LCD TV or Monitor

[Bycrick]

Be a little careful about “12v” tv/monitors that run off a wall-wart power supply. A spec of “12v” often doesn’t means that they should run on a 12v boat system where voltages when charging batteries can easily reach 15 volts. My old monitor, when I checked with the manufacturer, was rated for “12 v +/- 10%.” I solved the problem with a Micrel 28000 low-dropout voltage regulator chip ($3 from Digikey) and two capacitors mounted on a 2” x 2” piece of metal. That regulates the output voltage to 12v until the battery gets down to about 12.1 volts where you should probably be doing something else than watching tv.

[lawrence_craig]

So I’ve gone and cut the transformer off of it. It was a wall plug/transformer type.

It’s the type of wire with one wrapped around the other which is inside another wire.

Quick question, how do I determine which is positive and negative?

[stormalong]

Originally Posted by lawrence_craig

So I’ve gone and cut the transformer off of it. It was a wall plug/transformer type.

It’s the type of wire with one wrapped around the other which is inside another wire.

Quick question, how do I determine which is positive and negative?

Quite often the connector symbol at the power input is marked. Take a look. It might also be on the power supply. Of course it would have been easier if you had checked with a multimeter before cutting the cable.

[pcmm]

Originally Posted by lawrence_craig

So I’ve gone and cut the transformer off of it. It was a wall plug/transformer type.

It’s the type of wire with one wrapped around the other which is inside another wire.

Quick question, how do I determine which is positive and negative?

Check on the wall wart ( the transformer) It will be marked, might be hard to read if its just molded in but it will be there. Usually the one wrapped around the other will be the ground.

[Dsanduril]

Originally Posted by lawrence_craig

Ah fair enough, sounds good, it wasn’t too expensive

So I just cut the plug off and wire it to the battery supply? Even though the plug says 19v?

If the plug says 19V then it is likely a 19V monitor, not 12V. It may or may not run on 12V, but probably won't last long. There are still monitors and TVs out there with 12V bricks, but they are getting rarer. 19V seems to be becoming more "standard" in the electronics industry.

The large majority of laptop chargers/batteries now seem to be 19V or 20V. We've used a 12V monitor directly wired for a long time, when it finally died we ended up with a 19V monitor (much better picture than any of the 12V units we could find in stock) and use an old DC-DC laptop charger that was lying around to keep it fed.

For the other question, both the monitor and the charging brick should have a little polarity diagram. Or you can get out a voltmeter, plug in the brick, and test it. Usually the center is positive, but this is far from universal.

[lawrence_craig]

Originally Posted by Dsanduril

If the plug says 19V then it is likely a 19V monitor, not 12V. It may or may not run on 12V, but probably won't last long. There are still monitors and TVs out there with 12V bricks, but they are getting rarer. 19V seems to be becoming more "standard" in the electronics industry.

The large majority of laptop chargers/batteries now seem to be 19V or 20V. We've used a 12V monitor directly wired for a long time, when it finally died we ended up with a 19V monitor (much better picture than any of the 12V units we could find in stock) and use an old DC-DC laptop charger that was lying around to keep it fed.

For the other question, both the monitor and the charging brick should have a little polarity diagram. Or you can get out a voltmeter, plug in the brick, and test it. Usually the center is positive, but this is far from universal.

That’s really great, thanks.

What do you think will be the issue for running a 19v monitor off 12v? Think I would be better using an inverter?

[Dsanduril]

Originally Posted by lawrence_craig

That’s really great, thanks.

What do you think will be the issue for running a 19v monitor off 12v? Think I would be better using an inverter?

It all depends on what voltage the monitor is actually using. Pretty much all of them have their own internal DC-DC converter that will take the input and drop it down to 3.3 or 5V to run the electronics in the monitor. They may also output other voltages for some functions, such as 9V or 12V. I have not yet come across a monitor where you can actually get the specs for those internal voltages.

So the brick drops things to 19V DC, then internally the monitor drops the voltage yet again to one or more even lower voltages. The question becomes what is the input range for the monitor's own internal power converter? Again, not something that gets published. If it is 10-30V input then it should run fine on a 12V system, but if it is 16-22V then not so much. There are industrial monitors out there with all that data actually published, but they cost far more than we want to spend.