Fluorescent desk lamp repair

**caphair** · 03-11-2017, 11:54 AM

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

thank you

**petehall347** · 03-11-2017, 02:05 PM

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

just for fun here is an idea to use lamps that look worn out but arent . can be used on good lamps too .

**redwire** · 03-11-2017, 02:39 PM

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

Quote:

Originally Posted by redwire

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

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

Quote:

Originally Posted by redwire

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

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

Quote:

Originally Posted by eccerr0r

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

Resistors are cheaper than proper fuses.

**petehall347** · 03-12-2017, 05:23 PM

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 .

**budm** · 03-12-2017, 05:32 PM

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

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

did the tube go this time?

**caphair** · 12-12-2021, 02:02 PM

Quote:

Originally Posted by eccerr0r

did the tube go this time?

Yes but even after replacing it’s not working

**caphair** · 12-13-2021, 09:33 AM

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

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

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

strike voltage probably comes from an inductor or cap discharging

03-11-2017, 01:46 PM	#2
kamel84 New Member Join Date: Feb 2016 City & State: now My Country: islam I'm a: Knowledge Seeker Posts: 9	Re: Fluorescent desk lamp repair thank you

03-11-2017, 02:05 PM	#3
petehall347 Badcaps Veteran Join Date: Jan 2015 City & State: worcester My Country: United Kingdom I'm a: Knowledge Seeker Posts: 3,215	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

03-11-2017, 02:09 PM	#4
petehall347 Badcaps Veteran Join Date: Jan 2015 City & State: worcester My Country: United Kingdom I'm a: Knowledge Seeker Posts: 3,215	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 .

03-11-2017, 09:46 PM	#7
redwire Badcaps Veteran Join Date: Dec 2010 City & State: Alberta My Country: Canada Posts: 2,474	Re: Fluorescent desk lamp repair I say you can use a larger value OK. But it will pop... larger... If the transistors are bad.

03-11-2017, 10:44 PM	#9
eccerr0r Solder Sloth Join Date: Nov 2012 City & State: CO My Country: USA Line Voltage: 120VAC 60Hz I'm a: Hobbyist Tech Posts: 7,089	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.

03-12-2017, 05:14 PM	#11
cpt.charlie Badcaps Veteran Join Date: Sep 2013 City & State: Torrelavega My Country: Spain Line Voltage: 230VAC 50Hz I'm a: Knowledge Seeker Posts: 255	Re: Fluorescent desk lamp repair Resistors are cheaper than proper fuses.

03-12-2017, 05:23 PM	#12
petehall347 Badcaps Veteran Join Date: Jan 2015 City & State: worcester My Country: United Kingdom I'm a: Knowledge Seeker Posts: 3,215	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 . Last edited by petehall347; 03-12-2017 at 05:25 PM..

03-12-2017, 05:32 PM	#13
budm Badcaps Veteran Join Date: Feb 2010 City & State: S.F. Bay area My Country: USA Line Voltage: 120V 60Hz I'm a: Knowledge Seeker Posts: 39,206	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. __________________ Never stop learning Basic LCD TV and Monitor troubleshooting guides. http://www.badcaps.net/forum/showthr...956#post305956 Voltage Regulator (LDO) testing: http://www.badcaps.net/forum/showthr...999#post300999 Inverter testing using old CFL: http://www.badcaps.net/forum/showthr...er+testing+cfl Tear down pictures : Hit the ">" Show Albums and stories" on the left side http://s807.photobucket.com/user/budm/library/ TV Factory reset codes listing: http://www.badcaps.net/forum/showthread.php?t=24809

12-12-2021, 12:30 PM	#14
caphair Badcaps Veteran Join Date: Nov 2011 Posts: 1,110	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

12-12-2021, 01:55 PM	#15
eccerr0r Solder Sloth Join Date: Nov 2012 City & State: CO My Country: USA Line Voltage: 120VAC 60Hz I'm a: Hobbyist Tech Posts: 7,089	Re: Fluorescent desk lamp repair did the tube go this time?

12-13-2021, 09:33 AM	#17
caphair Badcaps Veteran Join Date: Nov 2011 Posts: 1,110	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?

12-13-2021, 10:43 AM	#18
eccerr0r Solder Sloth Join Date: Nov 2012 City & State: CO My Country: USA Line Voltage: 120VAC 60Hz I'm a: Hobbyist Tech Posts: 7,089	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.

12-13-2021, 11:01 AM	#19
caphair Badcaps Veteran Join Date: Nov 2011 Posts: 1,110	Re: Fluorescent desk lamp repair What would cause it to be low? The tube is small and circular probably 12” give or take

12-13-2021, 11:45 AM	#20
stj Great Sage 齊天大聖 Join Date: Dec 2009 City & State: Europe My Country: some shithole run by Israeli agents I'm a: Professional Tech Posts: 26,450	Re: Fluorescent desk lamp repair strike voltage probably comes from an inductor or cap discharging

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