HP Pavilion vf15 (15" Flat Panel LCD Monitor)

[budm]

Re: HP Pavilion vf15 (15" Flat Panel LCD Monitor)

Glad to hear, one less monitor to the landfill. If I did not see that brown spot, it would have taking more time, glad that good pictures help remote troubleshooting.

[jdmillard]

Re: HP Pavilion vf15 (15" Flat Panel LCD Monitor)

Man, I tried my best to get good pics. It's tough when the only time I have available is at night (bad lighting) and the best digital camera in the house is the one in my phone. lol. You did great with them.

I'm looking into those part numbers of the voltage suppressors. Digikey and Mouser are both out of stock. Any suggestions? I could search their inventory for products of the same dimensions and specs...

[budm]

Re: HP Pavilion vf15 (15" Flat Panel LCD Monitor)

You can try one volt higher, 8.5 if you can get it.

[jdmillard]

Re: HP Pavilion vf15 (15" Flat Panel LCD Monitor)

So my search for specific part numbers was futile. So I tried using the search options for TVS diodes. I'm not at 100% with understanding the specs so correct me if I'm wrong.

What we care about is:
-Operating voltage close to 7.5V
-Breakoff and Clamping voltages close to the ones in the pdf you sent me (under the assumption that the pdf does indeed define a component very similar to mine... which is a pretty good assumption).
-Max peak pulse surge current needs to be able to withstand at least what the last one could (or at least what the specs show in the pdf you sent me).

So the SMA6F7.5A TVS diode would work if I'm not mistaken. It has the right operating voltage, it's breakdown and clamping voltages are close to the ones in the spec sheet, it can take a LOT of surge current (more durable right?), it's unidirectional, it's surface mount... I haven't looked at physical dimensions yet. Is there something I'm missing?

EDIT: I looked into the spec sheet of the link above and now I'm thinking that the SMA6F6.5A would be better because its standoff voltage and breakdown voltages align themselves with your pdf better. This PDF as my source.

[jdmillard]

Re: HP Pavilion vf15 (15" Flat Panel LCD Monitor)

Okay, so in my quest to find a good zener diode to replace the one that got charred to medium-well I've done some research. At first I thought that zener diodes and TVS diodes were basically the same thing but according to one source:

"TVS diodes are designed and characterized for transient voltage suppression, while Zener diodes are designed and specified for voltage regulation."

But from what I can tell, either should get the job done for my purposes right? The charts of current vs voltage seemed to suggest that either type of diode would accomplish what is necessary for my particular circuit as long as it's properties match the original. I'm I way off here? If so, help me to understand it a bit better.

[budm]

Re: HP Pavilion vf15 (15" Flat Panel LCD Monitor)

I would not use lower voltage, the diode is connected between 12v and the 5V so the difference is about 7V which you measured, I looked through a bunch of HP service manual and see they use that typical setup with the TPSMP (post34) and some just use Zener diode. The key point is that the diode should not go into conduction unless the voltage goes up way up above the voltage the circuit can handle, it is a secondary protection in case the power supply failed big time, power supply already has control feedback to maintain out put voltage and over current draw protection.
I would try to match the clamping voltage of what I think the TPSMP7.5 is. or you can try 8.5V or 9V 2Watt (or higher) Zener through hole device since you do have the room to install. If you go lower voltage, the diode will conduct all the time which we do not want, it should only conduct if there is big problem.

[jdmillard]

Re: HP Pavilion vf15 (15" Flat Panel LCD Monitor)

From what I understand: If Zener diodes are regulators, their goal would be to make sure that an output is a specific voltage. They would be more fine-tuned with tighter tolerances because they would be actively responsible for maintaining the right voltage. If TVS diodes are suppressors, they would aim to take care of things in case stuff gets crazy (like a psu failure or simple transient response). They would not need to be exact, rather they would need to activate at an arbitrary voltage higher than that which is expected, thus suppressing the overage only when it's needed.

So either:
1)that diode was rightly intended by HP designers to be a Zener diode that is actively involved (always regulating). If this is the case, I'll want a zener voltage of something pretty close to 7 and is rated for the right wattage (you suggested 2W)
2)or it was meant to be a suppressor as a backup just in case things get crazy (as you suggest) and they probably should have not called it "zener" because it's misleading from it's intended purpose. If this was the case, I'll want a diode (Zener or TVS) that activates at some arbitrary voltage above 8V-ish. Either Zener or TVS should work in this case as long as the breakdown voltage satisfies this one condition and the chosen diode is rated for the appropriate wattage (you suggested 2W).

How can we know which one it is? The fact that I removed "zener" diode makes me think the first one above is correct. Not to mention, that I measured 7.6V across the solder points (should it be "zener-regulated" down to 7.0V?... 12v-5v=7v). EDIT: Then again, I can see why it would be the second one because I would find it questionable to try to simultaneously voltage-regulate two different outputs with one diode (because it would only be able to regulate the difference between them, not necessarily guarantee both their values)... so it does make more sense that it would be there strictly for the reasons that you have presented (as a "just-in-case" protection feature).

I'm just trying to make sure I understand the system.

Anyways, would THIS zener diode work? It's Vz is 8.2V and has 2W power dissipation.

[jdmillard]

Re: HP Pavilion vf15 (15" Flat Panel LCD Monitor)

Do want a lower power dissipation so it doesn't heat as much?
or
Do we want a higher power dissipation because it's rated higher and can withstand higher loads if necessary? (I'm guessing it's this one but I really don't know).

[budm]

Re: HP Pavilion vf15 (15" Flat Panel LCD Monitor)

Higher Wattage can handle more current but with bigger body, the use in this case is for it to conduct only if there is problem such as the 12V goes way too high, on normal operation this diode should never turn on. The inverter circuit itself can easily operate at 18~20V (see the spec of the PWM IC of the inverter circuit), so there is lots of head room. If you look at other monitor circuit design, most do not even has this diode in place and the monitor still works fine.
This SCH of this HP 15" uses 6.8V ZENER (SML4736) instead of TVS, power supply has 12V and 5V. You can also see that ACER uses 9.1V so as you can see it is around 7~9V range.
The lower the voltage, the more chance of it being conducting all the time instead of when it is only needed during power supply failing mode.

[jdmillard]

Re: HP Pavilion vf15 (15" Flat Panel LCD Monitor)

Got it. Excellent.