> Far be it for me to criticise a ciruit that has
> apparently been around and in use since
> the 1950s,
Why not you? <g> Nothing's perfect, criticise away. But it's
almost certainly older than that, the date I gave was the first
time I saw it. In those days the capacitors were massive, but
you could apparently buy them in your local ironmongers!
> The other coil will never have the capacitor discharge
> through it - it can only be used to charge the cap.
> Therefore the only initial "oomph" it will get is from
> what the power supply can give it, rather than the
That's true, and why I didn't describe it as a "CDU", although
I see someone else tried to. This design needs an adequate
power supply. It doesn't make up for a feeble one.
In fact a power supply which is only barely adequate without
this circuit probably won't work with it -- you need a decent
sized transformer. I covered this point in the discussion on the
7mm Yahoo group (the reason I originally prepared the diagram).
The main advantages of this circuit are:
a) you can use any simple 2-way switch: toggle, slide, rotary,
home-made, whatever -- it doesn't require a momentary contact.
b) that means that you can also use it with relays and logic
to control the points.
c) most such switches usefully indicate the points direction
on the panel diagram. Especially if you use a rotary switch
with a pointer knob (but make sure it is a break-before-make
d) it is impossible to burn out the point motor coils. (Unless the
capacitor fails -- make sure the capacitor voltage rating is at
least 25 volts for rectified 16vac supply.)
But a CDU it ain't. Or at least, only in one direction. If the points
are stiffer one way than the other, it's worth arranging that to be
the discharge direction.
p.s. The circuit actually described by Dave requires a 4-terminal
point motor (i.e. independent coils). The circuit at:
works the same way, but allows for 3-terminal point motors with
a common coil connection.