Download Testing LCD displays

Testing LCD displays
No real test fixture required. Power and a signal source.
Power requirements
All the LCD displays I have run into so far run off of +12 Volts. 18” or 19” displays seem to draw
around 3.5 to 4 Amps. On my bench I have a 6 Amp variable supply with metered Volts and Amps. It
serves me well yet again. Getting power to the LCD is another story. Some have the power plug exposed
and some don’t. On those that don’t I have rigged a “Y” cable that plugs into the connector going to the
fan.
Video signal requirements
Standard Video (if there is such a thing). VGA, 640 x 480, 31.5 KHz Horizontal, 60 Hz vertical.
On most CM125 or CM2125 this is Recall 7, but it may not necessarily be on your setup.
Common failures
CCFL Inverter
Bent pins on the VGA connector
Testing CCFL Inverters
Not much of a test fixture required. Just connect lamps and apply power. This also gives us a test fixture to
test the lamps themselves.
5-pin Inverters for Williams Kristel LCD Displays
1 – +12 V @ about 1.5 Amps
2 – Ground
3 – Brightness (Ground is highest, +12 V is lowest, or just ground it)
4 – (n/c)
5 – On / Off Control (+12 V is On, floats off, or just run it to +12 Volts)
How much current should we have at the input? These assemblies are blowing 3 Amp fuses, and
suggestions are made to replace the 3 Amp fuse with a 5 Amps when it blows. To find out how much
current we should have at the input of the Inverter we start with what we cal tell from a data sheet and work
our way back to the input. Not having a data sheet for the exact tube we have to make the assumption that
all CCFL tubes are about the same if they are the same diameter and length. Running down a pile of data
sheets on various CCFL tubes I get that a tube this size would run at about 860 Volts and draw about 5 mA.
That may not be exact, so we won’t bother with doing exact math.
Running voltage for the tubes is about 860 Volts, AC, 30 KHz, at 5 mA per tube. There are two tubes on
each side. Volts times Amps gives us about 4.3 Watts per tube, times four tubes for a total of about 17
Watts at the output. With an input of 12 Volts our input current should be about 1.5 Amps (Watts divided
by Volts gives us Amps), assuming a bit less than 100% efficiency. We can’t have any more power at the
output than we have at the input. So we can make a good guess that our input current should be around 1.5
Amps (with a little leeway).
Sure enough, when we bring one up under power on the bench it comes up at around 1.5 Amps. I suggest
using real tubes. Others suggest resistors. Nothing wrong with resistors at all. They are a reliable static
load. Using real tubes gives a lively dynamic load that behaves exactly like the tubes do.
If you use resistors R. E. Sult suggests a string of 50,000 Ohm, 10 Watt resistors. An excellent load. 860
Volts at 5 mA comes to about 170K Ohms. Since we will be testing various Inverters with an assortment of
lamps a string of five 50,000 Ohm resistors makes good sense. Hats off to Mr. Sult.
So why are we blowing 3 Amp fuses if our expected current should only be 1.5 Amps? Why does the
vendor suggest replacing the 3 Amp fuse with a 5 Amp?
CCFL lamps
Unlike HCFL (Hot Cathode Fluorescent Lamps), CCFL (Cold Cathode Fluorescent Lamps) do not
have heaters at the end to ionize the gas inside the tube. HCFL have heaters that run at a few volts to heat
the gas up. Once heated and ionized it conducts and the starter cuts out of the circuit. CCFL lamps have no
heaters. To ionize the gas we apply a high voltage. The voltage depending on the length of the tube. The
longer the tube the higher the voltage. 200 or 300 volts for a tube 4 or 5 inches long, up to 800 or 900 Volts
for the longer tubes 10 to 12 inches long. The diameter of the tube dictates current. 2 mm to 5 mm is typical
diameter, and gives us 5 mA to 9 mA of current. Each tube has a fairly wide operating range and we can
get a small variety of color and brightness by changing the voltage.
The resistance of the tube is dynamic. That is, it changes as conditions change. Initially it has a
high resistance until the gas ionizes. That 860 Volts may require over 1,000 Volts for a few cycles
(typically less than 10 cycles). Once the gas ionizes the voltage of the tube drops down to the operating
voltage range and the current stabilizes. At lower temperatures the tube may become dimmer because a
higher voltage is required to ionize the gas. Remember all we do when we apply a voltage is apply energy
to the system. At lower temperature we need to apply more energy to raise the gas to the ionized state.
Like most CCFL tubes we have electrodes running through the ends of the tube. Over time this
point leaks gas. As the tube heats up and cools down this seal degrades. This happens with HCFL and
CCFL alike. As the tube gets older and the gas inside goes bad it takes a higher voltage to ionize the gas.
The Inverter normally has a resistor in parallel with the tube to monitor the tube current. When this current
decreases the regulator increases the voltage to the tube to keep the brightness consistent. We could also
use this circuit to monitor for excessive current and lower the voltage to the tube.
Ah! What causes the tubes to increase in current? The gas inside the tube is a Mercury vapor.
CCFL (as well as HCFL) need AC going through them. DC currents will drastically decrease the
resistance. Normally in parallel with the tube we will find a capacitor. Something small, in the 15 pF to 22
pF range depending on the size of the tube and rated at around thousands of Volts for noise. As these
capacitors age DC leaks into the system and current increases, just as it does in HCFL DC Ballasts.
EEFL
Just to keep up on coming trends, the next following rage in HCFL and CCFL tubes is EEFL
(External Electrode Fluorescent Lamps). The electrodes are a metal wrap around the ends of the tube where
the tube is thinner. This does away with the seal that is the weakness CCFL and HCFL lamps. Where
CCFL and HCFL may have life expectancies in the tens of thousands of hours (2 ¼ to 3 ½ years) EEFL
may have life expectancies in the hundreds of thousands of hours. This gives us lighting that would last the
life of the game, around 10 years.