Author Topic: EEFL backlight circuits  (Read 1041 times)

Offline klugesmith

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EEFL backlight circuits
« on: November 08, 2020, 03:08:15 AM »
A dumpster-diving friend just gave me the backlight unit from a LCD TV set or monitor.
18 straight, thin lamps which I have learned are not CCFL, they are EEFL
(with external electrodes, and nothing but glass touching the plasma).

The external electrodes are all connected in parallel, which puzzles me.   
Each lamp's intrinsic glass-dielectric capacitors will offer substantial impedance at the operating frequency,
which I guess will serve to balance the currents. But how do they get all lamps to start?
Really high operating voltage on the shared AC bus?

Along with the panel I got one inverter unit, very similar to the one pictured here:

The part number on mine appears to have a Slave suffix, for use with a Master board and possibly other Slaves.
Is there a politically correct word-pair that conveys the relationship so well?

Anybody know if those are easy to operate on the bench?  Maybe just DC power and an Enable signal on the correct pins.
It wouldn't be too hard to trace the power path, which has a HV transformer and I guess an H-bridge of transistors in SO8 packages.
The one in picture (snipped from ebay) seems to have six of those transistors, so maybe a half-bridge.
« Last Edit: November 08, 2020, 03:16:50 AM by klugesmith »

Offline klugesmith

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Re: EEFL backlight circuits
« Reply #1 on: November 08, 2020, 07:42:29 AM »
Learned things from Internet, starting with a 2007 paper about work sponsored by Samsung.
* Study_on_the_equivalent_model_of_an_External_Elect.pdf

Scope view shows peak voltages around +/- 2500 V, and current on the order of 100 mA for whole array of lamps.


Still haven't found info about how to fire up an inverter module.

« Last Edit: November 08, 2020, 07:45:44 AM by klugesmith »

Offline klugesmith

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Re: EEFL backlight circuits
« Reply #2 on: November 11, 2020, 05:40:19 AM »
It was educational to see 18 luminous tubes in parallel all strike and glow with the same brightness, driven by a plain old 60 Hz NST. 
It's the series capacitance at each end of the EEFL's that limits and balances the current, with enough bus voltage to start every tube.   


In this case I stopped turning the variac at current of about 0.5 mA for the whole array.  About 28 microamps per lamp.   Indicated voltage is 5380 volts.  Couldn't go much higher without spurious arcs between lamp holder metal and the sheet metal panel.

Now that voltage comes from a combination of capacitive and resistive impedances.  For simplicity, assume i is sinusoidal and E = i/jwC +  i*R.
If we guess 28 pF per lamp end, whole array has 252 pF, so there's 5263 V across the equivalent capacitance and 1116 V across the barely lit plasma columns.
Guessing 30 pF per lamp end, 270 pF for array, E_C is 4912 V and E_R is 2195 volts.

How would 252 pF of equivalent capacitance figure into normal operation?
Say we want 180 times more current (5 mA per lamp, 90 mA for whole array).
If frequency is higher by factor of 900 (54 kHz)
then the voltage to push current through capacitor is down by factor of 5, to 1053 V.
I bet this is in the right ballpark.



Next exercise could be firing up the inverter which was salvaged with the backlight panel.
I found connector pinout (24 V DC, ground, on/off, and brightness control voltage) for a Master unit.
The one at hand is a Slave unit, I think with the control pins unused.


« Last Edit: November 11, 2020, 07:06:27 AM by klugesmith »

Offline Mads Barnkob

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Re: EEFL backlight circuits
« Reply #3 on: November 11, 2020, 09:39:49 AM »
Very long ago, 10 years or so, I experimented with CFL drivers like these.

Back then I just found some enable pins that was pulled high by the power supply in the monitor, not much else to get them running.

One of the funny applications I could see for CFL tubes was that they ignite from the same end of the tube and once you get duty cycle/voltage low enough, you can make it light up partially. I played around a bit with making a VU meter with a row of CFL tubes, but that project died when the tubes one day was broken by a dropped plier :(
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Offline klugesmith

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Re: EEFL backlight circuits
« Reply #4 on: November 13, 2020, 12:44:47 AM »
Bad pliers!  At least its fall didn't destroy a loaded ICBM, as in the Damascus accident (Arkansas, USA, 1980).

A closer look at my inverter board shows a transformer primary winding that's bundled fine wire, maybe even Litz wire.

.
The four main transistors say RSS100, which has standard pinout (SSSG,DDDD).
Driver says OZ9981GN, and I found an application circuit that looks promising.

.
from https://www.520101.com/html/internationaltv/002018461.html.
Is the text on that page Korean?
Does any reader here have a datasheet for the OZ9981GN?
« Last Edit: November 13, 2020, 12:52:41 AM by klugesmith »

High Voltage Forum

Re: EEFL backlight circuits
« Reply #4 on: November 13, 2020, 12:44:47 AM »

 


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