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-   -   Fluorescent desk lamp repair (https://www.badcaps.net/forum/showthread.php?t=60987)

caphair 03-11-2017 11:54 AM

Fluorescent desk lamp repair
 
2 Attachment(s)
Turned it on and heard a loud pop, opened it and a small 10uf 50v electrolytic capacitors had blown up.

Is it safe to replace with a higher value like 47uf 100v? Or does it have to be exact value of old?

kamel84 03-11-2017 01:46 PM

Re: Fluorescent desk lamp repair
 
thank you

petehall347 03-11-2017 02:05 PM

Re: Fluorescent desk lamp repair
 
i would put a regular ballast in and starter if it doesn't already have one . less to go wrong

petehall347 03-11-2017 02:09 PM

Re: Fluorescent desk lamp repair
 
just for fun here is an idea to use lamps that look worn out but arent . can be used on good lamps too .
http://2.bp.blogspot.com/-yXvNUmOyxY...ht-circuit.png

redwire 03-11-2017 02:39 PM

Re: Fluorescent desk lamp repair
 
2 Attachment(s)
That capacitor is probably for limiting base-drive, so if it popped I think a transistor is shorted. Usually the lamp fails open circuit and then the PSU dies.

These schematics are similar to your circuit but not exact - it's a two transistor resonant oscillator. The Philips/Osram one has no 10uF 50V cap.
I don't see filament connections in that desklamp, only 2-wires leaving the PSU.

caphair 03-11-2017 02:59 PM

Re: Fluorescent desk lamp repair
 
Quote:

Originally Posted by redwire (Post 732546)
That capacitor is probably for limiting base-drive, so if it popped I think a transistor is shorted. Usually the lamp fails open circuit and then the PSU dies.

These schematics are similar to your circuit but not exact - it's a two transistor resonant oscillator. The Philips/Osram one has no 10uF 50V cap.
I don't see filament connections in that desklamp, only 2-wires leaving the PSU.

Thank you. So could I use a higher value capacitor or has to be exact?

redwire 03-11-2017 09:46 PM

Re: Fluorescent desk lamp repair
 
I say you can use a larger value OK. But it will pop... larger... If the transistors are bad.

caphair 03-11-2017 10:14 PM

Re: Fluorescent desk lamp repair
 
Quote:

Originally Posted by redwire (Post 732630)
I say you can use a larger value OK. But it will pop... larger... If the transistors are bad.

Tried one nothing happened, no light as well. Does that mean transistors are ok since it didn't blow?

Curious when is the exact capacitor value replacement necessary? What circuits can't be altered by higher capacitor values than original

eccerr0r 03-11-2017 10:44 PM

Re: Fluorescent desk lamp repair
 
petehall347, I'm sorry, I'm tired of those magnetic ballasts that flicker at 100/120 cycles/second... The electronic ballasts, though quite a bit more fragile, are smaller, lighter, more power efficient, and usually starts lighting faster.

caphair, likely you will have to carefully check each of the transistors for shorts, as well as making sure the input fuseable resistor didn't fry. Unfortunately all of the electronic ballasts I've had when they do go, tend to fry transistors and then input circuitry - a big domino effect.

caphair 03-11-2017 11:16 PM

Re: Fluorescent desk lamp repair
 
Quote:

Originally Posted by eccerr0r (Post 732638)
petehall347, I'm sorry, I'm tired of those magnetic ballasts that flicker at 100/120 cycles/second... The electronic ballasts, though quite a bit more fragile, are smaller, lighter, more power efficient, and usually starts lighting faster.

caphair, likely you will have to carefully check each of the transistors for shorts, as well as making sure the input fuseable resistor didn't fry. Unfortunately all of the electronic ballasts I've had when they do go, tend to fry transistors and then input circuitry - a big domino effect.

Why are fuseable resistors used and not regular fuses?

cpt.charlie 03-12-2017 05:14 PM

Re: Fluorescent desk lamp repair
 
Resistors are cheaper than proper fuses.

petehall347 03-12-2017 05:23 PM

Re: Fluorescent desk lamp repair
 
might not flash the same running off dc voltage . i have to try it soon so will let you know if it works out ok .
we are on 50hz here so its worse for me .:smile:

budm 03-12-2017 05:32 PM

Re: Fluorescent desk lamp repair
 
Why fusible resistor?
http://powerelectronics.com/passive-...t-applications

The flickering rate will be 100 Hz,
Electronic ballast uses much higher frequency.

caphair 12-12-2021 12:30 PM

Re: Fluorescent desk lamp repair
 
Revisiting this. Changed the cap and one of the transistors previously and it worked all this time.

Just recently quit again, this time no caps blew up and no shorts on the transistors or diodes or any of the caps (tested in ohms settings)

Any suggestions? I should just replace it but curious to get it going again

eccerr0r 12-12-2021 01:55 PM

Re: Fluorescent desk lamp repair
 
did the tube go this time?

caphair 12-12-2021 02:02 PM

Re: Fluorescent desk lamp repair
 
Quote:

Originally Posted by eccerr0r (Post 1091139)
did the tube go this time?

Yes but even after replacing itís not working

caphair 12-13-2021 09:33 AM

Re: Fluorescent desk lamp repair
 
How many volts should I see at the lamp terminals when first powered on? I’m measuring 126v then drops to 86v

Does that mean the circuitry is working and possible bad replacement lamp?

eccerr0r 12-13-2021 10:43 AM

Re: Fluorescent desk lamp repair
 
OCV it should be really high and then drop down fast when the light strikes. 126V seems a bit low depending on the length of the tube.

caphair 12-13-2021 11:01 AM

Re: Fluorescent desk lamp repair
 
What would cause it to be low? The tube is small and circular probably 12” give or take

stj 12-13-2021 11:45 AM

Re: Fluorescent desk lamp repair
 
strike voltage probably comes from an inductor or cap discharging


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