Lighting Leds for models ?

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Lighting Leds for models ?

Postby snoko » 23 Oct 2012, 15:52

I have some leds I picked up ( 8 gas lamps ) but I dont have a power source /railtrack to run it off ( Im just focusing on the diorama aspects for now ) the leds take up to 20mA per led / 3 volts max so I should be ok if im using at least 4 lamps i.e. 12/4 = 3 volts

I read that 300 ohm resistor ( 12/3 ) will give me 0.04 , if thats right then 600 ohm will reduce it to the required 0.02 but Id still have 80 ma needed dealt with i.e.

( 2 amps ) 2000 / 20 = 100 ma

100ma - 20 = 80 ma

According to this calculator 80ma should give me 150 ohm

So am I correct in assuming 4 750 ohm resistors is what I need to wire 4 lamps in parallel ?

thanks

snoko
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Re: Lighting Leds for models ?

Postby Burgerman » 23 Oct 2012, 16:53

What is it for?

What voltage do you start from?

What spec are LEDs?
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Re: Lighting Leds for models ?

Postby snoko » 23 Oct 2012, 19:58

Burgerman wrote:What is it for?

What voltage do you start from?

What spec are LEDs?


It is for model railway scene basically ( and some basic practice at my first wiring attempt ) , the lamps / leds have maximum of 3 volts ( DC only ) / 20ma per led , and theres 8 of them. see here

The voltage Im starting from is 12 dc / 2 amps.

The guy supplied me with 8 620 ohms for 12 volts .

snok
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Re: Lighting Leds for models ?

Postby snoko » 23 Oct 2012, 20:22

Actually 750 would be too low , Im guessing I would need something like 820R at least ?

snok
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Re: Lighting Leds for models ?

Postby LROBBINS » 23 Oct 2012, 21:59

If the LEDs are connected in parallel, you will need a resistor with a voltage drop of 9V at 20 mA. 9/0.02=450 ohms. With the supplied 620 ohm resistors, the current will be limited to 9/620 = 0.0145A = 14.5 mA which is well below the maximum allowed and should give pretty long life for the LEDs. If the LEDs will be located at different places, it would probably be easiest to have one resistor for each LED. In that case, the power dissipation is 0.0145 X 9 = 0.13W so 1/4 W resistors are fine. You could also have multiple LEDs in parallel with one resistor, but you would need a resistor with a higher W rating - for 8 leds, it would be 8 x 0.13 = a bit over 1 W so you would need a larger-than-1W resistor.
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Re: Lighting Leds for models ?

Postby snoko » 24 Oct 2012, 01:12

LROBBINS wrote:If the LEDs are connected in parallel, you will need a resistor with a voltage drop of 9V at 20 mA. 9/0.02=450 ohms. With the supplied 620 ohm resistors, the current will be limited to 9/620 = 0.0145A = 14.5 mA which is well below the maximum allowed and should give pretty long life for the LEDs. If the LEDs will be located at different places, it would probably be easiest to have one resistor for each LED. In that case, the power dissipation is 0.0145 X 9 = 0.13W so 1/4 W resistors are fine. You could also have multiple LEDs in parallel with one resistor, but you would need a resistor with a higher W rating - for 8 leds, it would be 8 x 0.13 = a bit over 1 W so you would need a larger-than-1W resistor.
Ciao,
Lenny


I was hoping to have the lamps far apart ideally in different places , well that was the idea , , but I was hoping there might be a way I could also used some of that wasted power on something else too if i wanted it ie a building .

So just clarifrey then, are your suggesting I wire in parallel then like the following diagram with a resistor being used for each led and having the 12 volts go directly to each led ?

Image

Or do you mean having it in series like below but with a resistor between each led off the split voltage ?

Image

Because the other option you suggested for 8 or more leds sounds to me like you mean this diagram below.

Image

thanks again

snok
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Re: Lighting Leds for models ?

Postby Burgerman » 24 Oct 2012, 01:28

Any of the above, or combinations of the above can be used.

As suits or fits with what you are doing. If in any doubt, just use higher resistance, if too dim, use less. Less power is wasted on series diodes than running each in parallel. Resistor size isnt critical in the slightest. Too bright though and they dont live as long. Use pots if in doubt and adjust to suit. Sticking an electrolytic capacitor across the supply will stop them "strobing" and make them more efficient/longer lasting. Anything with higher voltage than 16 and say 1k m/farad or bigger will work.
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Re: Lighting Leds for models ?

Postby snoko » 24 Oct 2012, 02:32

Burgerman wrote:Any of the above, or combinations of the above can be used.

As suits or fits with what you are doing. If in any doubt, just use higher resistance, if too dim, use less. Less power is wasted on series diodes than running each in parallel. Resistor size isnt critical in the slightest. Too bright though and they dont live as long. Use pots if in doubt and adjust to suit. Sticking an electrolytic capacitor across the supply will stop them "strobing" and make them more efficient/longer lasting. Anything with higher voltage than 16 and say 1k m/farad or bigger will work.


Using a pot is an excellent idea , so something like a 20k ohm rotary or slider should be ok ( my memory on pots has gone bit foggy but if 6v = 10k Im assuming 20 is fine ) ?

snok
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Re: Lighting Leds for models ?

