Hi Eric (and MrRB!)
Appologies for not writing earlier, have been playing on big steam trains with sons and grandchildren!
' Very clever! Thank you I have never seen that done before!
And me neither Mr RB! There are many circuits designed for Constant Lighting on DC Train Layouts. I prefer mine (above), simples. I use constant current devices to source leds in engine and trains. These being connected to the track prior to the dropping diodes to the motor.Of course in carriages, some form of circuit for changed polarity has to be implemented. Again I cheat!
As the LZ334 (low current device) has a reverse breakdown voltage rating somewhat higher than the max applied dc motor V, these are connected in a sort of inverse parallel again with no bridge but 2 diodes. This giving a Vdrop of only approx .7v whatever polarity is applied.
'As an alternative you could use a low dropout 3.3v regulator, which will give 3.3v out even from as little as 3.5v in. Then you probably need to add a zener on the regulator input as most of these regulators are limited to a Vin of 9v or 16v etc.'
Interesting MrRB. I must confess I have never used a Zener in anything, although it was part of a Btec unit I did many moons ago. But, if I recall, arn't Zeners used in fixed V circuits i.e. when the input and output voltages are knowns and not variable? The use of a 3.5v fixed reg is an option to consider thou.
Eric.
Many thanks for your time and effort.
'If your circuit uses the back EMF from the motor, the LM317 smoothing cap could load the back EMF, its a point to watch out for.'
Back emf is normal in closed loop PWM controllers. As we are using pulses that rise and fall approx 120 hz there is no where to sample and hold (nominally using a cap) the back emf. Normally this is monitored in PWM controllers when the pulse is off. Ie , if no pulse voltage is present , then any voltage detected between pulses is back emf. So no back emf.
You cannot connect the LM317 0V, to the Motor Controller 0V
Can you explain that to me please? The bridge is the only means of isolating the LM317 from the Motor Controller 0v( sort of). Is this not exceptable? I would also be very gratefull if you could explain reasons for R1 (10meg) and R 2 (10k). Essentially I believe I am looking at the circuit from the bridge D5 -D8 onwards.
Again thanks and regards,
Bill