Re: (FED and other bat drain in meter) digest 1564

[P3D Bob Howard <bobh@xxxxxxxxxxxxxxxxxxxx>]

RE: John B. says his FED might have high dark current as he has to
change batteries. This after someone lost lens caps on FEd and then was
told that meter is not behind lens, so caps don't matter unless on photo
cell window!
The replacement for selenium meters in cameras was the CdS cell, which
is highly IR and Red sensitive but acts as a resistor with high
resistance if in dark, hence a closing the case over the cell window
cuts off the current..on lens cap with behind the lens meter. BUT
BEWARE!
More modern "silicon blue" cells used in behind the lens meters are NOT
photo resistors, but like selenium, actual current generators. THEY NEED
AN AMPLIFIER and this is on if the meter is turned on regardless of lens
caps or light level. So most cameras have an on off switch these days.
BobH
(I doubt if the FEDBOY got to silicon blue but who knows?)


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