J
Jeremy D. Grotte
Guest
I work B-52's, which use a DC fuel quantity indicating system, capacitance
sensing probes, indicators, wiring, C/B's, etc.etc.
I guess over the years, we've gotten so 'stupid' that now all we do is
troubleshoot down to bad parts and swap them out, and when that fails, we do
'swap-tronics' until it's fixed. Anyways, I want to learn more about this
type of indication system. So here goes...
I remember reading somewhere that the system sends a negative going DC
square wave pulse of 20v down the wires at 6200hz, 50% duty cycle, very low
current. So, I can assume that charges the capacitors (probes). The
indicator takes the pulse on the return line (reading the probes discharge?)
and does some 'magic' on it, and basically what you end up with would be
some sort of wheat-stone bridge type of circuit using phase-shift and
capacitors on the motor driven re-balance side instead of resistors and
motor driven re-balance resistors. The probes themselves have 3 wires going
to them, I assume a center, a positive return and a negative return, or a
ground of some sort, and on the circuit card attached to the probes are a
couple of diodes and resistors (if that helps anybody).
Am I even close on this subject? Does anybody have any better ideas on how
this works? I'm getting up in years in my career and need better info to
pass along to my youngsters just getting started in the job and of course,
knowing the guts of the system would sure help for troubleshooting. (i.e.
one leg open/shorted will cause the indicator to drive one way, the other
line open/shorted, drives the other way, etc.etc.)
JDG
sensing probes, indicators, wiring, C/B's, etc.etc.
I guess over the years, we've gotten so 'stupid' that now all we do is
troubleshoot down to bad parts and swap them out, and when that fails, we do
'swap-tronics' until it's fixed. Anyways, I want to learn more about this
type of indication system. So here goes...
I remember reading somewhere that the system sends a negative going DC
square wave pulse of 20v down the wires at 6200hz, 50% duty cycle, very low
current. So, I can assume that charges the capacitors (probes). The
indicator takes the pulse on the return line (reading the probes discharge?)
and does some 'magic' on it, and basically what you end up with would be
some sort of wheat-stone bridge type of circuit using phase-shift and
capacitors on the motor driven re-balance side instead of resistors and
motor driven re-balance resistors. The probes themselves have 3 wires going
to them, I assume a center, a positive return and a negative return, or a
ground of some sort, and on the circuit card attached to the probes are a
couple of diodes and resistors (if that helps anybody).
Am I even close on this subject? Does anybody have any better ideas on how
this works? I'm getting up in years in my career and need better info to
pass along to my youngsters just getting started in the job and of course,
knowing the guts of the system would sure help for troubleshooting. (i.e.
one leg open/shorted will cause the indicator to drive one way, the other
line open/shorted, drives the other way, etc.etc.)
JDG
