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From: Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net>
Newsgroups: sci.electronics.design
Subject: Re: Photocell connection
Date: Sun, 10 Mar 2024 13:45:41 -0000 (UTC)
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Liz Tuddenham <liz@poppyrecords.invalid.invalid> wrote:
> I've noticed that when gas-filled photocells were used in valve
> equipment, they were nearly always supplied with a low-impedance source
> of +ve voltage to the anode and the signal was taken off a resistor in
> the negative return.  There is a blocking capacitor between the
> photocell cathode and the grid of the valve, so the standing current and
> DC conditions don't appear to be relevant.
> 
> This means the photocell has to be connected by a 2-core screened cable,
> which was an expensive luxury in those days.  It also has to be
> thoroughly screened to prevent hum, whereas the cathode half-cylinder
> would partly screen the anode and reduce the amount of extra screening
> needed;  so what was the advantage of taking the signal from the cathode
> instead of the anode"?
> 

At a guess, it’s because it saved headroom to have a positive-going signal.
There’s a well-defined dark level. 

Of course, in a general AC-coupled system, that doesn’t help as much unless
the photocurrent waveform is known to be very asymmetrical. 

 Cheers 

Phil Hobbs 

-- 
Dr Philip C D Hobbs  Principal Consultant  ElectroOptical Innovations LLC /
Hobbs ElectroOptics  Optics, Electro-optics, Photonics, Analog Electronics