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A simple circuit providing a constant low voltage drop for a wide current range is described. The invention uses the association of two usual silicon diodes and a transistor.
English (United States)
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Wide Current Range Low Voltage Limiter
A simple circuit providing a constant low voltage drop for a wide current
range is described. The invention uses the association of two usual silicon diodes
and a transistor.
In some applications, it is necessary to have a device that creates a low
voltage drop, as constant as possible, for a wide current range.
This can be the case for telephone handset switch-hook detector, where the
described device is put in parallel with the detector in order to limit the current
flow into the detector inputs.
In that case, the requested clamping voltage is around 1.9 volts, and PTT
national requirements are difficult to fulfill in that respect with usual silicon diodes.
The more usual solution is to put 3 diodes in series. It works correctly for low
currents. Unfortunately, for larger currents, VD is too important. The proposed
invention uses one base emitter junction of a transistor.
One portion of the current flowing through D1/D2 is used to polarize the base
emitter junction of T. Then, the current that transistor T is able to drive is IB
multiplied by the transistor gain; thus, it is the main part of IT. It means that the
current flowing through D1 and D2 remains low and maintains a low voltage drop
through D1 and D2.
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