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Constant Current Driver with Minimized Transistor Power Dissipation

IP.com Disclosure Number: IPCOM000079369D
Original Publication Date: 1973-Jun-01
Included in the Prior Art Database: 2005-Feb-26
Document File: 2 page(s) / 36K

Publishing Venue

IBM

Related People

Davie, NR: AUTHOR

Abstract

In the circuit shown, transistor T1 is a constant-current driver with a current regulator circuit 10. Normally, the current driver would drive current through the load 12 in series with resistors R1 and R4. To maintain constant current through this series circuit, a variation in load or variation in the voltage +V, which would cause a variation in voltage on the elements R1, R4 or load, must instead have the voltage variation absorbed across the transistor T1. Thus, in certain situations, the Vce voltage demand and hence the power dissipation of transistor T1 can become inordinately high.

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Constant Current Driver with Minimized Transistor Power Dissipation

In the circuit shown, transistor T1 is a constant-current driver with a current regulator circuit 10. Normally, the current driver would drive current through the load 12 in series with resistors R1 and R4. To maintain constant current through this series circuit, a variation in load or variation in the voltage +V, which would cause a variation in voltage on the elements R1, R4 or load, must instead have the voltage variation absorbed across the transistor T1. Thus, in certain situations, the Vce voltage demand and hence the power dissipation of transistor T1 can become inordinately high.

Transistors T2 and T3 are provided to change the effective resistance between the load and the voltage source +V, so as to effectively maintain a constant current through the load, T1, and R4, even though the load may vary or the voltage source +V may vary. In effect, the Vce or transistor T1 is held fixed, and voltage variations in the series circuit are absorbed by changing the effective resistance between the load and the voltage source +V.

This function is performed by using the Vce of transistor T1 to control the conduction of transistor T2. In other words, Vce of T1 is limited to Vbe of T2. As T2 comes on harder, the voltage at node 14 goes down, tending to turn off transistor T3, and effectively increases the resistance between the load 12 and the voltage source +V. Conversely, if the voltage from collect...