Equipment connected to the public switched-telephone network (PSTN) sometimes needs to detect polarity reversals of the battery feed. One common application is the Direct-Inward-Dialing (DID) service that uses what is termed as "loop-reverse battery signaling." For more details on the implemention of DID, refer to ANSI document T1.405-1996. One of the requirements for DID operation is that the telephone-line loop current be monitored for current direction. The direction of current is directly related to the battery polarity applied to the circuit. One possible implementation uses a Clare LDA201 dual optocoupler (www.clare.com) as a solid-state current sensor (see the figure).
Normally, TIP is positive with respect to the RING lead on the telephone line. The nominal open-circuit voltage from the PSTN is 48 V. RSENSE is selected such that a particular threshold current can be set before the reverse or forward signal is asserted. For example, suppose you want to make the current detection occur when the loop current is ≥10 mA. Using the typical value for VF in the LDA201 data sheet:
R = VF/I
R = 1.2/10 mA = 120 Ω
With RSENSE set to this value, the FORWARD line will activate when the loop current exceeds 10 mA. If TIP and RING polarity is reversed, the REVERSE line will activate when the loop current is greater than 10 mA. Note that 1.2 V is the typical value for VF. If we take into account the minimum and maximum VF (0.9 V and 1.4 V respectively), the detection threshold can vary from 7.5 to 11.7 mA with a 120-Ω resistor. Also, VF changes with temperature as shown in the characteristic curves from the LDA201 data sheet. Pull-up resistors, R, should be selected based on the minimum current-transfer ratio (CTR) specified in the data sheet, and the minimum LED current for the application. Note that the LEDs in the LDA201 are rated for a maximum current of 100 mA, so series-limiting resistors for the LEDs are unnecessary.