I want to repair a 2 x 9W PL-S lantern at friends. The original choke is 13W PL-C and is damaged. The closest thing i have on hand is 18W PL-C chokes, and i wonder if that is okay for the replacement

9W PL on 1 x 9W choke works at 170mA

On the 13W choke it is stated 2 x 9W 140mA, cannot measure resistance (choke is open)

On the 18W choke it it stated 18W 220mA, measures 107 ohm coil resistance



Intuitively i think it is going to be pretty close, but i dont know the induction value of the chokes to really calculate this exactly. Anyone know any better ?

Maybe it was overloaded from 2x9 instead of 13w. It may have a lower current than the 18w ballast since it will underdrive the lamps. I think it would be ok to use the 9w lamps in series on a 18w choke, but before you install, be sure the ballast doesn't heat up too much. 

The 13W PL choke is the "correct" to run 2x9 as in - it is the one rated for them, though it underpowers them (140mA instead of 170mA on the proper 1x9). The same choke run 1x13 at 190mA and heat more with the 1x13 accordingly

The choke was damaged when the lantern was installed - the installer pulled the wires and ripped the terminal block out a bit, so pulled on the wire from the coil that comes from inside. The choke still worked for several yers in this condition, but now the wire is snapped off - possibly due to something hitting the lantern and giving it that one last pull

I'd say use the 18w choke as to make the lamps work at closer to proper operating current. I've heard that underdriving lamps is bad for them, so would using the 18w blassast be better?

Trying to estimate (for better precision i use the 230V as in the choke ratings, and not 240V as in the actual line voltage) :



1x9 Choke : 9W 170mA PF0.37

9 * 1.15 / 170mA = 61V

230V * 0.37 = 85V on lamp + choke wire resistance - so 24V on choke wire resistance
24V / 170mA = 141 ohm wire resistance (and it comes consistent with the measured value 143 ohm)

230V * sqrt(1 - 0.37^2) = 214V on choke magnetic coil
214V / 170mA = 1260 ohm coil reactance



1x13 Choke as 1x13 : 1x13W 165mA PF0.49

13 * 1.15 / 165mA = 95V

230V * 0.49 = 113V on lamp + choke wire resistance - so 18V on choke wire resistance
18V / 165mA = 110 ohm wire resistance

230V * sqrt(1 - 0.37^2) = 200V on choke magnetic coil
200V / 165mA = 1210 ohm coil reactance



1x13 Choke as 2x9 : 2x9W 140mA PF0.57

2 * 61 = 122V

230V * 0.57 = 131V on lamp + choke wire resistance - so 9V on choke wire resistance
9V / 165mA = 64 ohm wire resistance - not very consistent with the 1x13W case.... somethig is not very precise. But oh well, the coil resistance is not the main part of the impedance anyway

230V * sqrt(1 - 0.37^2) = 189V on choke magnetic coil
189V / 140mA = 1350 ohm coil reactance - ok this is better



1x18 Choke as 1x18 : 1x18W 220mA PF0.5

18 * 1.15 / 220mA = 94V

230V * 0.5 = 115V on lamp + choke wire resistance - so 21V on choke wire resistance
21V / 220mA = 95 ohm wire resistance (and it comes consistent with the measured value 107 ohm)

230V * sqrt(1 - 0.5^2) = 199V on choke magnetic coil
199V / 220mA = 905 ohm coil reactance



Now assume i connect the 2x9 PL on the 18W choke. Lets try to estimate :

(230V)^2 = V(lamp + wire resistance)^2 + V(coil reactance)^2

230^2 = (122 + 95 X)^2 + (905 X)^2

52900 = 14884 + 23180 X + 9025 X^2 + 819025 X^2

828050 X^2 + 23180 X -38016 = 0

X1 = 0.200 = 200mA
X2 = -0.228 (the second solution of the equation that have no meaning here)



So, we get 200mA (thats what the math says, i didnt test). About same 30mA difference but now to the high power side. Will it affect the lamp much ?

On the 18W choke it it stated 18W 220mA, measures 107 ohm coil resistance

It would overload the lamps. The PL-S9W are rated at 170mA, so the 18% (200 vs 170mA) overload is too much (getting even worse during preheat).
Just any "7..11W PL-S" ballast works fine (the arc current will be in the 0.14A range, but more important for the life is the preheat current and that remains the same as with only one lamp - filaments are nearly a short circuit with respect to the ballast impedance)...

Update :

I put this up to the test, with Eltam EEI=B1 18W PL-C choke and 2x brand new Philips 9W PL-S

Work current : indeed 200mA spot on

Preheat current (i really measured choke across the mains, without the lamp filaments there at all) : 300mA on 18W choke, 200mA on 13W choke (old 13W choke in the other identical lantern)



I installed it in that lantern i had to repair - Back porch light at friends home, where it is used everyday with some variations. We'll se how long it last...