Lima Class 37 Police

[Ard Lochan]

I have a Lima class 37 Police loco that i wish to equipe with flashing blue 3mm led`s.

I have some led`s, On the packet it states 12v dc. Is there a suitable place on a 8 wire decoder that will output 12v DC? Regards, Ard

[Howbi]

If you have an 8 pin decoder such as the Hornby R8249 it has a Green wire known as function 3 and a purple flying lead known as function 4. The blue wire is the positive feed to the LED so you can use the purple lead as negative. This is then switched on your controller via F2. If you prefer to use the green wire, this is switched via F1.......Now the voltage output may well be as much as 15 volts DC so as a precaution a small series resistor may be required to protect the LED........more expert advice will be forthcoming shortly..........HB

[96RAF]

I have 'blues and twos' on my TTS Class 37 RAF Police tractor. There is a silly u-tube video of it knocking around somewhere.

 

The pair of blues on top of the cabs are simple 2mm flashing leds with a 1K resistor in series operated by the green wire (normally F1 on a 3/4 function decoder), but in my case operated by F25 on TTS) and the horns are operated by TTS F17 on a special one-off Police car version.

 

Note that the decoder socket blue wire is positive and the green/purple wires are switched negatives so arrange your led anode/cathodes to suit.

[Chrissaf]

The blue positive function wire on Hornby DCC decoders is derived directly from the output of the on-board bridge rectifier and will be in the region of 14.5 volts +/- 0.5 volts DC. There is no on-board 12 volt function output capability on the basic Hornby decoders.

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LED's that are marketed as 12 volts will normally be safe to use on voltages up to 15 volts, but with increased brightness. If they are too bright then their operating life may be reduced. LEDs are current based semi-conductor devices not voltage based ones. Thus it is the current and not the voltage that is the critical design characteristic.

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If you measure the current (with a mA multimeter) that the LED draws on the function wire. Then it will probably be OK if the current measured is less than 30mA, but it would be even better for longer term reliability if the current was between 10 and 15mA or lower....see next para. Not only that, but by reducing the current drawn by the Function outputs to the bone, then that will put less stress on the power handling of the decoder and make more current potentially available for the motor.

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So measure the current and then add a supplementary series resistor to bring the current down to less than 15mA. This will increase the life span of the LED significantly. Personally, I would go significantly further and reduce the current to less than 10mA.

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Personally I really cannot see the point of paying extra for 12 volt LEDs. They are basically standard low voltage LEDs in the 1 to 3 volt range with a tiny resistor built into the LED package. Standard low voltage LEDs are typically cheaper, more readily available, and more flexible in terms of where they can be used. Sorting out a suitable resistor to use is not that onerous, it is a simple mathematical formula:

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Resistor value = [supply voltage minus LED Voltage] divided by Current.

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Example for a 3 volt Blue LED:

Resistor = [15 volts - 3 volts] divided by 0.008 Amps (8mA).

Resistor = 12 volts divided by 0.008 = 1,500 Ohms.

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Example for a 12 volt Blue LED:

Resistor = [15 volts - 12 volts] divided by 0.008 Amps (8mA).

Resistor = 3 volts divided by 0.008 = 375 Ohms (nearest standard value = 370 Ohms).

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[Ard Lochan]

Thank you all for your input and help. After thirty radiotherapy sessions and half a dozen chemo I am unable to hold the soldering iron steady so I am going to try to teach the Mrs to solder, I will let you know how I get on, Regards, Ard.