Testing CCFLs

[OldLes]

I have been looking at "the other forum" where I came upon a reference to a tester for CCFLs, which I see can be purchased from Chas Hyde in the UK. At £100, rather more than I am inclined to pay.
I am sure somebody will have taken an old inverter board, cut it down somewhat, and found a way to use it as a CCFL tester.
I have a 42" Sharp with at least three faulty tubes, but with 18 in there, not easy to locate which ones, dismantle and remove and replace the correct ones.
So, has anybody made such a tester, and if so, what was involved?
OldLes.

[fix2003]

try ebay
400349118892

[Keith]

Hi Oldles,

I have one like the one Fixitnow has linked to and it is only good for up to 20"

This is better I think, 150948891826 on Ebay. I am in fact waiting for one to turn up.

You could modify an inverter board by disabling the protection circuits, again which I have, I just want something more easier / convenient to use.

Testing CCFLs is not 100% with any tester. These testers, including the £100 ones, only really show how the lamp lights. If the lamp lights with pink or blue tinges it duff! But the lamp may be drawing excessive current which will cause a protection fault and  the drive IC to shut down. So it is also necessary to measure the current for each lamp. Also, these testers can not detect a slow starting tube. The drive IC expects the tubes to strike and start within a specified time, if one doesn't, again the drive IC will go into protection.

When mine turns up I will let you know if works OK

Keith

[downunder]

Go to www.shop.television-magazine-forum.co.uk and check out their EMR Diagnostika.
Available for 40 pounds in the UK.

[Keith]

The EMR Diagnostika only checks that the inverter circuit is working, in the same way that if you put your scope probe near the transformers to detect any EMR. It doesnt test CCFL tubes.

Keith

[dynomo]

try ebay
400349118892

Looks exactly the same as the ones used in pc's to light cold cathode tubes. Only they added a 12 volt transformer to run it off the mains and made some leads with croc clips. Not sure if they would be any use to run larger tubes. Like Keith suggests Ok for 20 inches and under i expect.

[OldLes]

Hi guys, thanks for replies. I told my pal last night, and he sent me a link to a you tube video.
http://www.youtube.com/watch?v=CK7rlLRHO3U
I have drawn out the circuit (below, subject to a recheck, and not sure about the transformer, but maybe almost any off a board will do).
After reading Kieth's Ebay link, I somehow don't think the Utube one would work off a PP3.
If I build it, I will probably add a milliameter (or a voltmeter across a resistor). Maybe different voltages for different length CCFLs.
Please comment.
OldLes.

[fix2003]

i run my tster from my 12 v bench power unit this way i can check the current (using its built in display)of each ccfl as i have had them come on but using a lot more current

[OldLes]

I have just been looking at CCFL specs. Approx values as follows:--

500mm tube length. Striking voltage 1,000. Running voltage 850 at 6mA current

900mm tube length. Striking voltage 1,500. Running voltage 1,300 at 6mA current.

Having regard to the points Kieth makes earlier regarding colour, current and start up time, it seems that a suitable tester should at least have a short/long length switch.

Looking at inverter boards, it seems that typically the HV secondary is centre tapped. Perhaps use one half only for short, end to end output for long lamps?
Any comments? Any suggestions for the best inverter unit to bodge, and how to over-ride the protection circuits etc.

It seems 40-60Khz is optimum frequency for operation. Would it matter for testing? I wonder what frequency that circuit I posted yesterday runs at? Probably less than 1000Hz? I built something similar 40 years ago to power a car radio when the vibrator "inverter" packed in, but I think that only ran at about 30Hz. Obviously the transformer will be important for frequency determination.
Les.

Edit. A quick calculation suggests that with 100% efficiency, a variable supply of 6-12v and adjustable from 0.5 to 1amp would be needed, rather than the PP3 of that earlier circuit.      Les.

[OldLes]

Maybe it was a mistake starting the Samsung inverter thread separately, time to bring it back together, but I will be referring to the circuit posted there.
With 12 v applied (-ive to 1,3,5,6,8 and 9.   +ive to 10, 11 and 12) application of the +ive to pin 4 via 10K fires up the inverter for about 2 to 3 seconds. I have not found any effect of using pin 2 via a resistor to anywhere.   (Above ref Nos. all CN1)
Using a 100:1 probe, measurement across CN2 yields 2,700vpp, equiv to 950V rms.
Using two probes in add mode, from output at blue capacitor end of CN2 to same point off CN3 yields 7,800vpp, or 2,760v rms. (Not sure that makes total sense, but it does suggest one output pair OK for 20" CCFLs, and possibly the two "hot" outputs of CN3/CN4 will drive the 42" ones).
Hopefully I will test it out on some CCFLs tomorrow, and if OK, I can look again at my Sharp LC-42DH77E mentioned in its own thread.
More input welcomed.
Les.

[OldLes]

I tried the samsung inverter for real this morning, but I only had CCFLs from 32" and 40" sets.
The "standard" output (950v rms off load) did nothing whatever on either size of tube.
Trying the two high level outputs in series as explained yesterday (2760v rms off load) succeeded in lighting up both ends, perhaps a little over 1/3rd of the length, but the centre 1/3rd remained unlit. Same for 32" and 40" set CCFLs.
I did try varying the supply up to about 14v, but saw no significant change.
I will keep this for shorter CCFLs, and try something else for longer ones.

I have the two inverter modules from a JVC set I scrapped (cracked LCD panel) for which I need info, but will open a separate thread for that.
Les.

[OldLes]

I pulled the CCFLs from a Toshiba 23WL46B, which turn out to me two tubes in series, so effectively equivalent to a 46" set's CCFL.
Once more, the Samsung inverter would not light them up, BUT, using the two outputs in series as mentioned, I could get one half to light up perfectly. The off load level of 7,200vpp fell to 2,450vpp (approx 865v rms), with the original sine waveform now distorting during its rising period. The current passed was 5.5mA. The supply current increased from 0.4A quiescent (no CCFL connected) to 1.25A, suggesting a 50% conversion efficiency. Now I just need to light the full length of the two tubes.
I have the inverter board from the Toshiba, but so far can't persuade it to produce any output.
I will search for a circuit for that next.
Les.

[sabre504]

Hi Les
assuming i have the right ones these may help

[OldLes]

Sabre, thanks for that. It seems to be generic sheet covering panel and inverter, but there is a difference with my inverter. Mine has pins 4, 5 and 6 with no connection. I have tried feeding supply via 10K and 22K to both pin 7 (yellow) and pin 8 (White), so far with no response. Maybe further study of this will lead to a result.
I know it is OK, as I tested it out 5 mins before dismantling. It had one vertical line on screen, but backlights etc were good.
Les.

[OldLes]

A bit more study of Sabre's info has lead me to the simple circuit attached. It will now fire up the two series connected CCFLs, so should be OK for the 42" set I need to test out the individual CCFLs before replacement.
These inverter boards seem to be quite common, so maybe they could be OK for others to use.
I may change the 2K2 resistor for a 3.3V zener, and may need minor changes to get it to switch off consistently. This could allow the supply voltage to be reduced for smaller CCFL tubes.
I will measure the off load and on load voltages as well as the current thro the tube.
Circuit now modified. Test results will follow.
Les