VS too high !

[plpsanchez]

Hi !

I have been strugling with a 51 inch Samsung plasma ( PL51E8000 ).

Replaced both Y-MAIN and X-MAIN boards, then before puting everything to test, I decided to check PSU voltages because I knew VS was about 212V, above the specified 207V .

So I kept the PSU disconected from everything else, put a jumper connecting PSON to ground and another connecting VSON to 5V STDBY.

Measured VS, it was about 212V. I left the board on for a few minutes and VS got to 223 V !!!

I don´t think it is a good thing to have such a high voltage. If not for anything else, highest voltage capacitors are 250V, so go figure... Any ideas of what is going on and how to fix it?

Thanks,

[plpsanchez]

Re: VS too high !

By the way, I have been unable to reduce VS using the VS-ADJUST trimpot. The trimpot is already at the minimum.

[tom66]

Re: VS too high !

It seems like the adjustment / regulator circuit may be faulty. What is the BN44-###### number on the board?

[drussell]

Re: VS too high !

By the way, I have been unable to reduce VS using the VS-ADJUST trimpot. The trimpot is already at the minimum.

You often need to add some load to the VS to get a proper reading. The bleed resistors on many PSUs aren't enough to keep the idling PWM supply from drifting upwards, depends on the design and whether it will completely stop pulsing or has a minimum duty cycle on the pulse width, etc.

You could use a bunch of power resistors if you have them or more easily, a lightbulb or two. Not sure of your line voltage but a single, say, 100W 220v bulb or a couple 40W/60W or so 120v bulbs in series should suffice to simulate a power load and get a real reading before you connect it back up to the TV if you want to verify the voltage and test the regulation.

This will tell you if it just needs some load or if the PSU is faulty.

[plpsanchez]

Re: VS too high !

Thanks!

@tom66 , the part is a BN44-00511A.

@drussell, I was wondering about that. But a voltage supply that drifts that much, with such bad line regulation, does not seem a good design to me...

[plpsanchez]

Re: VS too high !

I mean, bad load regulation.

[drussell]

Re: VS too high !

Thanks!
...
@drussell, I was wondering about that. But a voltage supply that drifts that much, with such bad line regulation, does not seem a good design to me...

Well, some designs do regulate fine when disconnected (and I'll agree I much prefer those) but some do not.

It does make sense though that it really doesn't matter because if they are always expecting to have at least SOME load, they can design it that way.

If a PWM controller is set up to never go below, even a very low % duty cycle, the voltage on the capacitors with no load is going to slowly creep up even from those narrow pulses with nothing to pull it back down a little. Draw even a few watts out of it and you should at least start to see at least SOME action from the Vs control pot.

When I built my own 1000W car amplifier (20+ years ago) was the first time I really ever designed a PWM supply (used a TL494, IIRC) and since there is always some bias on the amplifier output transistors, I had it set up this way with the built-in minimum duty cycle of something pretty high like 5%... There was no reason to add extra circuitry to disable the TL494 on slightly high voltage because it simply didn't matter. Even on some kind of fault, the transistors and main supply capacitors had more than enough voltage headroom that they would never have a problem even if the supply was WAY high.

Of course if I were designing a TV PSU, I'd make it run properly with no load, but manufacturers are really trying to engineer every little last bit of cost out of EVERYTHING these days, unfortunately...

[plpsanchez]

Re: VS too high !

Of course if I were designing a TV PSU, I'd make it run properly with no load, but manufacturers are really trying to engineer every little last bit of cost out of EVERYTHING these days, unfortunately...

Manufacturers are also fully exploiting planned obsolescence in a way that I prefer call it programmed self destruction. So maybe it can be regarded as an advantage anything that tends to disrupt a working device.

Perhaps in societies where perfectly functional equipment is thrown away in little more than one year just because there is a new model in the market, really doesn't make sense produce anything durable.

Unfortunately...

[plpsanchez]

Re: VS too high !

The board is rated 207V at 1.1A.

Tested the board using a 500R, 2.5A, rheostat as a load. See picture.

Adjusted the rheostat to 415 ohm to get a expected load current of about 0.5A.

The power source works nicely, 207V at 0.5A, and I was able to adjust all voltages to the correct values.

Still does not know what is the problem with the TV. The problem is not the SMPS for sure, so I will start another thread.

Thanks for the help!