SparkFun Forums

[magnethead]

What would be the best way to reduce a 13.8V automobile battery down to a constant 12V to drive a device (mobile 15" TV) drawing 4 amps?

[lyndon]

If it's a mobile TV, it will probably work just fine from 13.8V

[magnethead]

If it's a mobile TV, it will probably work just fine from 13.8V

I say mobile TV that's what it was advertised as. It's a 15" computer monitor/video monitor/ATSC TV. It has a power jack input that is 12VDC tip positive.

[skimask]

I'd go with it. Under load, that 13.8v will drop off a bit anyways, and you'll end up less than 12v quicker than you think.

[magnethead]

I'd go with it. Under load, that 13.8v will drop off a bit anyways, and you'll end up less than 12v quicker than you think.

With the alternator and engine running? I don't think so.


The idea is to be able to have it for out in the field and what not, or as a cheap display for remote camera systems. I know I can just use a lead-acid (which is how i tested that it works), but that's a bit bulky. That said, the lead acid was at 12.8 volts.

I'm wondering if the 12 volt input gets further refined through regulators, or if it actually feeds something that's voltage-specific. It's an LCD, so the CCFL backlight and Tv Tuner should be the 2 most voltage-untolerant pieces.

[waltr]

You need to read the actual specs on the input Voltage range not what is marked on the panel.
Like the other posters I'll bet that directly off an automotive system of 12 to 15V is fine.

[skimask]

I'd go with it. Under load, that 13.8v will drop off a bit anyways, and you'll end up less than 12v quicker than you think.

With the alternator and engine running? I don't think so.

DOH!
Then again, who's watching TV while they're driving?

[Mee_n_Mac]

Hopefully not the driver !

[magnethead]

Is it not realistic to have the engine running while watching a video monitor in the field (R/C, robotics, entertainment while on a HAM post)?

[Mee_n_Mac]

No doubt there's a way to reduce that small voltage differential to a more regulated 12V for the monitor. But it's probably going to take some $$s to do and so the first question to ask is "Is it really really necessary ?" Perhaps a good look at the device or it's specs might yield an answer.

If not you could just try it and risk the device. If that's not desirable then you might find a switching converter that works with those voltages and the 4A max (probably overstated). I cant think of one OTTOMH. Google will be your friend. Given I suspect the 4A is overly generous, I might be tempted to make my own linear regulator and use it with a series power dropping resistor. Then again if this monitor came with a wallwart, power-brick ... you might look at getting a low power, cheap $$s, inverter and plug that into the cig lighter socket and the wallwart into the inverter. Kinda Rube Goldberg but cheap $$.

http://www.walmart.com/ip/Schumacher-El ... r/15167385
http://www.walmart.com/ip/Bravo-View-25 ... t/15529721

[magnethead]

I'm thinking to just put one of these in series- 2 volt drop rated to 10 amps. Would get 14.4 to 12.4 and 13.8 to 11.8. 12.8 engine off would be too low, but at least under-voltage can't damage things whereas overvoltage can.

http://www.mouser.com/ProductDetail/NXP ... cCvgDFo%3d

[Mee_n_Mac]

W/o looking hard at the specs ... that might work. I'd expect lower than expected drops because I doubt the 4A is the common use #. You'll also need some heatsinking as each diode will be dissipating at least a couple of watts IMO. At 60 degC/W (no heatsink), that's a 120 deg C delta from ambient to junction at 2 W. If the ambient temp is room, about 25C, then the you're at the 150 C max junction temp. You should aim for a 125 C junction temp or even lower (110 for long life).

[skimask]

Is it not realistic to have the engine running while watching a video monitor in the field (R/C, robotics, entertainment while on a HAM post)?

True.
But, what is unrealistic is for the rest of the world to continually guess at your end application...

If it was me, I'd plug it in and go with it, engine running or not.
I don't think it'll be the ~14v -ish that would kill it.
I think it would be all of the other electrical noise, inductive kicks, spikes, etc. that are present in an automotive electrical system that'll wipe out something on the front end of it's PSU.

And as far as under-voltage not damaging anything (vs. over-voltage which almost certainly will damage something somewhere)...well, 'taint necessarily so.

As far as using those diodes, check the graphs (Fig.5 of the datasheet)...
That 2volt drop is only at certain points, certain loads at certain temps.
Less load, less Vf. Higher temps, less Vf.

[falingtrea]

Does the manufacturer of the monitor have a Automotive adapter available? Check into it. You may find it is just an power jack with a fuse, and the TV was design to handle a Automotive input directly and they just supplied a 12VDC power supply for Home use.