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Current Limiter and Short Circuit Protector

IP.com Disclosure Number: IPCOM000087258D
Original Publication Date: 1977-Jan-01
Included in the Prior Art Database: 2005-Mar-03
Document File: 2 page(s) / 58K

Publishing Venue


Related People

Actor, EJ: AUTHOR [+2]


Circuits are described for protecting computer digital outputs (DOs) from overloads and short circuits.

This text was extracted from a PDF file.
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Current Limiter and Short Circuit Protector

Circuits are described for protecting computer digital outputs (DOs) from overloads and short circuits.

The basic circuit (Fig. 1) consists of a Darlington power transistor 1, a field- effect diode 2 (current regulating diode (CRD)) and two resistors. Field-effect diode 2 conducts a constant current through resistor RB. This provides a constant voltage of VB. Transistor 1 will conduct until VBE + ICRE is equal to VB, at which time transistor 1 cuts off, limiting the current.

The circuit of Fig. 2 provides a visual indication of current flow with consistent brightness over an extremely wide range of load current. This is achieved by the substitution of a light-emitting diode (LED) 4 for resistor RB and the addition of biasing components.

A short circuit at the load imposes the full source voltage across the current limiting transistor. This results in a great deal of power being dissipated by the transistor which may be beyond its design limits. The amount of power dissipated by the transistor may be controlled by the addition of a photo-coupler 6, which includes an LED 7, to the circuit, (Fig. 3). When the current limit is reached, the voltage across transistor 8 starts to rise, causing current flow in the input of photo-coupler 6. The output resistance of photo-coupler 6 decreases, which lowers the voltage at the base of transistor 8, lowering the collector current and limiting the power dissipation until the short is...