DCC and Magic Train

[peterdk]

Hi all ! 

 

I am about to begin converting a Oe Fleishmann Magic Train diesel locomotive, to something that looks americanish. The rebuild will include a new paintjob, adding exhaustpipe, bell and horn, I will also be installing rear lights and changing the front light to LED. The last job will be fitting a DCC decoder.

 

I should like to know if anyone knows the output voltage, on a DCC chip, for the lights. Any help will be greatly appreciated.

 

Happy moddelling - Peter

[Gordon H]

Hi all ! 

 

I am about to begin converting a Oe Fleishmann Magic Train diesel locomotive, to something that looks americanish. The rebuild will include a new paintjob, adding exhaustpipe, bell and horn, I will also be installing rear lights and changing the front light to LED. The last job will be fitting a DCC decoder.

 

I should like to know if anyone knows the output voltage, on a DCC chip, for the lights. Any help will be greatly appreciated.

 

Happy moddelling - Peter

 

It is likely to be approximately one volt lower than the track supply voltage (whatever that might be), possibly slightly less.

DCC decoders simply rectify the track waveform and use the output for the local positive and negative supplies. Function outputs switch the negative terminal of their loads, so each load to be switched has its positive terminal connected to the positive supply.

[peterdk]

It is likely to be approximately one volt lower than the track supply voltage (whatever that might be), possibly slightly less.

DCC decoders simply rectify the track waveform and use the output for the local positive and negative supplies. Function outputs switch the negative terminal of their loads, so each load to be switched has its positive terminal connected to the positive supply.

 

Thanks Gorden 

 

As I remember, the track voltage is about 14 V (I'm using the Lenz system - so I guess I'll be needing resistors that can reduce the voltage from 13 to 1.5 V. (de voltage required for LEDs) Is this correctly asumed ?

 

Best regards

 

Peter 

[Gordon H]

As I remember, the track voltage is about 14 V (I'm using the Lenz system - so I guess I'll be needing resistors that can reduce the voltage from 13 to 1.5 V. (de voltage required for LEDs) Is this correctly asumed ?

 

The answer is yes to resistors, but not for the reason you suggest.

 

The resistors are there to set and limit the current the LEDs pass, not to reduce voltage as such, though that is another consequence of their action.

 

Elsewhere, several contributors to this forum have been trying to get the message across to other readers that the first consideration when dealing with standard LEDs should not be their often quoted 'voltage rating', but the current that they need to pass for normal operation. The 'forward voltage rating' (or 'Vf') is only ever used as part of the resistor value calculation to account for the fact that being a semiconductor diode as well as a lamp, the LED happens to drop a certain voltage when operating.

The 'Vf' value varies between LED types and colours, but only minimally with the current you pass through them.

 

For this reason, direct drive of standard LEDs from an unlimited supply is not a good method to use at all, as the current can vary dramatically from LED to LED, and with only small variations in the supply voltage.

 

Note however, that this does not apply to LEDs with built-in resistors - which is, of course, why they are built-in in the first place. All you have to do with those is run them from the specified supply voltage (usually 5V or 12V). Even with these you have the option to add an extra resistor to reduce the light output by further reducing the current if deemed necessary.

[AMJ]

Most wiring for LED's off the shelf for DC and DCC use I usually use a standard 1K Ohm resistor, you can check this using Ohms law

[peterdk]

Thanks for your help guys ... 

[Phil S]

if you don't already have one, I recommend a Kemo led tester: #m087 available from many places like maplins and under a tenner probably (I bought mine many years ago)

this offers a range of outputs  from 1mA, through 2.5mA, 5mA, 10mA, 10mA,  20mA to 50mA, and runs from a 9v battery inside it - which lasts for years.

 

I use it to decide on the led current I want, as this varies a lit between leds - modern types are much more efficient than earlier leds, and 5mA may seem very bright!

then you can choose an appropriate resistor value - i use from 1k (occasionally) to 4k+ .... and might still get the comment that the lighting is bright ... this also depends on the ambient lighting you will be operating in!   ('too much' internal light shows through the white plastic of Bachmann dmus vety easily - requiring them to be lined internally with foil!

 

the Kemo battery tester seems to be reliable* across many led types (Vf) - whilst another make, from rapid, behaved more as if it was a simple resistor in series, and gave different current with different colour leds   \  *a proper current source internally ??

 

Because I work in both 00/H0/HOm/H0e and G scale, I tend to ensure that they will survive the full 24v possible in dcc (or an H&M '12V' output!!).... aiming for 5mA usually ensures this.

a reminder, if using pre-made Led lighting strips in coaches - they are usually for 12Vdc ie 12.00Vdc, at 20mA/50mm length ... increasing their supply to `14Vdc doubles the current and the heat outout ! -- I add either another series resistor or some diode drops to ensure no problem, and reduce the default brightness  (I use 16Vdcc measured by rrampmeter, indoors, and 22V for outdoor g scale). If half-wave rectification is used, only partial dc compatibility is maintained, but power consumption is halved.