repair desktop fluorescent lamp - Page 1
Forum section: electronics / repair

author: senderj
author_rank: Newbie
timestamp: June 04, 2015, 06:40:53 am
content: I have a National desktop fluorescent lamp that failed to work. Every time I press the power button, it blink once and doesn't light up. When I then press the button again (intended to put it to off state), it blink once again after 8 seconds. I trace the PCB and found that it is almost the same as this one.except that the values of the components are different. The C3 is 1.25kV 5.1nF while the C6 is 400V 470nF. I suspect that it is the C3 that failed. Photo hereI don't have exact replacement, but I have two 630V 10nFand thinking of connecting them in series to replace C3. I think the calculation is correct, but I don't know if the type of capacitor and the current loading match as replacement or not. The 10nF is much smaller in size. Can somebody please help me on this.

author: DmitryL
author_rank: Regular Contributor
timestamp: June 04, 2015, 08:59:10 am
content: Connecting capacitors in series in order to increase working voltage is a bad idea (uless you have resistors in parrallel with each capacitor). The resulting capacitor will have the same safe voltage rating as the lowest from the pair...its like connecting fuses in parallel to increase current rating, bad idea

author: senderj
author_rank: Newbie
timestamp: June 04, 2015, 09:38:21 am
content: "unless you have resistors in parrallel with each capacitor"Thanks for the reply DmitryL. Can you please explain why having resistors in parrallel can reduce the risk? Is it has something to do with the internal resistance of the capacitors?

author: DmitryL
author_rank: Regular Contributor
timestamp: June 04, 2015, 11:00:05 am
content: Yes, there will always be a difference in capacitors' impedance and leakage currents.Imagine that you have one capacitor with leakage curent 0.1uA and another worse capacitor with a leakage current 0.5uA. Then you connect them in series and start charging for 2xtheir nominal voltage.

author: edavid
author_rank: Super Contributor
timestamp: June 04, 2015, 02:34:45 pm
content: Quote from: senderj on June 04, 2015, 06:40:53 amWhy? It looks fine in the photo.

author: IanB
author_rank: Super Contributor
timestamp: June 04, 2015, 02:56:45 pm
content: Quote from: edavid on June 04, 2015, 02:34:45 pmLooks can be deceiving:https://youtu.be/qGc9-ToEiIQ

author: SeanB
author_rank: Super Contributor
timestamp: June 04, 2015, 06:35:55 pm
content: More likely that R6 is open circuit, or the lamp has failed. Blinking is typical of the lamp having lost emission, and failing to start up is also typical of that or the resistor R6 going open circuit

author: senderj
author_rank: Newbie
timestamp: June 05, 2015, 03:32:06 am
content: Thanks for all the helps.@DmitryL, I use 2 cap in series so each bears 1/2 of the voltage in the circuit. The original cap is labelled 1.25kV, so 1/2 of it is 625V which is within the 630V of the replacement. But current leakage is what out of my knowledge.@edavid, I suspect C3 because of this articlehttp://www.pavouk.org/hw/lamp/en_index.htmlsaid most likely failure is C3.@SeanB, the lamp should be fine since it's new and I tested it before I buy. If R6 open, there shouldn't be any startup blink. Will measure it tonight to confirm when I return home. From what I observed (see my first post) the tube can startup and cannot sustain, so I suspect it's the resonant circuit C3, C6, L1, TR1(1-4). For Q1, I measured the resistance of each leg and is the same as Q2. So I guess it is fine.

author: senderj
author_rank: Newbie
timestamp: June 08, 2015, 06:48:10 am
content: The problem turns out to be in C4 !! It's output was 293V when I start trying to resolve the problem so I didn't suspect it has any problem. Since I am not sure whether I can use 2 capacitors in series to replace C3 which sounds dangerous (that was why I ask here), I started to replace the easy ones first. My first attempt is C4 since I have a 100uF 400V capacito on hand and it seems of lower risk. With this replacement, the output now is 303V and it works. I think the old one may be dried out causing internal resistance to drop and hence the output is insufficient to drive the rest of the circuit.Thank you for all the helps here.