Repairing a CFL that stopped working after a few hours of use

Thread Starter

seanspotatobusiness

Joined Sep 17, 2016
210
A long time ago I bought a multipack of 11 watt CFL bulbs and I no longer have the receipt. I put one of them into service and after a few hours of use (and about three ignitions) it has stopped working. I've opened it up and I see nothing obviously wrong. What parts can I test? How many hundreds of volts does the energised ballast circuit produce?
 

DickCappels

Joined Aug 21, 2008
10,153
One of the diodes --the black onw which is a rectifier should have less than about 5 microamps of reverse leakage at room temperature. The blue one is (probably) a diac and with less than its firing voltage (usually 28 to 40 volts) should have less than about 10 microamps reverse leakage at room temperature.

Is that little TO-92 transistor the only one?
 

SLK001

Joined Nov 29, 2011
1,549
I don't see the switching transistors (there should be at least one). Maybe they are surface mounted on the back?
 

DickCappels

Joined Aug 21, 2008
10,153
Years ago Philips developed some TO-92 transistors for these low power lamps but I have only seen them used in pairs. For higher power lamps it is common to see a pair of husky versions of the MJE13003, which is much larger.

The TO-92 looks like a transistor, maybe you can use your DVM to tell whether it is ok or not.
 

Thread Starter

seanspotatobusiness

Joined Sep 17, 2016
210
I don't know whether this video is clear enough but there are two TO-92 transistors.


In this video I test the transistors and the diodes (not the diac) and I think the transistors are okay but the diodes do not resist reverse current.

 

Alec_t

Joined Sep 17, 2013
14,280
If the low impedance were intentional and very low, then yes. But if the low impedance were due to some other, failed component then a healthy diode could be mis-interpreted as a dead diode.
 

DickCappels

Joined Aug 21, 2008
10,153
Sorry if this is a stupid thing to ask but wouldn't a low impedance in parallel with the diode make the diode pointless?
It can only be a stupid question if you don't ask it.

There is a possibility of the timing transformer appearing across the diode and looking like a short to the ohmmeter but looking very different to high frequency signals.
 

Thread Starter

seanspotatobusiness

Joined Sep 17, 2016
210
The diodes are actually fine so I still need to find what is causing the short circuit.

Incidentally I have a capacitor which I may have overlooked before but looks damaged to me and has a measured capacity of 1.2 nF:

 

Thread Starter

seanspotatobusiness

Joined Sep 17, 2016
210
Okay, I will; thanks.

Unfortunately, there is still a short circuit across where the diodes are (and it's not the diodes at fault) so I think there must be another flaw somewhere. Based on my reading, it looks like it will not be trivial to test the DIAC since it needs to be used at high voltage.
 

DickCappels

Joined Aug 21, 2008
10,153
Take note if the markings on the capacitor. I have a strong suspicion that it has a safety rating of "X1" or something similar. In that case it should be replaced with a capacitor with the same rating.

The capacitor almost certainly failed because it was defective to begin with. There should be a fuse resistor in series with the capacitor at the AC input to the circuit that is normally a few to a hunderd ohms. The failure if the capacitor may have caused that resistor to fail open.
 

Thread Starter

seanspotatobusiness

Joined Sep 17, 2016
210
Thanks DickCappels, I'm still trying to interpret the circuit but I think it might roughly match this schematic with some slightly different values. The 47 nF capacitor is the one next to the fluorescent lamp. Edit: it can't match the schematic because in my circuit I think the diodes and transistors are in parallel across collector and emitter (assuming the middle leg is the base) and also the diodes are not in series.



In the below pictures I've already removed the capacitor, one transistor and disconnected on leg of the two through-hole diodes. Also, I did flip one of the images horizontally in order to line them up for the overlay. I'm wondering whether the short circuit across the diodes doesn't matter if the circuit is a high frequency (I mean if it's part of the design)?




 
Last edited:

R!f@@

Joined Apr 2, 2009
9,918
The short might exist if the diode has a low value resistor across it or if it is across a coil.
Remove it and check
 

Thread Starter

seanspotatobusiness

Joined Sep 17, 2016
210
The short might exist if the diode has a low value resistor across it or if it is across a coil.
Remove it and check
I've removed one leg of both diodes and they are not at fault. I'm thinking that the 220 resistors are maybe 22 ohms rather than 220? Then the path is as below; it goes through some coil thing which presumably resists AC and therefore isn't really a short circuit over the diode?

 
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