[Amps] Filament monitoring - LOOKING FOR ELMER

[Steve Thompson]

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.
>