Date: Wed, 03 Mar 1999 10:11:51 -0500

Author: Gerald Zani

Subject: Re: RC Flash Lamp PIRA 5F30.60


Thank you kindly Brett. My and I faculty made use of your explanation today.
Indeed, the answere is that the average ionization potential of the neon,
36 Volts, prevents the capacitor from discharging to zero in the relaxation
oscillator demo, PIRA 5F30.60.

Cheers, Jerry

At 11:15 AM 3/2/99 -0800, you wrote:
>Jerry -
>Neon is not inherently conductive at such a low voltage (36 volts). If you
>raise the voltage on a neon lamp slowly, you'll find that it begins to
>conduct around 60-70 volts (can't remember exactly). If you then lower the
>voltage it won't shut off until somewhere down around the upper 50's. The
>reson is that once the lamp begins to conduct the neon becomes ionized
>(thus the conduction), and that ionized neon support enough current flow to
>keep the gas ionized, even though it is at a lower voltage than where it
>kicked in. As you lower the voltage further the current flow becomes too
>low to support ionization and the current stops.
>I suspect on the flashing neon bulb the gas will continue to conduct below
>50 volts because all of the ions created during the flash have not yet been
>swept out of the gas. It takes a finite amount of time to "clear the air"
>(well, neon), thus allowing conduction below 50 volts for a time and the
>voltage continues to drop. Once the residual ions are gone (around 36
>volts for your particular lamp), the voltage is far too low to do any
>ionizing and must rise to the breadown voltage of 70 or so to fire again.
>At 03:52 PM 3/2/99 -0500, you wrote:
>>Question. The RC circuit with the neon flash lamp. After the neon flash
>>discharges it does NOT drop to zero volts, it drops to about 36 volts. The
>>repeating waveform for the RC charge and discharge curve rises to 70 volts
>>drops to 36 volts. What is this residual charge effect called?
>>After the neon lamp flashes why doesn't the voltage drop to zero? Is
there a
>>name for this effect? What is the cause for the residual charge in the
>>Gerald Zani
>>Lecture Demonstrator
>>Dept. of Physics #1843
>>Brown University Phone: (401) 863-3964
>>Providence, RI 02912 Fax: (401) 863-2024
>>Demonstrations for duty and humanity.