Author Topic: Trying to find the 12v rail on a LED TV board with built in power supply.  (Read 1217 times)

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Offline niffcreature

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Hi, I have a TV board for a cheap little LED TV. The AC/DC adapter is built onto the board. I have successfully swapped it into another TV that has a power supply with 12v rails (mainly using it for CCFL backlight power). However, I would like to power the new board with the other power supply, and I have tested 12v just about everywhere on the "COLD" side of the circuit so it seems plausible. Trouble is, I can't seem to find where the 12v comes in. I'm not getting anything from the leads on the "COLD" side of the transformer. What am I missing?

It's also probably worth mentioning, I wouldn't care so much except that the 24v (CCFL backlight power) rails from my separate TV PSU won't turn on unless the 12v rails are loaded. The 12v rails are also always on. There's also an "enable" pin on the PSU that I've gotten to work nicely with my new TV board so the whole thing can turn on with the remote control from the new TV board. I also rewired the LVDS cable and found a jumper to switch one pin from 5v to 12v panel power. All is working great, I've never done anything quite on this scale so you can understand that I want to finish this last puzzle piece and assemble things without wiring up a dummy load and the 120v power cords together!
 

Offline ZaphodBeeblebrox

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Difficult to see, but probably that D2PAK is a diode? Right after that would be the place to inpput your +12V raill. This net should be connected to the "+" of the bulk ELCOs (just a means of verification).

 
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Offline niffcreature

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Difficult to see, but probably that D2PAK is a diode? Right after that would be the place to inpput your +12V raill. This net should be connected to the "+" of the bulk ELCOs (just a means of verification).

Thanks! Yes it looks like the big d2pak is a ps20u100bct rectifier diodie. Forgive my ignorance but by ELCOs you mean caps? EDIT: Found my answer to that!
« Last Edit: January 06, 2020, 11:00:47 am by niffcreature »
 

Offline ZaphodBeeblebrox

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Forgive my ignorance but by ELCOs you mean caps? EDIT: Found my answer to that!

Yes. The "EL" comes from "ELectrolytic" en the "CO" comes from "COndensator", which is dutch for capacitor.

Sorry for using a somewhat obscure abbreviation (in dutch is is perfectly clear of course :-) ). I guess that's what happens when a group of engineers work together for long enough: they tend to develop thier own "language" :-)
 

Offline niffcreature

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This is even more n00b of me to ask but, should I find a good place to connect the ground too? The board and the PSU are sharing chassis ground but the PSU has a bunch of ground wires next to the 12v.
 

Offline ZaphodBeeblebrox

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Difficult to answer on-the-fly, but I would just make the ground connection right next to the place where you make the +12V connection. Then you twist both wires to form a sort of twisted pair (keeps the wires together and is good for radiated immunity).

The reason you have a chassis ground and extra ground wires is probably twofold:
  • Safety (LVD): by grounding the secondary side to the chassis, you are placing the product in a specific category, which *could* save testing time to pass international standards. (note: I'm not a safety expert - I just picked up some things along the way)
  • EMC: grounding the secondary side of a SMPS is, in my experience, a good way to alleviate a lot of emission issues. By grounding the secondary side you also get complete control of where the current of, say, an EFT of ESD pulse will flow. This makes the system easier to predict and hence makes it easier to comply with the EMC directive.
 
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Offline niffcreature

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OK, I'm more confused now. The ground tab on the ps20u100bct d2pak reads 12v too?
 

Offline ZaphodBeeblebrox

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What I meant was: you will probably find some place physically closeby to where you found your +12V point. Use that spot to connect the GND of your power supply. That is to say: connect the GND wire as close as possible to where you connect the +12V (probably there will be a GND plane on the other side of the PCB: that is an ideal place to make a connection).
 

Offline niffcreature

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ok, but shouldn't I be reading 12v somewhere else, not the ground tab of the diode?
 

Offline niffcreature

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Re: Trying to find the 12v rail on a LED TV board with built in power supply.
« Reply #9 on: February 23, 2020, 10:58:25 pm »
anyone else have input on this?? I'm still stumped
 

Offline Jwillis

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Re: Trying to find the 12v rail on a LED TV board with built in power supply.
« Reply #10 on: February 24, 2020, 01:33:18 am »
I have a similar problem. I haven't found any 12V  supply anywhere on the board  and the board won't stay powered without CCFLs.  There are  2 dual mosfets driving the HV transformer and look like they're outputting together 40VAC p-p. Still  checking the frequency ( which means I'm still learning how to use this DSO) . Some of my multi meters won't pick up the AC square wave voltage but are detecting the frequency at 90KHz . But that  seems pretty high .  And I can't be sure what the output voltage  is. I know it's a lot because some of my  DMMs that do show a voltage don't care for it set at 750VAC. The true RMS DMMs won't show any voltage including the Fluke.

 I got a 5m strip of 24V white LEDs. The light looks good at 20VDC through the diffuser. So I think I'll just pull the  HV transformer and run off the one Mosfet . They are outputting a 50% duty cycle for bright  and about 10 or 15% for low back light.   
I may even have to rewind the HV transformer to a lower output voltage. I'm not sure if the LEDs will run at 90KHz (if that's what it is) or have enough impedance .
 

Offline Jwillis

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Re: Trying to find the 12v rail on a LED TV board with built in power supply.
« Reply #11 on: February 24, 2020, 10:27:16 pm »
anyone else have input on this?? I'm still stumped

Have you looked into these kits. https://www.ebay.ca/itm/533mm-Universal-LED-Backlight-Strip-Kit-Update-15-24-CCFL-LCD-Monitor-to-LED/133216814800?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2057872.m2749.l2648
Friend of mine just upgraded a Curtis LCD TV with one of these kits and it works great .

On your board .where's the output to the inverter board that drives the CCFL's

Ah rats. after re reading your post I understand now your looking for a way to power the whole board. the only thing I can suggest is to  remove the flyback run a couple 12V buck inverters direct to the low side .
« Last Edit: February 24, 2020, 10:50:47 pm by Jwillis »
 

Offline niffcreature

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Yeah I have an existing power supply board that has a 12v rail and I am trying to power another TV board from it that's basically just the video signal board and audio amp.
 


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