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Old 03-11-2017, 11:54 AM   #1
caphair
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Default Fluorescent desk lamp repair

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?
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Old 03-11-2017, 01:46 PM   #2
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Default Re: Fluorescent desk lamp repair

thank you
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Old 03-11-2017, 02:05 PM   #3
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Default 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
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Old 03-11-2017, 02:09 PM   #4
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Default 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 .
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Old 03-11-2017, 02:39 PM   #5
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Default Re: Fluorescent desk lamp repair

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.
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Old 03-11-2017, 02:59 PM   #6
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Default Re: Fluorescent desk lamp repair

Quote:
Originally Posted by redwire View Post
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?
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Old 03-11-2017, 09:46 PM   #7
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Default Re: Fluorescent desk lamp repair

I say you can use a larger value OK. But it will pop... larger... If the transistors are bad.
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Old 03-11-2017, 10:14 PM   #8
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Default Re: Fluorescent desk lamp repair

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Originally Posted by redwire View Post
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

Last edited by caphair; 03-11-2017 at 10:15 PM..
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Old 03-11-2017, 10:44 PM   #9
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Default 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.
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Old 03-11-2017, 11:16 PM   #10
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Default Re: Fluorescent desk lamp repair

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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?
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Old 03-12-2017, 05:14 PM   #11
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Default Re: Fluorescent desk lamp repair

Resistors are cheaper than proper fuses.
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Old 03-12-2017, 05:23 PM   #12
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Default 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 .

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Old 03-12-2017, 05:32 PM   #13
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Default 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.
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Old 12-12-2021, 12:30 PM   #14
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Default 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
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Old 12-12-2021, 01:55 PM   #15
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Default Re: Fluorescent desk lamp repair

did the tube go this time?
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Old 12-12-2021, 02:02 PM   #16
caphair
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Default Re: Fluorescent desk lamp repair

Quote:
Originally Posted by eccerr0r View Post
did the tube go this time?
Yes but even after replacing itís not working
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Old 12-13-2021, 09:33 AM   #17
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Default 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?
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Old 12-13-2021, 10:43 AM   #18
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Default 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.
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Old 12-13-2021, 11:01 AM   #19
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Default Re: Fluorescent desk lamp repair

What would cause it to be low? The tube is small and circular probably 12” give or take
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Old 12-13-2021, 11:45 AM   #20
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Default Re: Fluorescent desk lamp repair

strike voltage probably comes from an inductor or cap discharging
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