Postby Burgerman » 24 Oct 2012, 09:34

Remember to add up the load and use one with excess watts just in case.

20k sounds a bit high. 1k or less sounds closer depending on how many leds and what configuration.
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Re: Lighting Leds for models ?

Postby snoko » 26 Oct 2012, 01:45

Burgerman wrote:Remember to add up the load and use one with excess watts just in case.

20k sounds a bit high. 1k or less sounds closer depending on how many leds and what configuration.


I.8K OR 2.2K Is what their saying on railway forums ,so I have few to try anyway.

Not sure about the wire, the one on there looks 28 or 30 AWG .

what do you think ?

snok
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Re: Lighting Leds for models ?

Postby snoko » 26 Oct 2012, 02:08

snoko wrote:
Burgerman wrote:Remember to add up the load and use one with excess watts just in case.

20k sounds a bit high. 1k or less sounds closer depending on how many leds and what configuration.


I.8K OR 2.2K Is what their saying on railway forums ,so I have few to try anyway.

Also what about resistor material itself , can I assume metal film resistors are better choice than carbon ?

from what Ive read they tolerate heat better than carbon while carbon are more noiser , and what sort of wire would you use? ( the one on those lamps look about 30 AWG )

thanks

snok
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Re: Lighting Leds for models ?

Postby ex-Gooserider » 26 Oct 2012, 05:54

These days any modern electronics should use metal film resistors for under 1 watt applications - carbon resistors are kind of stone-age technology with no advantages... Noise isn't an issue in this application, but no reason to ever use a carbon resistor that I can think of today...

Wire gage choice is basically a mix of two factors - how much load (current) do you need to carry, and how much mechanical strength do you need...

Single LED's don't take much current, so you can probably run them off AWG 30 wire with no problem, but if you are putting more than one or two on a circuit keep an eye on the power consumption to make sure you don't overload it.

However AWG 30 is pretty fragile stuff (especially the Kynar insulated variety), so you may want to think about how much abuse it will take without snapping the wire or damaging the insulation. Going to a larger size generally gets you heavier insulation and more strength, and this can be an issue... Of course if you are doing a model scenario, the heavier wire is harder to hide, so that might be a consideration as well...

You can also do a combination - run a heavy gage "bus" wire for the main current carrying part, and then tap off of it with small wires that go to individual lights along the way...

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Re: Lighting Leds for models ?

Postby snoko » 26 Oct 2012, 14:52

Well i just picked up this 2 amp / 0.2mm wire, so we'll see how it goes .

m
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Re: Lighting Leds for models ?

Postby FredMan » 28 Oct 2012, 15:25

Have you already got them. The notes say that suitable resistors for 12v will be supplied with them.

If not, for 12v use most LEDs are OK with 1K (1000 ohms). Dont worry about the current rating of the wire, as long as you dont short them out, absolutely anything will handle 20mA, even 40swg / awg if you can work with it. 8 of these is parallel with only use 180mA. If you're using the 12v to power the trains as well then you've still got 1.8A for them.

I'd be very carefull about carbon pots. They are only rated at about 50mW, at full resistance a 1k pot dropping 9v will be carrying 9mA, thats 81mW, crank it round to 1/20th and thats 50ohms, which will be carrying 180mA which is about 1.6W. Any further and the carbon track inside will start to glow, followed by a nice acrid burning smell.

If you need to vary the light level, then connect a 1K pot in series with say a 470ohm fixed resistor. That way the resistance with vary from 470ohm to about 1500.
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Re: Lighting Leds for models ?

Postby FredMan » 29 Oct 2012, 00:18

Correction - 8 LED/resistor sets in parallel would only use 160mA.

I checked the maths on the pot wattage but didn't bother to check 8 * 20 !!

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Re: Lighting Leds for models ?

Postby snoko » 30 Oct 2012, 01:39

FredMan wrote:Correction - 8 LED/resistor sets in parallel would only use 160mA.

I checked the maths on the pot wattage but didn't bother to check 8 * 20 !!

Doh


it gets bit confusing , i have 0 ability to remember most calculations myself due to suffering dyscalculia , so all this is big challenge for me to do .. and bit annoying

still waiting on wires .

snok
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Re: Lighting Leds for models ?

Postby Burgerman » 30 Oct 2012, 02:59

Just remember this. Too much Ohms and they will be very dim. Not enough ohms and they will be very bright, but not last as long.

Experiment to see what looks best, and just go with that. Its really non critical.
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Re: Lighting Leds for models ?

Postby snoko » 30 Oct 2012, 03:36

Burgerman wrote:Just remember this. Too much Ohms and they will be very dim. Not enough ohms and they will be very bright, but not last as long.

Experiment to see what looks best, and just go with that. Its really non critical.


I can remember it vaguely , but its very short term ( hard 2 describe ) its like I cant process number related things in a way that makes sense .

I have to almost make up a visual story or fairytale so it makes sense otherwise i cant separate things , its like looking at models of the same brand that all look the same but with maybe different combinations of keys being the only difference.

hopefully i will get it right , im trying to use it for this diorama ( I used to have background in art ) before things awol for me.

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