Dell 2209WA Power Indicator

[Dubi123]

I have two Dell 2209WA monitors, one REV A01, other REV A02(?).
On first power indicator wokrs as expected, amber for standby and blue when power on.

On second (REV A02(?)) I get amber light in standby, but no blue light when power on. But blue light comes on after about one hour. I have not detected any LED flickering, just blue LED turns on after about one hour. If I switch it off now, I get amber light, if again on, get blue - it is OK, could repeat it form many cycles, seems OK. But if I leave monitor in standby for let's say one hour, I again dont get blue light after wakeup. Some people had problem with flickering blue LED, but I have never detected this flickering, just blue LED switch on on its own. I think it is not related to small temperature changes within monitor case (very minor change). Similar problem appears even when monitor is disconnected from PC, i.e. not PC related. Swithc it on and again no blue light on second monitor.
After some thinking I am not sure it is LED problem, could it be different firmware in each monitor? Flickering LED problem (but not this one as described) is reported for some other Dell LCD monitor, but this is not flickering LED. Seems as not important problem, but it becomes irritating. Any ideas?

 

[rclarke250]

Sounds like a classic cold solder joint, as the monitor is on for a while and heats up, allowing the board to flex, creating a better connection and led turns on. If you care to check this out, and feel like taking it apart, I would check not just the obvious LED, but also, I would check the cable connector at both ends of the control board. Could be anywhere in the control path supplying power or a flaky ground connection on the path.

 

[Dubi123]

Thanks. However an idea came om my mind. This problem appears on some small portion of Dell monitors, regardless of model and LED involved is always blue, with amber LED there is not reported problem (at least I don't know). So I researched a little about LEDs, particularly about voltage drop on LED. And data voltage drop on LEDs is following: 1,7-1,9 V for red, 2 V for orange and yellow, 3.4 volts for bright white, bright non-yellowish green, most blue types and 3.8 V for 430 nM bright blue types. Now. if there is slight shift in operating point of power supply or LED it may cause the problem. If there is cold solder joint, may be additional voltage drop is involved. Also LED operating point could probably change due to LED initial ageing (but it is just my guess). If IC that drives LED operates on 5 V, and there is some voltage drop on IC transistor and resistor, may be there is not enough left for LED with shifted operating point (if there is large sample of blue LEDs may be that its voltage drop was in some monitors initially greater than expected 3,4-3,8 V). So, for now, I think that new LED would fix a problem. I suppose thet power supply is OK as other monitor functions are still not affected.

Kind regards

Dubi

 

[rclarke250]

I do not follow your logic. There are several voltages that will be present from the power supply of the monitor. All of which have there own separate regulator circuits. Where would this power shift come from? the wall socket? the internal regulators will just shift to cover the difference, that's their job. So if you have it hooked up to 110va, And it shifts down to 100va, there will be no difference in the internal output of the power regulators, they are designed to compensate for the fluctuation in the line voltage. If the shift is internal after the regulator, or the regulator itself, the shift in voltage would most definitely effect the other circuits on that rail. Which would effect the operation of the monitor. Since it works after an hour or so, Heat buildup in the case is effecting it, and therefore it most likely is a bad connection that "fixes" itself when the heat allows the bad connection to connect the circuit because of flex in the circuit board. Also changing the led to a different operating voltage is a "band-aid fix" . You are changing the circuit dynamics, not repairing the underlying cause of the supposed voltage shift.

 

[Dubi123]

Hi! I don't say it is not thermal caused, although LED sometimes power up after just few minutes. What I say is that similar problems is reported in many Dell monitor types, even the most expensive 30'' models, but always with blue LED (all users reported amber OK, blue doesn't work correctly). I suppose it is one bicolor LED behind a switch button, but I am not sure, may be there are two LEDs. So why this bad soldering connection always happen just to blue LED? I think that it "may" be faulty design that doesn't consider expected voltage need of a blue LED (3,4 - 3,8 V). Other (conventiona) color LEDs need just around 1,7 - 2,0 V. I suppose that there is too small tolerance left to power supply variation or LED parameter change that causes this problem only with blue LED. Don't have scheme and don't know what is driving these LEDs, but I suppose it is some output from 5V powered IC like some kind of dedicated microcontroler (amber - sleep mode, blue power on after short initialization, back to amber after few seconds if no input video signal or remain blue if video signal is present, always just one color is lighted - there are some simple rules behind turning on and off of these lights). If one needs 3,8 V to power blue LED and 3,8 V is data for a typical blue LED (and LEDs probably disperse a little over large smaple and change a little over time), and some 0,3-0,7 V is "lost" at thr switching transistor in IC (just a guess, don't have exact value), something is lost at the resistor (if exists, don't have details of design) and even 4,5 V or so from power supply doesn't suffice any more (or quite at the limit). However monitor IC(s) still operates correctly at this voltage. Yet there is plenty of room for correct operation of amber LED. I don't know for single case of reported problem with amber LED (from few other, not so technical user forums, and not suitable for technical discussion of this kind), all with blue LEDs in this power indicator. Don't intend at all to change LED operating voltage or even play with power supply (quite dangerous), what I mean is that if one has a pool of let's say 10-20 blue LEDs most may work OK all the time. But for small percentage of blue LEDs this problem appear and may be related to small variations in power supply output voltage that again may be thermaly caused (or voltage drop change at a switching trasistor within IC, again thermaly caused).

Regards