B
Bill Sloman
200 V at 60 mA is 12 W! Must be a big neon lamp!> The trouble I have with this circuit is that at above currents
My guess is that the lamp exhibits negative resistance at this
current. You will find it pretty difficult to control the current
through a negative resistor. I think the only hope is to add
series resistance greater than the value of the negative resistance.
Negative resistance wouldn't be a problem. The circuit is a constant
current source, sensitive - in the first instance - to the current
through the sensing resistor. The voltage drop across the lamp does
control the base-collector voltage, but that has very little effect
(less than 0.1%) on the base-emitter voltage which the op amp sees.
If the discharge turned right off, then there would be a problem. If
the control loop is oscillating, and drove the controlled current down
to zero this could happen, but stopping the control loop from
oscillating would be the area to concnetrate on, rather than the non-
linearity of the lamp when operating way off the desired stable state.