John D Alford

US Patent:

42478499, Jan 27, 1981

Filed:

Mar 19, 1979

Appl. No.:

6/022090

Inventors:

Phillip D. Morris - Flower Mound TX
John H. Alford - Dallas TX

Assignee:

Beta Products, Inc. - Carrollton TX

International Classification:

G08B 2100

US Classification:

340660

Abstract:

A current limiting voltage sensing circuit especially suited as an annunciator input circuit is disclosed. It includes a radiation emitting device circuit, a current limiting circuit for maintaining the current supplied to the voltage sensing circuit at a substantially constant value while supplying the needed current to the radiation emitting device circuit, and a radiation sensitive device circuit coupled to the radiation emitting device and electrically isolated from it for converting from the radiation signal into an electrical signal. In one arrangement, the input to the voltage sensing circuit is a full-wave rectifier so that it can be used for detecting an ac voltage or a dc voltage in either direction. One arrangement of the current limiting circuit includes a field effect transistor with the drain-to-source forming a variable resistance current path for maintaining the current supplied to the voltage sensing circuit at a substantially constant value, and a circuit connecting the source of the field effect transistor in a feedback relationship to the gate. In one arrangement, the feedback circuit is a resistor in series with the radiation emitting device which is a light emitting diode.