S
Syd Rumpo
- Jan 1, 1970
- 0
Reposted here 'cos Ed didn't realise that sci.electronics isn't
available on many news servers, although he's had a couple of helpful
replies.
<quote>
I presently working on a GM detector. Unfortunately the source I'm
using has a fairly high count rate and the period between events seen
by the tube is much shorter than the dead time. I thought I would get
around the problem with external quenching; switching the supply to
the anode to below the starting voltage (750 V) of the tube for the
dead time (200uS) this period is given from a monostable triggered by
the event.
The problem seemed to me to be trivial but after trying various shunt
and series arrangements with FETs I have realised it is a bit more
difficult than I originally thought.
One problem is my power supply; it was originally designed for a PMT
so it cant provide that much current. This has forced me to use a 16M
potential divider to switch down to 750V and the big resistances are
giving me a large time constant even though the capacitances are quite
small. This makes the rise and fall of the voltage too slow (I was
aiming for 20 uS or so).
I cant drop the supply right down to deck because some voltage (not
sure how much - would be interested if anyone knows?) is required at
the anode to let the tube recovery from an event.
The other problem is miller capacitance across the FETs and emfs being
induced by the large voltages. The most successful circuits I have
made respond to the first pulse but as the tube is switched back on
again after the dead time a pulse appears on the cathode hence
retriggering the monostable and causing the tube to keep switching
off.
I found one circuit using valves on the net but it didn't make any
sense to me. Does any one/ has anyone done this with FETs ?
Any advice would be much appreciated; I'm pretty stuck on this one.
Cheers
Ed
</quote>
available on many news servers, although he's had a couple of helpful
replies.
<quote>
I presently working on a GM detector. Unfortunately the source I'm
using has a fairly high count rate and the period between events seen
by the tube is much shorter than the dead time. I thought I would get
around the problem with external quenching; switching the supply to
the anode to below the starting voltage (750 V) of the tube for the
dead time (200uS) this period is given from a monostable triggered by
the event.
The problem seemed to me to be trivial but after trying various shunt
and series arrangements with FETs I have realised it is a bit more
difficult than I originally thought.
One problem is my power supply; it was originally designed for a PMT
so it cant provide that much current. This has forced me to use a 16M
potential divider to switch down to 750V and the big resistances are
giving me a large time constant even though the capacitances are quite
small. This makes the rise and fall of the voltage too slow (I was
aiming for 20 uS or so).
I cant drop the supply right down to deck because some voltage (not
sure how much - would be interested if anyone knows?) is required at
the anode to let the tube recovery from an event.
The other problem is miller capacitance across the FETs and emfs being
induced by the large voltages. The most successful circuits I have
made respond to the first pulse but as the tube is switched back on
again after the dead time a pulse appears on the cathode hence
retriggering the monostable and causing the tube to keep switching
off.
I found one circuit using valves on the net but it didn't make any
sense to me. Does any one/ has anyone done this with FETs ?
Any advice would be much appreciated; I'm pretty stuck on this one.
Cheers
Ed
</quote>
