Bootstrapping 2 decoders together

[96RAF]

I have installed an Express Models lighting kit to several of my diesels, but where they use 3 leds to the front and 2 leds to the rear lights this total load of 5 x 20mA is too much for an R8249  or TTS decoder directional lighting function, which is limited to 100mA per function and 500mA total decoder normal output including motor. As a result the decoder function outputs are blowing, although obviously the rest of the outputs are OK such as motor control and sound.

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Has anyone tried strapping 2 decoders together to share function overloads i.e. 2 decoder sockets each with their own decoder, but with both sockets in the same loco sharing pickup wires and function output wires. I don't need twin motor outputs on the models I am having problems with.

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Both decoders would have the same address just as if they were in motor and dummy cars.

Rob

 

[Chrissaf]

Surely, the simpler option Rob is to add an extra series resistor in line with the white and yellow decoder function wires to reduce the current down below 100mA. Leave any existing lighting kit resistors in-situ, just add an additional one on each function wire.

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LEDs will light up more than adequately on currents of less than 5mA each. I've run LEDs on as low as 1mA to 3mA without any noticeable visual loss of brightness.

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Why after market sellers insist on running LEDs at 20mA is a mystery to me. The 20mA and/or 30mA values stated in the LED datasheets are the values that the LEDs will happily run at all day long at maximum brightness. After market LED product sellers seem to assume that these datasheet documented currents are the current values that they must design to. This is not so. I design all my LED circuits to run between 5mA and 10mA max per LED. My eyes can't tell the difference in brightness between an LED running at 20mA and the same one running at 5mA.

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Apart from which, the lower you can reduce the current draw per loco, the more current will be available on the layout to run other things as well as improving the overall long term reliability of the decoder itself (less stressed).

[96RAF]

Thanks Chris

I'll try splicing in a 1K resistor into each 3 led end. (1Ks are all I have in my box).

Rob

[96RAF]

@Chris

I have spliced a 1K resistor into each of my Express Models lighting kits and lo and behold I have my TTS lighting back, so the decoder hadn't blown its function outputs, it was merely self protecting even though the outputs have no self protection according to the blurb, only on the motor circuit.

 

The lights are also much less glaring than before.

 

So I would strongly advise anyone using Express Models lighting kits with either R8249 or TTS decoders to insert a 1K resistor into the blue wire on their circuit board for added protection.