OT - need advice from electronic guru

Hi guys and gals,

Sorry for posting off topic, but I'm still waiting for response on the MERG forum, and I know you guys would chip in quick.

I've breadborded a liniar pulese controller for my sons model railway outlined in the following MERG technical builletin http://www.micro-heli.co.uk/T27_1_1.pdf

However it doesn't actually produce a zero output because the injection of the 50hz pulses is still enough to cause some lococ to run at the minimum setting. Can anyone suggest a modification to the schematic on order to give a better control. I did think about connecting the 270K resitor to the wiper of the pot rather than after the 100K resistor, but I'm not sure if this will have the desired effect as it could be taking the half wave rectified pulse to ground via the 270K resistor when the wiper reaches the end ?

Hi, Malc

I'd suggest you to scope the 100 Hz 22v PP ... and see if it really reaches < 1.2 v level ...

In the "possibility" NOT, I'd suggest you to place a stupid 24 v 5 w bulb ( truck parts ...) just past the rectifier bridge.

You certainly noted there were no capacitors except the smoothing 22 µF ... there's an excellent reason to that !!! ... Hope you've respected it .

Ah ... last thing ... I did not see any pic in this design ...

Forum error ??? LOL

Alain

Thanks for the reply,

I've found that by increasing the value of the resistor from 270K to 660K slows the Dapol loco, so I guess that if I use something in order of 1M it will do the trick.

Uhm.. maybe using a PIC to generate the 50 hz pulse would also work.. and then this post would be in the right forum :)