power rails – Experimental Engineering

Posted on October 27, 2016 by The Engineer — Leave a comment

LG Flatron 22EA53VQ-P Power Issue

I was recently given a pretty nice LED backlit 1080p LG monitor, with the instruction that it wouldn’t power on correctly. The monitor would power on as far as the standby light, but when fully powered on, would flash the backlight momentarily then shut down. A power supply issue was immediately suspected.

I popped the covers off the monitor itself first, thinking that it was an electrolytic gone bad in the backlight DC-DC converter. Not to mention the fact that cracking into a wall-wart type of PSU is only occasionally possible without the use of anger & large hammers. (Cracking the glue with the handle of a screwdriver doesn’t work so well when the factory went a bit nuts with the glue/ultrasonic welder). As can be seen in the photo, there’s not much inside these monitors, the logic is a single-chip solution, the rest of the PCB is dedicated to supplying the power rails for the various circuits. On the left is the power input & the DC-DC converter for the backlight, along with the DC-DC converter supplying the logic circuits. None of the capacitors here are damaged, everything looks good.
I then measured the output of the PSU, which under no load was the correct 19v DC. However applying any load caused the output voltage to drop like a proverbial brick. Applying a full load of 1.3A saw the output voltage drop so severely that the PSU tripped on it’s UVLO.

At 200mA of load the factory PSU is already dropping to 18v, with a 5.3kHz switching frequency appearing.

At higher load the frequency increases to 11.5kHz & the output voltage has dropped to 11.86v!

750mA was as high as I could make the supply go without it tripping itself out – the UVLO circuit trips at 9v. 12.6kHz is now riding on the severely low DC at this point.

The power supply is supposed to be rated at 1.3A at 19v, however with this fault it’s getting nowhere near that. The LG brand is on this PSU but it’s contracted out to Shenzen Honor Electric Co. Ltd.

Here’s the problem with this PSU. The output electrolytic has ballooned. I don’t have an ESR tester, but this cap has gone way past it’s sell-by date. It’s position right next to the heatsink with the output rectifier diodes has probably cooked it. The PSU isn’t that badly built for a Chinese one – there’s plenty of creepage distance on the PCB & even a couple of isolation slots.

Posted on September 21, 2015 by The Engineer — Leave a comment

Dell E207WFPc Monitor 12v Conversion

I’m still on my crusade of removing every trace of 240v mains power from my shack, so next up are my computer monitors.

I have 4 Dell monitors, of various models, hooked up to my main PC.

The monitor here is a Dell E207WFPc 20″ widescreen model. There will be more when I manage to get the others apart to do the conversion. However I’m hoping that the PSU boards are mostly the same.

There are no screws holding these monitors together, the front bezel is simply clicked into place in the back casing, these clips are the only thing that holds the relatively heavy glass LCD panel & it’s supporting frame! The image above shows the panel removed. The large board on the left is the power supply & backlight inverter, the smaller one on the right is the interface board to convert the DVI or VGA to LVDS for the LCD panel itself.

Here’s a closeup of the PSU board, the connector at centre right at the top of the PCB is the main power output, and also has a couple of signals to control the backlight inverter section of the PSU, on the left side. The PSU requirements for this monitor are relatively simple, at 14.5v for the backlight & 5v for the logic board.

Here’s the top of the PSU board, very simple with the mains supply on the right side, and the backlight inverter transformers on the left.

Here I’ve hooked into the power rails on the supply, to attach my own 12v regulators. The green wire is +14.5v, and the purple is +5v. Black is common ground.

On doing some testing, the backlight inverter section doesn’t seem to mind voltages between 11.5-14.5v, so a separate regulator isn’t required there. Even running off batteries that’s within the range of both charging & discharging. The only regulator required is a 5v one to reduce the input voltage for the logic PCB.

On applying some 12v power to the regulator input, we have light! Current draw at 12.5v is 2.65A for a power consumption of 33W.

There’s plenty of room in the back casing to mount a 12v input socket, I have left the mains supply intact so it can be used on dual supply.

Here’s the 5v regulator mounted on the back of the casing, all wired up & ready to go.

Posted on August 26, 2015 by The Engineer — Leave a comment

Rigol 12v Power Supply Project Wiring Loom

As the crimp tool for the PSU connector in the Rigol scope is a very expensive piece of hardware, I decided to use pre-crimped terminals, from an ATX power connector. (They’re the same type).

Here’s the partially completed loom, with the 13 cores for the power rails. The 14th pin is left out as that is for AC triggering, and this won’t be usable on a low voltage supply.
A couple of the pins have two wires, this is for voltage sensing at the connector to compensate for any voltage drop across the cable. The regulators I am using have provision for this feature.

To keep the wiring tidy, I dug a piece of braided loom sleeving out of the parts bin, this will be finished off with the heatshrink once the pins are inserted into the connector shell.
The remaining parts for the loom have been ordered from Farnell & I expect delivery tomorrow.

More to come then!

73s for now 🙂