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'[EE]DC fluro inverter problem'
2011\02\12@214805 by cdb

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I am trying to track down the cause of an increasing problem with some IGT electronic DC ballasts (model UB-12415) for F15T8 fluros.

<http://www.tektronikenterprises.com/images/Igt%20White%20Ballast.jpg>

There are an increasing number that are being returned from the field which display the following symptoms.

At switch on they are slightly below full brilliance, after a variable period of time (between 3 -15 minutes) they suddenly become super bright before shutting down. Once the transistors are allowed to cool they can be restarted by re-applying power and the cycle begins all over again.

Sometimes the TIP41C pair of transistors begin to smoke, but not always.

It seems most likely that the transformer is the problem and probably the secondary windings, and yet after measuring the inductance between a working device and a failing one there are only a few uH difference. One suspect suggested was shorted turns, I suspect possible saturation of the transformer.

I've looked at the output under load of a fluro tube on a scope and there is a nasty dip on the rising edge of the sine wave near the peak level which gets deeper as the fluro goes into over brightness. A working inverter produces a near perfect wave and at a much higher voltage approx 800v (hard to tell as the scope is ancient and can only measure everything on the 50v range and the reading goes off the scale). Frequency doesn't change at about 19kHz.

These are simple devices - 24v @ 620mA in,  uses two TIP41's with each transistor attached to it's own centre tapped winding - in other words there are two CT windings one for each transistor on the same transformer, two chokes in the power feed to each, one 27nF cap for LC tuning and one 100nF on the secondary side.

These ballasts are over 5 years old and are in gaming machines run 15 hours a day, in very hot environments and regularly fail - normally the transistors short out and the 100nF dies. Replacing these does not solve the above problem.

The company refuses to purchase new ones and expect these to be repaired until literally the PCB falls to pieces due to  fibre glass failure.
From an academic point of view does any one have any thoughts on what could be the most likely cause?

Points to note - the transformers are not available in small quantities and the company will not deal direct with Korea. I have no choice in the oscilloscopes at my disposal at work.
Colin
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2011\02\12@221124 by Spehro Pefhany

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At 09:47 PM 2/12/2011, you wrote:


>  From an academic point of view does any one have any thoughts on what could
>be the most likely cause?


Flux walking?



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2011\02\13@091850 by Olin Lathrop

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cdb wrote:
> From an academic point of view does any one have any thoughts on
> what could be the most likely cause?

Without a schematic, probably not much.

> I have no choice in the oscilloscopes at my disposal at work.

There are always choices.  However, you could rig up your own high voltage
probe for the existing scope.

2011\02\13@151938 by Bob Blick

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On Sun, 13 Feb 2011 12:47 +1000, "cdb" wrote:

> These are simple devices - 24v @ 620mA in,  uses two TIP41's with each
> transistor attached to it's own centre tapped winding - in other words
> there are two CT windings one for each transistor on the same
> transformer,
> two chokes in the power feed to each, one 27nF cap for LC tuning and one
> 100nF on the secondary side.

I would look at your 24 volt power supply - perhaps the output
capacitors are bad, allowing the inverter too much freedom and that
causes the transistors to overvoltage, or maybe get insufficient
feedback and there is double switching.

You could try tacking on a biggish electrolytic cap on the 24 volt
supply and see if that makes a difference. If it improves things,
something's wrong.

Cheerful regards,

Bob

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2011\02\14@024340 by cdb

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:: I would look at your 24 volt power supply

The thing is, the power supply I use on the bench is a standard SMPS with fixed 24v output, and the ballasts behave like this in the field, the machines have their own multi rail SMPS which is nothing like the one I use.

The two input electrolytics are 100uF/25v each, and I've replaced them with 220uF/35v so total filter capacitance is 440uF.

I'll draw up a schematic and post it later.

Colin
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2011\02\14@040957 by cdb

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part 1 299 bytes content-type:text/plain; charset="us-ascii" (decoded quoted-printable)




:: Without a schematic, probably not much.

Schematic attached. Quality is as good as it gets from IGT themselves.

Colin
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part 2 36387 bytes content-type:image/jpeg; name="IGT Ballast for F15T8.jpg" (decode)


part 3 181 bytes content-type:text/plain; name="ATT00001.txt"
(decoded base64)

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2011\02\14@041535 by cdb

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part 1 360 bytes content-type:text/plain; charset="us-ascii" (decoded quoted-printable)





:: Without a schematic, probably not much.

Schematic attached. Quality is as good as it gets from IGT themselves.

Error in my original post the frequency is approx 30kHz.

Colin
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part 2 23962 bytes content-type:image/jpeg; name="IGT Ballast for F15T8.jpg" (decode)


part 3 181 bytes content-type:text/plain; name="ATT00001.txt"
(decoded base64)

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2011\02\14@050113 by alan.b.pearce

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> I have no choice in the
> oscilloscopes at my disposal at work.

