In cycling HPS there is sometimes small discharge remaining when the lamp is in the off (powered up but extinguished) period

Why does it remain when the big discharge extinguishes ?

Why doesnt it become a full discharge again ? (untill the lamp cools down)



I tried to power up, let warm, and quickly interrupt the power to 70W internal start HPS. When the lamp is extinguished, this discharge appears and disappears accordingly when i switch on / off the power - so it has nothing to do with impulses from ignitor or stuff like that, it happens at 240 V AC or below

It is a weak discharge from the ignitor.
When you cycle the power, cathodes cool down.
Then the ignitor pulse is sufficient to create the cold cathode discharge, but it is insufficient current to transition to hot cathode arc and the ballast OCV cannot overcome the high anode column drop (~60..70V on SON70W; the arctube is still pressurized) together with the cold cathode fall (~200..300V) to take over (like when cold).

As cathodes cool gradually, few seconds the ignitor is able to maintain the tiny discharge, then when they are still colder, it is not able to do so anymore, so the faint discharge disappear.

This discharge has nothing to do with the ignitor

I tried it with internal ignitor (glow bottle) 70 W HPS. This lamp has a thermal disconnect in series with the ignitor, which means that if i interrupt the power quickly the ignitor won't function at all for a while

And yes with this lamp there is this tiny discharge, just with 240 V and series inductor ballast that does nothing to boost volts due to lack of ignitor

Then only one thing came to my mind: Remaining ionization, where the insufficient voltage is not able to support it's avalanche breeding, so the ionization level only decay (= the discharge is not selfsustaining).

I observed this "afterglow", but I have experience only with setups HV ignitors, so I took the "ignitor theory" as most likely...

"so the ionization level only decay"

Doesn't it just decay de to falling temperature and reduction in amount of vapor ?