Browse Prior Art Database

Current Detector

IP.com Disclosure Number: IPCOM000083001D
Original Publication Date: 1975-Mar-01
Included in the Prior Art Database: 2005-Feb-28
Document File: 2 page(s) / 44K

Publishing Venue

IBM

Related People

Daniels, RJ: AUTHOR [+2]

Abstract

When operating high power-push-pull transistor circuits, it is important that the pull device not be switched on until current to the push device has reached zero and vice versa.

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Current Detector

When operating high power-push-pull transistor circuits, it is important that the pull device not be switched on until current to the push device has reached zero and vice versa.

The circuit of the figure will detect a current threshold within 0.2% of the active current flowing through the sensing circuit. Transistor 22 and resistor 12 referenced to VREF 1 form a current source which determines the current in transistor 21, which in turn fixes the voltage at node 1 by controlling the IR drop across resistor 11. Transistor 23 prevents transistor 21 from saturating.

Example resistor values of the resistors in threshold sensing circuit 10 to sense zero current are as follows: 13-400 ohms, 14-30 ohms, 15-500 ohms, 16- 30 ohms, 19-1,000 ohms and 20-5,000 ohms. Schottky diodes 32 and 33 prevent transistors 27 and 28 from saturating. When a positive current flows into the sense input, the emitter-base junction of transistor 28 is back biased. Transistor 28, therefore, does not conduct and the output is clamped through Schottky diode 32 at a voltage slightly above the voltage at node 1.

Ideally, when the sense input current in this example goes to zero, the current through transistor 27 is equal to the current in transistor 28, and the output voltage will drop to a voltage equal to the sum of the voltage at the base of transistor 26 plus the base-emitter voltage of transistor 25. The output, therefore, swings from a voltage near the supply voltage to a v...