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Measurements of Small Currents in CRT

IP.com Disclosure Number: IPCOM000100049D
Original Publication Date: 1990-Mar-01
Included in the Prior Art Database: 2005-Mar-15
Document File: 4 page(s) / 119K

Publishing Venue

IBM

Related People

Beanlands, P: AUTHOR [+3]

Abstract

A technique is described to accurately measure the cathode cutoff voltage of a cathode ray tube (CRT). Cutoff voltage is of fundamental importance both for correct monitor design and for assessment of the life performance of a CRT. Its measurement by conventional techniques presents intractable problems which are overcome in this disclosure by optical isolation, conversion to digital measurement, and allows accurate and repeatable measurements to be made.

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Measurements of Small Currents in CRT

       A technique is described to accurately measure the
cathode cutoff voltage of a cathode ray tube (CRT).  Cutoff voltage
is of fundamental importance both for correct monitor design and for
assessment of the life performance of a CRT.  Its measurement by
conventional techniques presents intractable problems which are
overcome in this disclosure by optical isolation, conversion to
digital measurement, and allows accurate and repeatable measurements
to be made.

      The operation of a CRT is characterised by fundamental
parameters, which relate the electrical conditions on the grids to
the current flowing from the cathode.  Cathode current flow is
related to the drive and cutoff voltages (for cathode drive) by the
equation:
   Ia = Kc x V(d) 7/2 / Vkco 2 where Kc is the cathode quality
factor (emissivity) of the gun, V(d) the drive voltage from cutoff,
and Vkco the voltage at which the CRT just ceases conduction.  The
light output of a CRT is directly proportional to its cathode current
so the parameters Kc and Vkco are of fundamental relevance to its
operation.  Both these parameters have ageing characteristics, so
that the current flowing for a given V(d) will change during the life
of the CRT.  Thus, in any characterisation or life test of a CRT, an
accurate measurement of both Kc and Vkco is required.  Measurement of
Vkco is difficult by conventional techniques because:
(a)  Very small currents (approximately 100 - 1000 pA) must be
measured directly.
(b)  These currents are flowing in a circuit whose potential is
typically 20 - 30 kV above ground.
c)  Optical methods of determining cutoff have proved inaccurate and
inconsistent. Apparatus (Fig. 1)

      The CRT anode voltage is supplied from a low impedance source
S, and returned through a current-sensing resistor Rs.  The voltage
is monitored by a probe whose equivalent circuit is represented by
the potential divider Rp1, Rp2.  A gas discharge device N is placed
in series with the EHT circuit.  When the voltage across N exceeds a
specific value (approximately 80 V in this application), it
"strikes", changing from its usual high impedance state, to
conduction, or low impedance, and light is emitted.  Fig. 2 gives an
equivalent circuit.  The EHT supply and probe are represented by Vs,
Rb, Rpl, and Rp2.  The CRT may be considered as a variable current
source, Ic, shunted by a capacitor C.  The potential at the anode of
the CRT is represented by Vt.  A light detector P is attached to the
gas discharge device N, and the signal from P is amplified and
threshold detected, producing a digital indication of the state of
conduction of devi...