Voltage and current on telephone lines

The voltage in the telephone line is 20V. When a call comes in, the voltage of the telephone line rises to 48V and the phone rings. When the phone is picked up to answer, the voltage of the telephone line drops to 10V.

Telephone lines have certain restrictions on current, and fuses are installed before entering the cable. The switching equipment supplies electricity to each module. The telephone line is powered by 48V, and a resistor is connected in series on the line , about 600 ohms at each end. If the load exceeds slightly, the switch will think that the phone is off the hook, which will affect communication . When the phone rings, the power consumption is the highest. The current of normal calls has been taken into account when designing the line. The average 48V DC power per household does not exceed 20 mA. When a user draws 100A of power for a long time, it will be detected by the monitoring equipment and the power will be automatically cut off to the user.

The DC feed voltage of the currently used program-controlled exchange is 48V. Since the lead-acid battery works by floating charge, the actual voltage is about 50V DC when the phone is on hook. When the phone is off hook, the DC circuit of the phone is connected and there is current in the phone. At this time, the voltage drops to 7-10V, which varies depending on the length of the line and the phone.

The current after the phone is off-hook is different, affected by the user line, telephone and internal switching equipment. Generally, it is 20-30mA. When on-hook, the current should be around 5 microamperes.

When the bell rings, the ringing voltage is an AC voltage of 75V with a frequency of 16-25Hz.

The original vertical and horizontal switch hanging 60V
program-controlled switch hanging 48V
ring current voltage 90V

Ordinary phone
On-hook 48V, ringing 90V, talk 8V
But the value is not accurate and will vary within a certain range. The general value is given

When the local telephone exchange is on hook, DC 48V.

Ringing: 20HZ, 75V or above

Call: Depends on the quality of the line and the phone, generally between DC 6-12V

Small switch: There are generally two types of DC voltage: 24 or 48.

The ringing is similar to the talking.

Although the voltage is above 36V, the current is extremely small and it is harmless to humans, so don't worry.