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Overcurrent Sensing Circuit

IP.com Disclosure Number: IPCOM000099593D
Original Publication Date: 1990-Feb-01
Included in the Prior Art Database: 2005-Mar-15
Document File: 2 page(s) / 59K

Publishing Venue

IBM

Related People

Najm, EM: AUTHOR

Abstract

Disclosed is an overcurrent sense circuit that can sense current on both positive and negative levels. This circuit provides an output fault signal if overcurrent occurs. Cost and parts count have been minimized.

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This is the abbreviated version, containing approximately 65% of the total text.

Overcurrent Sensing Circuit

       Disclosed is an overcurrent sense circuit that can sense
current on both positive and negative levels.  This circuit provides
an output fault signal if overcurrent occurs. Cost and parts count
have been minimized.

      Most power supplies have outputs of both polarities, and this
tends to complicate the design of the protection circuitry.  It is
usually necessary to combine the outputs from all the current sensing
circuits into a single fault output.  The presence of both positive
and negative voltages requires the use of level shifting circuits.
This adds cost and complexity to the protection function.  In
cost-sensitive applications, such as power supplies for monitors, it
is important that cost and parts count be minimized.

      The figure shows a circuit that meets these requirements.  This
circuit senses current on levels of both polarities and produces a
single fault output.  This output is activated in case of an
overcurrent on either level.  The power transformer T1 and the
secondary com ponents CR1, CR2, C2 and C3 are not part of the new
circuit. They are shown to illustrate how the new circuit connects to
the power supply.  The new circuit is not dependent on any particular
power supply topology.

      The current in the positive output level is sensed by R1.  When
the voltage drop across R1 becomes high enough, Q1 will turn on.  R3
limits the base current of Q1.  Q2, R2 and R4 operate in a simila...