[will]

Hello, I was not really sure where to put this topic but I think it will suit here. I have the Hornby Elite which has a 15V DC output, I would like to make use of this byt powering some LEDs from it. The LEDs run at 12V DC and I will have about 10-15 LEDs in each circuit (I am using a standard Hornby controller for one of the circuits). My question is, what resistors would I need to reduce to output to 12V DC or would adding more LEDs to the circuit work? I understand that there is a calculation for resistors but i do not know it. Also, each LED come with its own resistor for 12V DC so would this affect it?

Thanks.

[traincliff]

Will. I am not an expert on leds they are new technology to old techies like me but unlike filament lamps they are current rated and operate best within certain current ranges so resistors are put in series to suit. It depends therefore what leds you have and what their specified current range is. Electronics guys seem to have rules of thumb and no doubt one of them will come on here and advise you and correct me.

My guess would be that if the reistors you have are 1000ohms or more then the increase in current by going to 15V will not matter but if you only have 500ohms or so then need to know the led spec.

Hopefully that will kick opff a debate any way.

Cliff

[Teleman]

LEDs don,t take a lot of power but knowing what spec they are or even the value of the resistor you are using at the moment will help

EXTRA NOTE .......

If you like i have a circiut here on file (yes the paper type) for running LEDs as lights in locos on DC and giving a constant brigtness using a LM334Z to get the voltage stable ...PM me and i will send it to you

[Rog (RJ)]

If they are ordinary LEDs, what colours are they? If they are white, they need different value resistors compared to red or green.

Red and green LEDs usually have a "forward voltage" of 2 to 2.5 Volts, white ones about 3.3 to 4 Volts

The formula for the resistor value is: (Supply voltage minus LED voltage) divided by LED operating current.

LEDs are usually operated at 20 milliamps (20mA or 0.02 Amps). This gives adequate brightness and a long working life but they will give a reasonable light at lower currents and will last even longer.

So for a red LED operating at 20 milli Amps on a 12 Volt supply:
(12v-2.5v) = 9.5v
9.5v/0.020A = 475 ohms
We usually round up the caculated resistor value to the next highest available, which in this case would be 510 ohms, but as the calculated value is so close we could use a 470 ohm resistor. (the next lowest standard value)

If we do the calculations for a 15 Volt supply we get the following results:
(15v-2.5v) = 12.5v
12.5v/0.020A = 625 ohms
Rounded up we would use a 680 ohm resistor but again we are so close that we could use a 620 ohm resistor (the next lowest standard value)

A white LED on 15Volts would be (15-4)/0.02= 550 Ohms rounded up to 560 ohms.
If two or more LEDs are wired in series, add their voltages together and subtract from the supply voltage. For example two white LEDs on a 12 volt supply (12 - (4+4))/0.020 = 200 ohms

[will]

Thanks for all the help. Rog, that is very confusing but I am getting the jist - re-reading, it gets clearer. Sorry, I didn't realise that there were so many things to take into account. They are white LEDs and hopefully I can get about 15 of them on the 15V supply. If I had 15-20 White LEDs on 15V, (15-(4 x 20))/0.02 = -3250, does this mean that I have far too many LEDs on the supply or that i do not need any resistors and they will be dimmer?

[Rog (RJ)]

To get 15 - 20 white LEDs on 15 Volts you have two choices

1 Wire each LED in series with it's own resistor (560ohms) to the 15 volt output.

2 Wire 3 LEDs in series (that's one after the other) with one resistor [ (15volts - (3x4volts)) / 0.02Amps = 150 ohms ] Remember each white LED has a forwards voltage of about 4 volts. 3 x 4 = 12. You can't have 4 or more in series as thats 16 volts or more. You then make up 5 of these sets of 3 LEDS (with resistor) and connect the 5 sets to the 15volt supply

[will]

Ahhh, thanks Rog.

[tradman]

question, are the LED's 12v ie already have built in resistor ?, if so than they cannot be connected in series.

also one other option is available, use a voltage regulator such as 78L12 to get 12 v from the 15v supply . This may well be the cheaper solution.

Mike

[will]

I do not know if the LEDs have a built in resistor but the description says that they come with one for 12V DC. I do know that some of the LEDs that are on a strip come with built in resistor for 12V but I have a supply specially for them. I will have a look at the regulator. Thanks.