Sujet : Re: Photocell connection
De : jl (at) *nospam* 997PotHill.com (John Larkin)
Groupes : sci.electronics.designDate : 10. Mar 2024, 20:04:12
Autres entêtes
Organisation : Highland Tech
Message-ID : <vrsrui17pko17i4424464q7ddacig3ubk3@4ax.com>
References : 1
User-Agent : Forte Agent 3.1/32.783
On Sun, 10 Mar 2024 12:30:18 +0000,
liz@poppyrecords.invalid.invalid(Liz Tuddenham) 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"?
Tube systems generally had a positive power supply, with a bit of
ripple, so taking the small signal off the anode was a nuisance. A
phototube usually needed to be shielded anyhow, unless it was
measuring a very slowly changing light source.
I just hung a cathode follower next to mine, phototubes or PMTs. That
kept the speed up too.