How Does Rectifier Disk Work?

[gottcent]

I recently replaced the whistle control rectifier disk in an LW transformer, a simple job which I've done several times before. But I have no idea of the physics involved in this device. How does this stationary disk turn AC into the DC bursts that operate the whistle solenoids? Can anyone explain this in non-technical English?

John Gottcent

[servoguy]

http://en.wikipedia.org/wiki/Metal_rectifier

[lionelsoni]

It's been too long since my physical electronics course in college for an explanation of the physics; but I'll try to explain the electronics for you, if that will help.

The copper-oxide rectifier diode in the transformer, like any rectifier diode, is a sort of electronic check valve.  It lets current flow in one direction but not the other.  This is hinted at in the schematic symbol used, which is a blunt arrowhead touching a perpendicular line:

The arrow points in the direction of current flow.  The left end of the diode is the anode.  The right end is the cathode, and is usually marked by a ring painted around the diode package at that end.  The name diode comes from the tradition of naming electron tubes according to how many elements they contained--pentode 5, tetrode 4, triode 3, diode 2.  The electron tube diode was used for the same purpose as copper-oxide, silicon, and other kinds of rectifiers; so the name was propageted to them.

When you operate the whistle control, it places an extra 5-volt winding in series with the regular adjustable transformer winding, boosting the voltage by that much.  It also places the rectifier diode in series.  When the transformer voltage is positive (relative to the common terminal) the diode conducts, almost like a wire, except for a small voltage drop across the diode.  When the transformer voltage is negative, 1/120 second later, the diode does not conduct, in effect disconnecting the transformer from the output terminal.  The result is that the track sees every other half-cycle of the full 60-hertz alternating sinusoidal waveform.  A mathematical analysis of this half-wave voltage shows that it has a non-zero average value.  This counts as a DC component.  It also has a diminished 60-hertz component and numerous higher-frequency components at multiples of the 60 hertz frequency that we started with.

When you move the control all the way, it connects a resistor in parallel with the diode.  This has no effect during the positive half-cycles when the diode is conducting; but it allows some current to flow during the negative half-cycles, around the non-conducting diode.  This reduces the average voltage, that is, the DC component, and restores some of the original 60-hertz AC component.  The assumption is that, after the whistle relay has operated, it doesn't need as much DC to keep it operated.

Lionel schematics always showed the diode connected backwards.  This may have been someone's rebellion against the convention of measuring current as flowing in the direction opposite to the flow of electrons in the wire.  Benjamin Franklin started that convention, before anyone had a clue as to just what was flowing in an electrical current.  We do electrical engineering just fine that way simply by defining the electron to have a negative charge.  But the US Navy unfortunately trained many military technicians the other way, causing confusion to this day.

[gottcent]

Thanks, guys. I appreciate the web link and the info.

John