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Load Impedance Sensor

IP.com Disclosure Number: IPCOM000080095D
Original Publication Date: 1973-Oct-01
Included in the Prior Art Database: 2005-Feb-26
Document File: 2 page(s) / 30K

Publishing Venue

IBM

Related People

Commander, RD: AUTHOR [+2]

Abstract

The figure shows a circuit for sensing departure of a driven load from nominal impedance in fault conditions. The circuit is essentially a latch which is arranged to come up in the set state (T1 on and T2 off) when the +24 volt line rises. In the particular application shown, the load is the energizing coil of an electromagnetic brake with a nominal impedance of 110 ohms.

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Load Impedance Sensor

The figure shows a circuit for sensing departure of a driven load from nominal impedance in fault conditions. The circuit is essentially a latch which is arranged to come up in the set state (T1 on and T2 off) when the +24 volt line rises. In the particular application shown, the load is the energizing coil of an electromagnetic brake with a nominal impedance of 110 ohms.

As the +24 volt line rises when power is switched on, T1 base voltage is equal to line voltage (T2 off), and T2 base voltage is approximately 1/7 of line voltage (T1 off) because of potentiometer R2, R3. Hence, T1 base voltage rises seven times as fast as T2 base voltage and T1 will turn on before T2, holding T2 off via R2 and R3 as the voltage rises towards +24V.

When power is fully on T1 is saturated and T2 is off, allowing R4 to provide T1 base current. In these conditions, the load current passing through R1 drops sufficient voltage to turn on T3 via R5 and provide a down level on the output.

If the brake impedance falls due to a short circuit, latch T1, T2 will reset. This can occur in two ways: a) If T1 has a current gain of less than 70 at the resultant higher current, T1 will have insufficient base current to remain saturated, its collector will rise and turn on T2 via R2, which will turn off T1. This is guaranteed to occur when the brake impedance falls below 33 omega (worst-case tolerance).
b) If T1 has a current gain of significantly higher than 70 at this higher...