's the best way to get a clean DC supply to the LED?

Do they require DC? Try them, with a suitable resistor direct to DCC track voltage. They will work fine, all my LED's are using DCC. On AC there would be some flickering but with DCC the frequency is too fast and flickering is not visible at all.

Below is a photo of the route indicator panel for my storage sidings, taken in full sun. Ultrabright LED's running off DCC, switched via LGB suplementary switches.
Alan

 

[Greg Elmassian]

Nothing inferred, I stated in post number TWO: (first response)

(hopefully at this point you are entertaining 12v bulbs, unless you want to go into the heating business)

use 12 volt power for 12 volt bulbs that are drawing a lot of current, they are effectively TEN TIMES the current of LEDs.

LEDs from the "AC" of DCC won't visibly flicker.

Yes, use high power LEDs, but I would run from a special LED power supply made exactly for this.

(google LED power supply, although many are 12v, that's another explanation)

Greg

 

[Andrew_au]

you can get high-intensity LEDs (which gives a lot more light, at a lot less current, than even standard LEDs.

Are these a variety of standard hobbyist LEDs or something quite different?

The hobbyist LEDs seem to run about 2v / 30mA. In contrast, the "LED power supplies" that turn up on a Google search seem to be 12-24V 1-5A (and designed for driving room lighting, etc).

---

Back to hobbyist LEDs:

In terms of resistor, I notice that a lot of LED deployments use some fairly high value resistors (e.g. 600-1000 Ohm). For example, my calculation of 20V -> 2V / 30 mA resulted in power consumption of 518mW for the 620 Ohm resistor. Is heat an issue here?

If I used a voltage regulator to pull the supply down to 2V, should I still add a resistor to manage current?

If I connect a LED directly (including resistor) to a DCC supply and just use half the duty cycle, do I need to add a separate diode for current or can the LED itself do this? Is there a brightness issue given that I'm only powering the LED half the time?

 

[PhilP]

Are these a variety of standard hobbyist LEDs or something quite different?

Google (or EvilBay) is your friend:
High intensity LED
Will give you plenty of hits..

The hobbyist LEDs seem to run about 2v / 30mA. In contrast, the "LED power supplies" that turn up on a Google search seem to be 12-24V 1-5A (and designed for driving room lighting, etc)

A different animal, to what you require.
Home-lighting LEDs, tend to be arranged in '12V groups' (cut-to-length tape, etc.) which makes installation simpler..
You are driving a few, at milliamps each, not a couple of hundred.

A bridge rectifier (if needed) and a small adjustable 'buck' regulator, is all you should need.

Each colour of LED, will have a different forward voltage, and light output. Using individual resistors, will allow you to 'tweak' the brightness a little. - You do not need to run at the full rated current, and as with all things, running under, will lengthen the life of the LEDs.

In terms of resistor,

You do not need to worry about these sort of wattages, especially if you are running outside. - Heat-soak, from the sun might stress things a little.

If you drop the voltage, still go for more than you need. - LED forward voltages vary, around 2V to 3.5V, if you choose 12V then you have some 'wriggle-room' with resistor values, and will also reduce the heat-dissipation in the resistors.

If I connect a LED directly (including resistor) to a DCC supply

If necessary, tweak the resistor, a little..
You won't see any 'flickering'. - You may notice a strobe effect on any video you record? Though either I am noticing this less, or the systems have been altered? - A whole different thread/subject!

PhilP