Correction, sorry. Opamp U2A adds x2.5 gain so nominal current in
the heaters will give ~+5V at FIL_MON. D25 prevents it going -ve
and D24 clamps the output at 5V, both presumably to protect
whatever the output feeds into, should the opamp output swing
beyond 0-5V.
Steve
Don't rule out an intermittent in the heater wiring, socket
connections or even inside one of the tubes. A bad joint or
dodgy connection will show up as low current and cause a trip.
Your guess at the operation of the current monitor is correct.
3A through 0.3ohms gives 0.9V of AC across the resistor. The
two 1N5711s rectify and double the AC to give a bit over 2V DC,
then the opamp adds x1.5 gain so normal heater current gives
something over 3V at the monitor output.
Steve
I am taking every opportunity to learn more and working on my
Alpha 87a always provides a good lesson. I have been getting
intermittent faults on the filament monitor for the 3CX800A7's
showing the filament current is low - suggesting that one of
the tubes has an open filament. I know this is not the case as
it is intermittent fault and when not faulted I can get full
legal power. I figure it's an issue with the filament
monitoring circuitry. I understand the basic concept of current
monitoring using a parallel resistive shunt load and feeding
that resistive voltage drop into an op amp to provide a
corresponding proportional voltage that could be read by a
microprocessor analog input. The 87a filament monitor per
manual is looking for 3 states, less than 2 amps or more than 4
A and anything in between so it seems the current sensing
method would not need to be too precise.
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