GB1402561A

GB1402561A - Measured-value converter circuit

Info

Publication number: GB1402561A
Application number: GB3746172A
Authority: GB
Original assignee: Danfoss AS
Current assignee: Danfoss AS
Priority date: 1972-08-10
Filing date: 1972-08-10
Publication date: 1975-08-13

Abstract

1402561 Bridge arrangement for remote twowire telemetry system DANFOSS AS 10 Aug 1972 37461/72 Heading G1U The remote unit of a two-wire telemetry system of the type in which the current flowing to and from the remote unit comprises the sum of a datum current (about 4mA) and a signal current (from 0 to 16mA), includes a resistive bridge R 3 , R 4 , R 1 ; R m , R2, Fig. 8 the resistance R m varying in accordance with a parameter being measured, and two distinct control circuits the first of which maintains the current leaving the bottom of the bridge via resistors R 1 and R 2 constant at I 1 , by comparing T 13 , T 1 4 the sum of the voltages across these resistors with a Zener Z 1 reference voltage and adjusting an impedance T 1 , T2 accordingly, and the second of which uses the amplified T3, T4 voltage across the diagonal of the bridge to control, via transistors T 5 , T 7 , T 8 , a current I 2 , tapped off from the fixed arm of the bridge so that the diagonal voltage is maintained substantially constant. The current I 2 is then proportional to the parameter being measured, and is added to the datum current Ii to form the current drawn from, and returned to, the central station. Clearly other parts of the unit, for example the Zener Z 1 reference voltage, consume current, but these amounts can be kept very low (about 0.3mA). In a modification the datum current comparator Ti 3, T 14 is replaced by a single stage T 12 , Fig. 5, and coupling resistors R 21 , R 22 are omitted. Further, the bridge diagonal-voltage balancing circuit is augmented by an additional stage T9, T 10 , so that the bridge diagonal voltage can still be maintained substantially zero even when the parameter being measured causes the resistance R m to rise (rather than to fall from a datum level as was tacitly assumed above), though in this case, although the bridge is properly balanced, no signal current I 2 flows. The bridge may include complementary strain gauges in both arms Fig. 6, (not shown).