[bill7437]

I have recently fitted reversing white / red lights to a Hornby/Jouef SNCF TGV made in China with a their copy of the old Lima circular motor. As an experienced modeller I have done this many times using 1.8mm LED's, 1N 4001 diodes and 2.2Kohm resistors and not had any problems. With this model there are no problems with the dummy power car, the lights reversing with the direction of the train but with the power car when in the reverse direction when only the red diodes
should show as the train moves there is also flickering intermittent lighting of the white diodes. I have checked wiring, replaced 1N4001's, removed chokes and suppressor fitted across the motor terminals but the problem still exists. Can anyone tell me what I can do to eliminate this flickering in the reverse direction so that only the red diodes show.

 

[zmil]

Hi Bill

I had the same problem with some reverse headlights (running on DC) flicking on occasionally . Caused by back voltage from the motor like BEMF , cured by putting a diode across the LED but in reverse

Regards

Zmil

 

[bill7437]

Thanks for your quick response. I understand the principle of back voltage from the motor but wonder where the diode should go - across the white leds or across the red leds ?? What value diode , would a 1N4001 do and how should it be wired????

 

[Brian Considine]

Just to be safe I would fit diodes across both LED's - not too sure of the type though.

 

[zmil]

Hi Bill

wired in parallel to the LED but reverse polarity, the same type you have used (1N4001) would be OK

Regards

Zmil

 

[bill7437]

Thanks for your reply Znil. The experts tell me that this problem is probably caused by poor current collection and dirty track and is common to the Lima pancake motor and its Hornby Chinese copies. Having inspected the model which is new I discovered the front bogie which forms part of the pickup did not rest properly on the track as it was held in place a la Lima by a brass clip and a very strong coil spring. I have dismantled the bogie and fitted a slightly weaker
coil spring allowing the front bogie to maintain better contact with the track and so prevent the generation of reverse
polarity flickering the white leds. This is a substantial improvement without resorting to further wiring. One would have thought that the Chinese makers for Hornby would have realised that the fitting of this strong spring would not give proper track adhesion.??

 

[Richard Johnson]

***Bill, The "experts" aren't so expert electronically as they are right and also very wrong - right in that the problems of the loco make it worse, but wrong in that you cannot actually fix it without putting a diode in place - this is standard practice in any LED circuit that also contains any potential for reverse voltage.

The Back EMF condition exists on every loco, as it happens every time a brush passes a commutator slot... thats 3 to 5 times each motor revolution.

Even if you can no longer see the phenomenon, the back EMF voltage will still be there - its just "faster" and eventually, it will kill the LEDs as they have almost no tolerance for reverse voltage.

Richard