Rationale for "lamp error" detection

[Curious.George]

Hi,

This one is just for curiosity's sake...

Subject makes it sound silly -- why detect a CCFL failure as it should be obvious to the user (viewer)?

I was working on an Aquos LC20B2UA (an ancient set) and was surprised to see all the components (components == $$) dedicated to monitoring lamp current! Each of the 10 lamps are monitored! Though only a single "error" signal is reported to the controller (i.e., it can't tell if 1 lamp or 10 lamps are at fault).

[It looks like the circuitry detects an open tube or a missing excitation voltage. I'll have to look at it more closely tomorrow...]

Surely, this is overkill?

The set has two inverter boards: one drives 6 lamps (three dual converters) while the other drives 4 lamps (two converters). Each board is individually fused. I.e., a fuse blows and you lose 4 or 6 lamps. A converter blows and you lose two lamps. A transformer/tube opens and you lose one lamp. All of these situations should be visible to the user, right? (I should try this...)

N.B. The set has no blink codes! Lamp errors are counted and displayed in the service menu. Each shuts down the set. The fifth such error causes the controller to inhibit the set from being powered up, thereafter. Remember, there are no blink codes so the user just sees a dead set in this case!

This suggests the reason for monitoring each lamp has to do with a type of error that can plague a single tube (thus requiring you to monitor all of them) that the user would not "notice" while watching the set.

Furthermore, shutting down the set suggests that this error would be a safety issue -- especially in light of preventing it from powering up after five such errors!

So, my question: what is the nature of this "failure" that would merit adding these components (i.e., to monitor each and every lamp) -- yet not also warrant reporting the error to the user (blink code)?

Thanks!

[tom66]

Re: Rationale for "lamp error" detection

If you have an open bulb, you're no longer loading a transformer. This means its output can increase from the nominal 700V to 5~10 times that, which can damage the transformer, burn stuff, arc to the frame and make an unpleasant smell. Similar thing goes for short circuit failures. They want to minimise the risk of fires.

As for no blink code, it's possible that Sharp didn't think of doing that until their later models.

[Curious.George]

Re: Rationale for "lamp error" detection

If you have an open bulb, you're no longer loading a transformer. This means its output can increase from the nominal 700V to 5~10 times that, which can damage the transformer, burn stuff, arc to the frame and make an unpleasant smell.

Ah! I hadn't considered the transformers would have such a high output impedance! I'll have to drag out a HV probe and see what things look like unloaded...

Similar thing goes for short circuit failures. They want to minimise the risk of fires.

Presumably, short is reflected back to blowing the fuse on the converter's primary?

Bottom line: lamp error covers anything wrong with any of the lamps and/or related circuitry (e.g., blown fuses, converters, missing primary supply, open lamp, shorted output, etc.).

The fact that sets don't shut down after a single error suggests the detection isn't reliable (the controller could always shutdown a converter, verify the error goes away, then restart the converter to see if it is a "persistent" condition).

Thanks!

[tom66]

Re: Rationale for "lamp error" detection

Continuously attempting to light the backlight is a recipe for fire/smoke. You don't want that.
http://www.dailymail.co.uk/sciencete...mit-smoke.html

[Curious.George]

Re: Rationale for "lamp error" detection

Continuously attempting to light the backlight is a recipe for fire/smoke. You don't want that.
http://www.dailymail.co.uk/sciencete...mit-smoke.html

The user turning the set on again effectively tries to relight the backlights. The point of my comment was, why give the set 5 errors before locking up? I.e., if the detection circuitry was effective/robust, the first detected error should be enough to cause the set to refuse to start (why give the user 4 more tries?)

If the error is signaled due to an inverter/fuse failure, then the second (third, fourth, and fifth) attempts will also fail.

If the error is signaled by an "open" lamp, the same should apply.

The only way for the error to appear and then disappear (without an intervening service call) is if the failure is intermittent or the detection means are inconclusive.

I just don't understand why "once isn't enough". Like replacing a fuse five times before looking to see why it's blowing in the first place!

[tom66]

Re: Rationale for "lamp error" detection

Because they said so. Do you want a better answer? I doubt there is one.
They probably did a failure analysis and said, "in the average lifetime of the TV we expect three random failures to occur, so allow up to five to limit harm."

Or, perhaps it's related to five errors of any type, and some programmer neglected to change it. And it's stuck. The five error buffer is very useful on these sets that's for sure.