>Rich, AG6K,
>
>You recommend a fil undervoltage trip based on a 723. Can you expand on
>this? Is the 723 (which is a regulator) providing a reference source for
>a comparator, with the filament volts rectified to provide the other
>input to the comparator?
>
- The 723 is a 7.1v reference plus a comparitor, Peter. By rectifying
and filtering the AC heater V, feeding the resultant DC to a
potentiometer whose output connects to an input of the 723, the output
of the 723 will change markedly when the heater V crosses the threshold V
setting on the potentiometer. However, the output of the 723 is capable
of only about 15mA. An optoisolator can be used to amplify the output
current and isolate the 723 from the circuitry that switches the
amplifier to operate mode whenever the heater V is insufficient for safe
operation of the cathode.
- - Caveats: the 723 has a current gain of roughly 1M. Thus, it is quite
vulnerable to RF unless 2, c.100 k ohm resistors are connected in series
with the 2 input pins, close to the IC. It's also useful to RF-bypass
the 723's c.15v power source with a 1uF tantalum cap. in parallel with
disc ceramic bypass caps of lower, differing values, such as 2000pF and
0.02uF. If the amplifier is for 6m, add a 400pF cap.-- if for 2m, add a
100 pF cap. Remember: Above it's self resonant frequency, an 'RF
bypass' capacitor exhibits XL, like a RFC. IMO, it might be best to
float both sides of the heater instead of grounding one side of it, and
to float the 723 and it's supply as well.
- Rich -
R. L. Measures, 805-386-3734, AG6K
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