DCC/Elink Signal Lamps

[240SQD]

I see from the forum that resistors should be placed between the R8247 decoder and the lamp which can be variable to suit the required brightness. 

As my bulbs flicked for a second only on switch on and now are unworkable...Being unaware of this requirement for resistors and no advice about it from Hornby, I am a bit 'put out'.

However, could you advise me on a suitable ohm range and wattage for the resistors please?

I have taken voltage readings for comm1 on the decoder and stll have -13.68v and +13.68 v output, so hopefully no further damage done.

[96RAF]

A 1K ohm resistor in each lamp wire will be suitable but the fact that your lamps flashed once means one of two things.

1. The R8247 is set to pulse not steady.

2. The LEDs have burnt out due to powering them without a resistor.

 

You don’t say which signal lamps you have but the Hornby R406 LED ones have built in resistors.

 

Edit - although the title says eLink I will leave this in the DCC section as it is not directly related to RM, just basic DCC electronics.

[Chrissaf]

Being unaware of this requirement for resistors and no advice about it from Hornby, I am a bit 'put out'.

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Since Hornby only officially support interconnection of their own products. Then it is down to the user to research the technical requirements of connecting unofficially supported products and components.

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I have taken voltage readings for comm1 on the decoder and still have -13.68v and +13.68 v output.

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The R8247 has three terminals the + terminal which is not a positive voltage, the C common terminal which is a positive voltage and the - terminal which is the same voltage as the + terminal. The + & - terminals are 'switched negative' voltages.

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PS - I assume by comm1 you meant "Port 1".

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However, could you advise me on a suitable ohm range and wattage for the resistors please?

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LEDs are current devices not voltage devices. The written spec for an LED will quote a current value (typically about 30mA). This is not the current they require to operate, this is the max current that you should design for in a circuit when calculating a resistor value. This is subjective, but I base my resistor calculation values on a nominal current of 5mA with a max of 10mA. LEDs are voltage polarity sensitive and will only light up when the operating voltage is applied the right way round.

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The function of the resistor is to manage the current passing through the LED so that it is a value within the LED supported range. If you don't include the current limiting function of the resistor, then the LED becomes, in effect, a fuse and will blow.

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The resistor value is calculated using Ohms Law.

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Ohms Law is defined as  Resistance = Voltage (Volts) divided by Current (Amps).

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The Voltage in the formula above is the Voltage of the Supply minus the Forward Voltage of the LED. Different coloured LEDs have different 'Forward Voltages'.

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The four main colours that you will probably need to work with are:

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White LED 'Forward Voltage' typically 3.2 volts.

Red LED 'Forward Voltage' typically 1.7 volts.

Green LED 'Forward Voltage' typically 2.2 volts.

Yellow LED 'Forward Voltage' typically 2 volts.

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So if you round up your 13.68 volts to 14 volts. This 14 volts would be your 'Supply' voltage.

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So using the Red LED as an example. The resistor value to achieve a 5mA current through it would be:

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Resistor value = (14 Volts - 1.7 Volts) divided by 0.005 Amps = 12.3 Volts divided by 0.005 Amps = 2,460 Ohms

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The Wattage value needed for the resistor is calculated using another Ohms Law formula:

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Wattage  = Voltage (across the resistor) times Current.

Wattage = 12.3 Volts times 0.005 Amps = 0.0615 Watts.

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Resistors have values that are in a standard progression range of values. So the nearest preferred standard [E24 series 5% tolerance resistors] value to 2,460 Ohms would be 2,400 Ohms. Typically LED resistors are 0.25 (1/4) Watt.

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If the LED is too dim, then decrease the resistor value in steps till you get the brightness you want, but do not go below 470 Ohms else you risk stressing the LED. Similarly, if the LED is still too bright, then increase the resistor value in approximately 1,000 Ohm steps until you get the brightness you require.

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I can recommend this excellent YouTube tutorial on LEDs and current limiting resistors:

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EDIT: Most commercially made Model Railway related LED products are fitted or supplied with a 1,000 (1K) Ohm resistor. This equates to a typical current of about 10 to 15 mA depending upon the supply voltage applied to the circuit and the colour of the LED.