Posted on Aug 7, 2012 11257
A sensitive-gate SCR provides a line-holding current of 20 to 40 mA, depending on loop resistance. It also lights an LED to give the user a positive indication that the telephone line is on hold. The 20 to 40 mA should prove sufficient to hold the majority of lines, but it might require increasing-by decreasing the size of Rl-in individual instances. When any receiver in the same loop is lifted, the low impedance of the offhook telephone set shunts holding current away from the SCR, thereby releasing the line and extinguishing the LED.
Zener diode D2 ensures that the line-holding current drops below the SCR"s minimum conduction current. If the calling party tires of waiting on hold and hangs up, the release of the central-office relays from the calling side also releases the line from the hold mode.