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I am also searching for the same. I connected two 18W FL in series with 36W Magnetic Ballast but not able to find suitable 110V or PHILIPS S2 starter in local market in India and failed to lit both FL.
Masters your suggestions/tips are highly required
So far, I've found that electronic ballasts have series LC resonant circuit where inductor goes in series with one side filament and capacitor goes in place of starter (connecting both filaments). If we connect two FL in series like below text
Pin1 --> Connect to inductor (inside the ballast)
Pin2 --> Connect to capacitor-1 FL-1 Filament-Right
Pin3 --> Connect to capacitor-1
Pin4 --> Connect to FL-2 Filament-Left Pin1
Pin1 --> FL-1 Pin4
Pin2 --> Second capacitor-2 of same value (extra component need to purchase) FL-2 Filament-Right
Pin3 --> Second capacitor-2
Pin4 --> Line Neutral
but according to online text this arrangement will make capacitor values to half (capacitor will appear in series) and will also half the voltage on TL. Half capacitor value will alter LC resonant frequency and may not produce the required ARC between filaments after preheat.
I think if we double the value of resonant capacitor (parallel same value capacitor) the LC resonance frequency will not alter and it will be able to produce ARC. I do not know how this will impact the filament life and preheating time.
If I connect single 18W FL to 36W ballast what will be the adverse effects?
Did you tried with 220V ones? Here they never fail in this circuit. Only tubes are are more randomly swithched by starters so it takes more initial time to fully glow.
Extra 12Vac voltage from a small transformer, fed to centre two elements in series can do the trick in circuit with electronic choke.