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The purpose of the circuit shown in the drawing is to derive DC current from line transformer TFR in modem or other applications.
English (United States)
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DC Current Bypass for a Line Transformer
The purpose of the circuit shown in the drawing is to derive DC current from
line transformer TFR in modem or other applications.
Its characteristic is to present a low impedance for very low frequencies and
DC signals, and a high impedance for the audio frequency range signals.
Consequently, the transformer no longer carries DC current, and thus it can be
as small in switched line applications as in leased line ones.
The circuit comprises a clamping arrangement comprising resistors R1 and
R2, the values of which are chosen such that R1 = nR2, and a Darlington
arrangement T1 and T2. The clamping arrangement causes a DC voltage drop
by letting the DC current imposed by the line flow into it.
In the AC condition, the impedance R1, C1 is high with respect to the modem
impedance to prevent the AC signal to be altered by the circuit.
Zener diode Z limits the potential transient overload, and capacitor C2 is for
surge protection. Its true value should be adjusted by experiment to avoid high
frequency resonance with the transformer.
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