Diode Controllers
Diode controllers (sometimes called passive
electronic controllers) are an alternative to resistance and
transistorized designs. One diode controller will be suitable for
a much wider range of motors than a resistance controller.
However the adjustability is quite limited compared with a transistorized controller.
Diode controllers do reduce motor power by turning some of the
available power into heat. In fact they produce exactly the same
amount of heat as a resistance controller, they will warm up
particularly with high current motors and with inexperienced drivers
who don't uses full power enough..
Here's a plot of voltage drop against current for a typical diode
What is a diode?
Basically it's a simple electronic device that passes electrical
current one way but not the other. When diodes pass current, they
don't do so perfectly, some of the power is wasted as heat. It is
this characteristic that is used for controllers. A typical
ordinary diode (for the technically minded silicon rectifier diode)
will drop about 0.6 volts at low current and about 1.1 volts at its
full rated current. The specs for diodes are not
identical, if you want to know exactly what a particular diode will do
you should consult the manufacturer's specification or measure it
yourself.
There is another type of diode (a Schottky diode) with less voltage drop, these could be used for finer adjustment of a controller. I would be interested to hear from anybody who has built one.
For controllers, one unattractive feature of
diodes when compared with transistors is that diode's voltage drop for
a given current is fairly constant, where as a transistor's voltage
drop can be adjusted. (For those into precision
measurement, diode voltage drops do change with temperature, but that's
not useful to adjust a controller)
One attractive feature of diodes when compared with transistors is that
rectifier diodes are usually design to take a huge overload current for
a short period, where as transistors are unlikely to survive even
brief bursts of current much beyond their standard rating. So
there is less risk of diodes failing.
How are diodes connected in a controller?
Here's a diagram showing correct wiring for a track wired to BSCRA rules with the control on the positive side and the negative side common. Some manufactures refer to this as positive connection. Note that only a positive wired diode controller will work with the track wired this way
Here's a diagram showing how a diode is connected for negative wiring track wiring ( with the control on the negative side and the positive side common.) Some home sets are wired this way. Note that only a negative wired diode controller will work with the track wired this way
If you want a diode controller to work on both positive and negative wired tracks, this can be done with a pair of diodes connected between each contact, as shown below.
