Hey all,

As you may know i have a 2xF15T8 preheat desk lamp. The power factor is about 0.60, would adding a capacitor across the lines create any positive effects such as a brighter tube or longer life? Just curious about this.

Why you need to increase the power factor to your desk lamp? This won't do anything helpful for you. Power factor don't affect lamp performance.
High power factor is only helpful for larger installation of fixtures, where it reduces current being draw from your electricity company by your fixtures.

Yeah improving PF doesn't really have that big of an effect since you're a residential user with only one light. Now if you had a ton of these, the lower current draw means you could have more lights on a circuit. If you're a commercial user it means you won't get penalized by the utility for poor PF.

Anyway if you want to improve the power factor, I would look adding a series capacitor on one choke to convert it to lead-lag operation. This improves your PF but it also means the lamps run slightly out of phase to each other which will result in less perceivable flicker.   

@lead-lag operation: But you would have to fine tune capacitor experimentally to match the ballasts you already have there. The reason is the wide range of currents these ballast are passing.
Not sure if it is worth.

Definitely power factor has nothing to do with the lamp brightness.
What you are likely looking for is a parameter called "ballast factor" (a ratio of the lamp brightness on the tested ballast vs brightness on a nominal reference ballast), but that is tied to the exact ballast model, so if low, you would have to replace the ballast chokes for some with ballast factor closer to unity.

As has been stated, for a domestic situation, power factor is of little importance. But some of us like our systems to be complete, and for that reason and that reason alone, I always fit PFC capacitors.

I don't care if they make a difference or not; to me, the fitting or ballast board or whatever doesn't look right without it.

YMMV.

I always prefer that my fixtures be HPF. You guys can crap on me all you want, however my reasons are I have a backup power system I use in blackouts. So it is a big help to have lights that are not as heavy of a load on the system than they need to be. As you know, LPF presents a heavier load on the power source than perceived, which will make the inverter work harder and drain my batteries down faster.

Hey all,

As you may know i have a 2xF15T8 preheat desk lamp. The power factor is about 0.60, would adding a capacitor across the lines create any positive effects such as a brighter tube or longer life? Just curious about this.

In order to increase the brightness of a fluorescent tube, you would need to increase its current draw by using a ballast designed for a fluorescent tube rated for a higher current operation. Ideally, it is best to run fluorescent tubes on the correct ballasts for safety reasons.

It's best to run any discharge lamp on the correct ballast. Including a PFC capacitor if one is specified by the ballast manufacturer.

I just bought a power meter, basically the UK equivalent of the Kill-a-Watt devices sold in the US. I've been meaning to get one for a while, but finally got around to it this week.

As a test, I ran up a 50W HPS lamp. Without a capacitor, the power factor was 0.35, which I think you'd agree is pretty poor. With the correct 8µF capacitor across the supply, this jumped to 0.91..!

Maybe it doesn't make a difference to the electricity bill, but it makes me feel better, and the installation looks complete 

It's best to run any discharge lamp on the correct ballast. Including a PFC capacitor if one is specified by the ballast manufacturer.

Well, with that desk lamp it actually means with no capacitor at all, because that is the way how the ballast was specified...

Hmmmm. But I'd be willing to bet putting one in would improve matters.

Hello,
You could compensate the desk lamp, I can also tell you the capacitor value however I need more data I need the Powerfactor, or system voltage, current and wattage. Personally I do not think it is necessary however I think it is good practice to bring systems to a power factor around cos(phi)=0,95 ind. As some people stated, unlike commercial installation a residential installation do not pay for reactive power. However i m a bit afraid that with smart meter that might change one day.

There a reason that I do not like the idea of compensating a desk lamp. A desc lamp is by definition a moveable appliance. If you take the plug out the plugsocket and the lamp was still on, even with a discharging resistor, you can still, for a brief moment be able to get a chock by the plug. Definitely a situation with might not kill you, but for sensitive people, especially people with heard problems it might be enough.

I read about on member adding a series capacitor to the ballast, I think that might reduce the risk for electric shock I wrote about above, however if the capacitor is not perfectly matched the lamp will be overdriven or under driven. I really prefer parallel compensation over any type of series compensation.



If you want I can show how you can calculate capacitor values for compensation.


Best regards,

Alex

The safety is indeed a point: on exposed plugs can not be higher than safe voltage (so 50V) more than about a second after disconnect. With few 100's nF (EMC filters in common SMPS's) that means about 1MOhm resistor (for 200nF). For 2x15W with series choke at 120V 60Hz the required capacitor would be around 6.3uF, which would require resistor around 50kOhm. And 50kOhm at 120V means extra 0.3W losses in the system.
It is not big, but it is extra losses not present when the capacitor is not used.
Losses that do not have to be there...

Given that the ballast losses are likely to be in the region of 10W or more, I don't think 0.3W is anything to be too worried about.