wincros@xxxxxxxxxxxxx writes:
<< This concern with VRs presumes that somehow the circuit components
will not change value with age and use. That just is not true. It
seems to me that if one follows the usual manufacturers instructions
to periodically service the camera, then a VR is a perfectly good and
rational solution the stability of which will probably outlast the
mechanical compenents' ability to function without cleaning and
lubrication.
>>
Although to some degree I agree with you from a field recalibration point of
view and a home maintainer of eqpt. And your point about other mechanical
components having reliability problems is well taken. But as an EE when I
design circuits professionally I avoid VR's unless forced by circumstance.
The reasons are cost (good potentiometers are expensive) , vibration
sensitivity (they shift if shocked), they have very high temperature
coefficients, they age much worse than fixed resistors, they are tempting to
adjust when something else is the problem, the cost of adjustment is high and
error prone and their failure rates are higher than fixed resistors. With
microprocessors now, the instruments almost always do a self test and self
cal at startup or have stored cal constants in a electrically programable
memory. Even where potentiometers (VR's) are used they are usually
electrically programmable rather than manually adjustable. The performance of
the latter is now much better than actual pots and the cost lower. In the
case of small programable memory chips the cost is much lower than actual
pots since many constants can be stored in a single chip that costs a little
less than even one cheap pot.
Regards,
Tim Hughes
>>Hi100@xxxxxxx<<
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