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Converting 120v TV to 12v using buck/boost converters
#1
I know this isn't exactly a battery topic but the goal here is to save battery power by not using an inverter in my RV. I picked up an "Insignia NS-24DF310NA19 24-Inch 720p HD Smart LED TV- Fire TV Edition" that has an internal power supply. The input is 120v at 0.32 amps AC. So this would potentially be around 12v and 3.2 amps DC. I've attached a picture of the power supply with some comments.

I cracked open the case and found the power supply converts the 120v AC to DC for all outputs. It sends 16.8v to the LCD panel and 12.8v to the mainboard. It appears, if I understand correctly, the mainboard feeds back 5v to the power supply when the TV is on which triggers the 16.8v for the panel.

My thoughts are to use a buck/boost converter to stabilize the +12V connection get 16.8v from the roughly 12v to 14v my battery will provide. I "think" I found one that also has a 5v on/off switch that seemingly would be perfect for turning the 16.8v output on. (amazon.com/dp/B017SM1O98) That is if I understand the "en" port correctly. This buck/boost seems like overkill at 15 amps but it's the only one I can find with the 5v "en" port. Snagged one off ebay for $5.

For the continuous 12.8v I would use another cheaper buck/boost converter that is always on. Unless multiple variable output buck/boost converters are readily available but I couldn't find one.

To avoid parasitic drain I was thinking about adding a switch between the converters and the battery. 
  • For those more electrically inclined does this all sound correct?
  • Any idea what the 4.9v dim pin is for? (See picture)

Important EDIT: I struck out some of my bad readings. The correct readings are in posts #13 and #15.


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#2
On the power board, you should probably check the voltage at the transformer output on the low voltage side. in the red/yellow circle below:



If the output is 12V (or there abouts) then you should be able to disconnect the transformer and power the board directly from there.

However, there is an opto ic just to the right of that. Not sure which direction it is facing. Does it trigger the high voltage side, or the low voltage side.
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#3
I wouldn't attempt to measure voltages at the transformer, they will be high frequency AC & most multi-meters won't cope with this.
Instead measure at the 3 points in yellow circles this will be the DC on the filter capacitors.

 

It sounds like you have three easy rails to make, +5, +12, & +16.8.
The "dim" pin is probably control, eg remote control flashes "on" signal to TV, power supply turns on main rails, TV works....
eg +5V might be "always on" for remote to work.
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#4
Ahhh yeah, that's right. High frequency. :face palm: I forgot about the frequency issue. Yes, I agree with Readpacket, after the filters.
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#5
I wondered if I could tap into the board but had no idea where to start. Are you saying it's likely transformed to 12v DC and later boosted to 16v for the panel?

Before reading the new replies I checked for 12v on the 5 prongs below the transformer but none had a reading. When reading the black chip to the right (I believe it's the opto ic) I read 11.2v on the left bottom prong and 10.2v on the right bottom prong. I measured the prongs on the 2 black chips below the transformer and found that the middle prong on the left chip had the roughly 12.8v. I'll check the black chip circled mid right later.

Trying to tap into the board may be a little more than I understand. I'll probably burn the board out somehow.

EDIT: Struck out some unneeded steps.
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#6
Because of the high frequency of the switching, a standard DMM won't get a voltage reading, or a reliable one, on the windings on the transformer. That's why Redpacket suggested the other chips to test from.

It's possible it's converted from 120VAC that is then rectified, then inputted into the transformer as 120VDC pulsed which then outputs as 16VDC pulsed, then smoothed out by the filter capacitors that Redpacket pointed out. (I'm guessing on the 16V output). Redpacket is the switching psu guru Wink
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#7
(02-21-2019, 10:59 PM)Korishan Wrote: It's possible it's converted from 120VAC that is then rectified, then inputted into the transformer as 120VDC pulsed which then outputs as 16VDC pulsed, then smoothed out by the filter capacitors that Redpacket pointed out. (I'm guessing on the 16V output). Redpacket is the switching psu guru  Wink

Yes, that's how it works.
Above the transformer there's a transistor on a small aluminium heatsink - this is switching the approx 160VDC (rectified 120VAC) into the transformer.
The transformer will have 3 windings,
- one makes the +5V rail
- one makes the +12V rail
- one makes the +16V rail
See the points I marked for you - those devices are all types of diodes.

Be careful with any part inside the hashed border area (left side mostly) as this has lethal voltages & if shorted, the flash/bang will be nasty. Also they don't become safe until several minutes after power off.
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#8
I'm a little confused. 

With the TV off, and using the case as the ground, the +5V is unpowered but the +12V and +16V are powered. If I use the ground wires, instead of the case, none are powered with the TV off.

Is the +5V coming from a rail on the power supply or is it coming from the mainboard? (I attached a picture of the mainboard) This makes me believe that once the TV is turned on the ground wires are grounded to the case which passes +12V to the mainboard that then bucks and sends back +5V to the power supply?

If this is true, what purpose does sending +5V back to the power supply accomplish? I'm also confused because some power must always be on in order for the TV to receive contact from the remote control.


EDIT: Struck everything out as my readings were bad. I removed the power supply preventing it from grounding to the case. My later readings in post #13 and #15 are accurate.
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#9
So I know that this isn't the answer you want - but really the way to do it is to look for a TV with an external power supply - as in, they pipe DC straight into it.

I have two LCD TVs that I have that I run on DC that I converted. The easiest is to just use an external power supply (BTW, TVs are so cheap now, it's almost worth it to just go and get one) One had an external power supply, and the other took in straight 120VAC, rectified it and bumped it down to 19VDC. I believe it had a transformer too to get to 5v and 3v for other goodies -. I tied into the 19vdc rail with a buck converter from my 12v supply and somehow it all magically works. It really shouldn't, but it works great. However, I think that's a fluke more than anything. Like how it somehow works its way down to the other voltages was a miracle more than it was intelligent decision (I have no idea what I'm doing) so for those out there interested, the easiest and safest way is to just aim for a TV with an external power supply.
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#10
(02-23-2019, 01:04 AM)MilkMan Wrote: I'm a little confused. 

With the TV off, and using the case as the ground, the +5V is unpowered but the +12V and +16V are powered. If I use the ground wires, instead of the case, none are powered with the TV off.

Is the +5V coming from a rail on the power supply or is it coming from the mainboard? (I attached a picture of the mainboard) This makes me believe that once the TV is turned on the ground wires are grounded to the case which passes +12V to the mainboard that then bucks and sends back +5V to the power supply?


If this is true, what purpose does sending +5V back to the power supply accomplish? I'm also confused because some power must always be on in order for the TV to receive contact from the remote control.

So your multi-meter black probe should be on one of the pins you've marked as "grounds connected when TV is on" ie the orange or green wire, this is the power supply's 0V rail.
With the red probe you're testing the 3x circled points I added to the pic.

The case of the TV might not be connected to 0V or might be switched by a part of the circuit.

Mikethezipper has a good point about picking a device with a single voltage external PSU.
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