| 摘要 |
A telephone subset circuit arrangement which includes a capacitor (C1) that provides a current source for the circuits' dialler chip (1). Initially, with the capacitor discharged, upon connexion of the line terminals (L1 and L2) to an exchange line, current flows via the line terminal (L1) and a bleed resistor (R1) to cause a first transistor (TR1) to switch on thereby rendering the circuits' line switch (2) conducting. The capacitor is charged via the operated line switch by line current and when the voltage level across the capacitor reaches the minimum operating voltage of the dialler chip, a circuit comprising a second transistor (TR2) switches the first transistor off which in turn switches off the line switch. The dialler chip now has sufficient power available to it to operate the line switch whenever the hook-switch (HS) is operated. |