Hmm, time to tell the boss that proper tools are needed to fix difficult faults ??? I wouldn't be surprised if a reasonable current probe for the scope wouldn't help as well.

Have you checked the state of the tubes themselves? If they are going soft or somehow screwing the inverter around it could result in slower switching of the transistors producing heating. What sort of heater wiring do the tubes have? If one heater wire is open the tube may still fire due to voltage peaks but may screw the transformer waveforms around due to not having the proper heater current draw.

Any bypass caps that have gone AWOL with extended time at the elevated temp - electros especially suspect, but could be any type of cap. Anything that affects a bypass point could again screw up the waveform producing slow switching.


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2011\02\14@050510 by alan.b.pearce

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> Schematic attached. Quality is as good as it gets from IGT themselves.
>
> Error in my original post the frequency is approx 30kHz.

Ah, tried the resonance cap in series with the tube? I have seen caps break down with strange results, but test OK out of circuit. Some form of voltage breakdown will screw the waveform there.
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2011\02\14@052255 by cdb

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:: Ah, tried the resonance cap in series with the tube?

Hmm, hadn't thought of that. The schematic shows it to be 15nF, but they actually come with a nasty yellow MKT 100nF/400v.
We replace them with an ill fitting orange poly 100nF/650v. Lower values tend to make the tube quite dim - in fact if colour can be watery it'd be a reasonable description.
Colin
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2011\02\14@052818 by cdb

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:: Hmm, time to tell the boss that proper tools are needed to fix
:: difficult faults

<sarcasm on> You are aware that the gaming industry is poor? <sarcasm off>

We are allowed $1K a month for all components (a failed notehead costs $300 for a mainboard alone), and no capital expenditure allowed.

Colin

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2011\02\14@070854 by Spehro Pefhany

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At 02:54 AM 2/14/2011, you wrote:




>:: Without a schematic, probably not much.
>
>Schematic attached. Quality is as good as it gets from IGT themselves.
>
>Error in my original post the frequency is approx 30kHz.

Has L1 changed? In particular, the DC resistance of L1.

Best regards,

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2011\02\14@083454 by Olin Lathrop

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cdb wrote:
> Schematic attached. Quality is as good as it gets from IGT themselves.

As you say, the quality sucks.  At first glance it looks like abuse of
transistors, and that both on could be a stable state.  A second more
careful look requires a better schematic that isn't as much of a pain to
read.


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2011\02\14@103446 by RussellMc

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Are your TIP41Cs TIP41Cs  ?
C is rated at 100V, A at 60V and no suffix may be 40V.
Peak voltage is ~~ 2 x Vsupply plus leakage inductance energy effects.

How old are the cores?
What are they made of?
Old powdered iron cores have increasing losses due to binder degradation due
to heat. This leads to more heat and more degradation and ...
Try swapping cores (whole inductors) between a good and a bad one.

How hot are the cores getting. Are the cores in bad units getting much
hotter than in the good ones.



 Russell



On 14 February 2011 09:19, Bob Blick <RemoveMEbobblickTakeThisOuTspamftml.net> wrote:

{Quote hidden}

>

2011\02\16@051506 by cdb

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:: Are your TIP41Cs TIP41Cs  ?

Definitely TIP41C's.


:: How old are the cores?

At least 5 years old.

:: What are they made of?

Ferrite.

:: Try swapping cores (whole inductors) between a good and a bad one

Tried this, it appears to solve the immediate problem, but not the cause or possible prevention.



:: How hot are the cores getting. Are the cores in bad units getting
:: much hotter than in the good ones.

Not very hot, they certainly run much warmer than an 'in service' ballast (though they all get hot in the field as witnessed by brown and twisted plastic cases.

The transistors get very hot - unlike other ballasts (using TIP41C's and 2SC2655's) the transistors don't disintegrate, they just short between C and B (when they die, but in the case of these malfunctioning units they just shut down), however much smoke curls from the PCB tracks.

Colin
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2011\02\16@091332 by smplx

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On Wed, 16 Feb 2011, cdb wrote:

{Quote hidden}

Is your 24v line being used to drive electromechanical devices like payout mechs maybe? Are these damaging your fluroecent divers over time?

Regards
Sergi

2011\02\16@101521 by Michael Rigby-Jones

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> -----Original Message-----
> From: TakeThisOuTpiclist-bouncesEraseMEspamspam_OUTmit.edu [RemoveMEpiclist-bouncesspamTakeThisOuTmit.edu] On
Behalf
{Quote hidden}

cause
> or
> possible prevention.

Then Russells theory is almost likely correct.  Powered iron cores
suffer an irreversible shift in core loss when heated for long periods
of time.  If the design was marginal at start of life, then the
increasing core loss could reduce the feedback signal to the two drive
transistors to the extent that both transistors could be biased on,
which would obviously let the smoke out very quickly.

Have you checked the waveforms on the transistor bases between good/bad
units?  Would it be possible to try adding some more turns to the
feedback winding on a bad unit, or is the transformer encapsulated?

Regards

Mike

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