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. |
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Here's a plot of voltage drop against current for a typical diode |
What is a diode? 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. You might
think the voltage changing as the diode warms up is disadvantage, it is
but not a very significant one.) Sometimes diodes are used in conjunction with transistorized controllers circuits. Some common transistorized controllers circuits give quite a large voltage step between the lowest part power contact and full power. This step can be split into a smaller step by inserting a bottom step connected to a power diode (often a Schottky diode)
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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 |
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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 |
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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. |
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