Hi Guys,

I have a problem with the backlights on a 17" LCD monitor going out
after about 30 seconds. Â*Turning off and back on causes them to come
back on but only for 10 seconds or so.

I took the thing apart and one of the two backlight PSU is getting very
hot very quickly. Any ideas anyone ?

Thanks in advance.
--
Best Regards:
Baron.

"Baron" wrote in message
...
Hi Guys,

I have a problem with the backlights on a 17" LCD monitor going out
after about 30 seconds. Turning off and back on causes them to come
back on but only for 10 seconds or so.

I took the thing apart and one of the two backlight PSU is getting very
hot very quickly. Any ideas anyone ?

Thanks in advance.
--
Best Regards:
Baron.

Either the lamps are failing and causing the ballast to overheat, or the
ballast itself is failing. Suggest swapping the ballast to the other lamp
and see if you maintain the same heat rise, if so then you know the lamps
probably are ok. Overheating ballast could be caused by failing
electrolytic capacitors or other devices.

Art wrote:



"Baron" wrote in message
...
Hi Guys,

I have a problem with the backlights on a 17" LCD monitor going out
after about 30 seconds. Turning off and back on causes them to come
back on but only for 10 seconds or so.

I took the thing apart and one of the two backlight PSU is getting
very hot very quickly. Any ideas anyone ?

Thanks in advance.
--
Best Regards:
Baron.

Either the lamps are failing and causing the ballast to overheat, or
the ballast itself is failing. Suggest swapping the ballast to the
other lamp and see if you maintain the same heat rise, if so then you
know the lamps
probably are ok. Overheating ballast could be caused by failing
electrolytic capacitors or other devices.

I thought electrolytics, but there is only one on this board and its
common to both PSU. I'll try swapping the lamps around and re-check.

Would a failing lamp be any dimmer ? The light across the screen seems
equal. No dark areas or bands. Turning down the brightness doesn't
alter the length of time they stay on. Another thing I noticed was
that the output capacitors, there is two in series for each lamp, seem
to be very small values. They are marked 18J 3Kv. I assume that is
18pf 5% 3000 volts working.

--
Best Regards:
Baron.

On Wed, 19 Mar 2008 20:57:29 +0000, Baron
put finger to keyboard and
composed:

Another thing I noticed was
that the output capacitors, there is two in series for each lamp, seem
to be very small values. They are marked 18J 3Kv. I assume that is
18pf 5% 3000 volts working.

An 18pF capacitor would have an impedance at 100kHz of ...

1/(2.pi.f.C)

= 1/(2 x 3 x 10^5 x 18 x 10^-12)

= 1/(108 x 10^-7)

= 100Kohm

If 100Kohm were in series with the lamp, then the lamp would be
drawing a miniscule amount of current.

- Franc Zabkar
--
Please remove one 'i' from my address when replying by email.

Franc Zabkar wrote:
On Wed, 19 Mar 2008 20:57:29 +0000, Baron
put finger to keyboard and
composed:

Another thing I noticed was
that the output capacitors, there is two in series for each lamp, seem
to be very small values. They are marked 18J 3Kv. I assume that is
18pf 5% 3000 volts working.

An 18pF capacitor would have an impedance at 100kHz of ...

1/(2.pi.f.C)

= 1/(2 x 3 x 10^5 x 18 x 10^-12)

= 1/(108 x 10^-7)

= 100Kohm

If 100Kohm were in series with the lamp, then the lamp would be
drawing a miniscule amount of current.

- Franc Zabkar
At 1000 volts and 100kohms there is 10ma current which is ten watts.
This is about what these lamps run at.

Franc Zabkar wrote:

On Wed, 19 Mar 2008 20:57:29 +0000, Baron
put finger to keyboard and
composed:

Another thing I noticed was
that the output capacitors, there is two in series for each lamp, seem
to be very small values. They are marked 18J 3Kv. I assume that is
18pf 5% 3000 volts working.

An 18pF capacitor would have an impedance at 100kHz of ...

1/(2.pi.f.C)

= 1/(2 x 3 x 10^5 x 18 x 10^-12)

= 1/(108 x 10^-7)

= 100Kohm

If 100Kohm were in series with the lamp, then the lamp would be
drawing a miniscule amount of current.

- Franc Zabkar

Thanks Franc,
I followed your maths perfectly.

--
Best Regards:
Baron.

Baron wrote:

Franc Zabkar wrote:

On Wed, 19 Mar 2008 20:57:29 +0000, Baron
put finger to keyboard and
composed:

Another thing I noticed was
that the output capacitors, there is two in series for each lamp,
seem
to be very small values. They are marked 18J 3Kv. I assume that is
18pf 5% 3000 volts working.

An 18pF capacitor would have an impedance at 100kHz of ...

1/(2.pi.f.C)

= 1/(2 x 3 x 10^5 x 18 x 10^-12)

= 1/(108 x 10^-7)

= 100Kohm

If 100Kohm were in series with the lamp, then the lamp would be
drawing a miniscule amount of current.

- Franc Zabkar

Thanks Franc,
I followed your maths perfectly.

I found a lot of references including the circuit diagram for my
particular monitor backlight inverter.

It seems that 18pf is indeed correct and that there are in fact four
back light tubes each with 18pf in series with it. The tubes need
about 1100 volts + to strike and then run at around 700 volts.

In response to another post, because there are two inverters I swapped
ends and the overheating stays at the same end. There is a 0.22uf 100
volt capacitor across each of the transformer primary which in the case
of one of them gets hot and changes value resulting in the current
limiter shutting them both down.

The bad capacitor rapidly looses value when hot air is blown on it. I
hope that replacing this will effect a cure. I intend replacing both
caps to make sure that I don't have to come back and replace the other
later.

Thanks for the support it's appreciated.

--
Best Regards:
Baron.

On Thu, 20 Mar 2008 15:52:52 +1100, Franc Zabkar
put finger to keyboard and composed:

On Wed, 19 Mar 2008 20:57:29 +0000, Baron
put finger to keyboard and
composed:

Another thing I noticed was
that the output capacitors, there is two in series for each lamp, seem
to be very small values. They are marked 18J 3Kv. I assume that is
18pf 5% 3000 volts working.

An 18pF capacitor would have an impedance at 100kHz of 100Kohm

If 100Kohm were in series with the lamp, then the lamp would be
drawing a miniscule amount of current.

- Franc Zabkar

After some research, it appears that 18pF makes sense.

The following datasheet uses an example with a 50kHz resonant
frequency, 5mA operating current, and a ballast capacitance
of 22pF:

http://focus.ti.com/lit/ds/symlink/ucc3972.pdf

The lamp specs (for a notebook) a

Lamp Length 250mm (10”)
Lamp Diameter 6mm
Striking Voltage (20°C) 1000V (PEAK)
Operating Voltage (5mA) 375V (RMS)
Full Rated Current 5mA
Full Rated Power 1.9W

Sorry for my error.

- Franc Zabkar
--
Please remove one 'i' from my address when replying by email.