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  1. #11
    gainer's Avatar
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    Sep 2002
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    CdS is usually a photoresistor. Ideally, it should have infinite resistance at zero illumination. Any corrosion or remaining solder flux, etc., may form a path around the cell that varies with age. Selenium is usually used in photoelectric mode. It's output can be sufficient to drive a meter directly as it does in my Leica meter. This leads to other problems because the meter must be a rugged, non-linear microammeter. The selenium cell can also be amplified by any of several (hundred?) integrated circuit operational amplifiers which can be made to have logarithmic output for linear input and in turn drive a more rugged linear meter or whatever circuits an automatic camera may present to it. Don't forget phototransistors. By the way, we are talking about analog computers here. Even d*****l cameras use them.
    Gadget Gainer

  2. #12

    Join Date
    Jan 2008
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    First and foremost, you CANNOT replace a mercury cell with and alkaline cell. The voltage of alkaline cells changes constantly as they discharge. The reason mercury cells were used was because they had a linear voltage ouput thoughout their life, then died almost immediately. The zinc-air cells can be used as a replacement, or get an MR-9 adapter and a silver oxide cell from C.R.I.S.

    CdS cells do age, which causes both accuracy and linearity problems. I have had much better luck with selenium meters.

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