Elenco Upgrade Kit SC500 to SC750 - Manual (Page 15)

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Set the meter (M2) to the LOW (or 10mA) scale. Place the fan on the
motor (M1) and turn on the slide switch (S1), the meter measures the
current on the other side of the transformer (T1).
As the DC voltage from the battery (B1) spins the motor, the motor
creates an AC ripple in the voltage. This ripple passes through the
transformer using magnetism. The diode and 0.1
μ F capacitor (C2)
“rectify” the AC ripple into the DC current that the meter measures.
Holding down the press switch (S2) connects the 470 μ F capacitor
(C5) across the motor. This filters out the AC ripple, so the current
through the meter is greatly reduced but the motor speed is not
affected.
Project #523
OBJECTIVE: To show how what a rectifier does.
Motor Rectifier
Project #524
OBJECTIVE: To show how an SCR works.
SCR Shutdown
In this circuit the press switch (S2) controls an SCR (Q3), which
controls a transistor (Q2), which controls an LED (D1). Set the
adjustable resistor (RV) control lever to the top (toward the press
switch).
Turn on the slide switch (S1); nothing happens. Press and release the
press switch; the SCR, transistor, and LED turn on and stay on. Now
move the adjustable resistor control down until the LED turns off.
Press and release the press switch again; this time the LED comes on
but goes off after you release the press switch.
If the current through an SCR (anode-to-cathode) is above a threshold
level, then the SCR stays on. In this circuit you can set the adjustable
resistor so that the SCR (and the LED it controls) just barely stays on
or shuts off.
!
WARNING: Moving parts. Do not touch the fan or
motor during operation. Do not lean over the motor.