Re: 12v TV

"gazz" <gazz@kampen~getlostspammers~wagen.co.uk> wrote in message
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"Tom" <nospam@xxxxxxxx> wrote in message
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The TV is a domestic one and the reason I bought it was
because it said 12v power supply on the box and I bought it
specifically for the caravan.

what was being pointed out is that the mains power supply
outpts 12 volts... usually 12.3 volt actuall, but it dosent put
out a millivolt extra,

hence the TV's components are made using the lowest tollerance
parts available, no need to use more expensive higher voltage
parts when the TV will never see more than 12.3 volts.

as a fair few people found out tho, plugging it into the
caravans battery directly... it's on when on battery alone,
highest voltage will be 12.8 and dropping, but connect the
charger and the TV will be getting upto 14.8 volts (depending
on charger) a lead acid battery dosent start to charge below
13.8 volts, so even if you had a really naff charger, your
putting 1.5 volts more than the Tv was designed to work on, not
to mention all the spikes, hash, noise etc on the line from the
charger, water pump switching etc.

the thing that goes is the backlight inverter, little puff of
smoke and you get no picture, the picture is there, but cant be
seen due to ne backlighting,

the flat screen tv makers are wise to backlight inverter
faliurs caused by overvoltage now, and reject a warrenty claim
when they see the components have failed in the way they
usually do when someone runs them on a caravan/motorhome 12
volt supply.

For about £40 you can get a DC to DC converter, i have one that
protects my Tv, dvd and sat boxes, takes from 10 to 18 volts in
and all the rubbish on the line, and outputs exactly 12.3 volts
of pure clean power, got all sorts of electronic protection in
it, unplug the dvd player and it'll shut down due to the tiny
spark you get then unplugging it, that's how sensative it is,
i have a large solar array on my vans roof, and it does an
equalizing charge every month, takes the batteries upto 16
volts for a few hours to balance them all out and stir up the
cells, my Tv would have let the magic smoke out first time it
did that for sure.

mines rates at 8 amps, and runs cool as a cucumber, but any
DC-DC converter will as it's not having to chop the voltage
down from 230 to 12, which is how switched mode psu's work, the
excess voltage/current is the heat you get in the power brick,
i imagine your TV's power brick is rated for 50 watts, the tv
can pull 50 watts, hence it gets hot, find a 100 watt power
brick and it'll run cooler.



Oh dear. With respect, if you don't fully understand it don't
pontificate.

First, a lead acid battery will charge if the incoming supply is
higher than its quiescent plate voltage. A fully charged battery
will provide 2.2V per cell or 13.2V unloaded. The bit about 13.8V
(or 2.3Vpc) is the point at which a battery will start to gas, so
if you charge it at, say, 13.5V, whilst it will not make full
charge it won't gas either - which must be safer.

In a car the alternator output is regulated at 14.4V or 2.4Vpc.
It matters not in such a situation if the battery is gassing as
the engine bay is open to the outside and the gas will disperse
quite easily. Not the same in a gas box on a caravan, hence
caravan PSUs (which incidentally are these days almost all
switched mode) are usually set at 13.8V or slightly less.

In respect of the TV, it would be useful to look at what it says
on the set about supply volts, not on the PSU. I would be very
surprised if the TV is not rated at 12-15V. I would also doubt
the PSU will be linear as the regulation of the transformer would
require it to be large and heavy and have a fairly large heatsink
on the regulator to dissipate the heat generated by the high
transformer output voltage that would be necessary. (Have you
ever looked at the size of a 5A regulated PSU for CB or similar
users?) The PSU will almost certainly be switched mode which
means it should run pretty well cold. If it is getting hot as
described by the OP then it is almost certainly faulty. It
shouldn't matter a jot whether it is rated 5A or 10A - it should
neither get hot nor get any hotter if the load is greater. It
would have been useful to know if the supply was getting hot when
not connected to the TV.

A SMPS doesn't quite work in the way to which you allude. The
incoming mains is rectified to d.c. and then chopped at high
frequency by a power transistor to feed a conventional
transformer. When hard on or hard off transistors dissipate
almost no heat, they only get hot when there is a voltage drop
across them as in a linear regulator.

Because of the high frequency the transformer losses are much
less and it is much more efficient so can be very small. In a
well designed supply there is a feedback circuit between the
input switching side and the output so that the secondary voltage
of the transformer (after rectification and smoothing) only
provides just enough headroom to keep the (linear) output
regulator working. The regulator thence will dissipate very
little heat and the whole unit will be barely tepid to touch. If
it is getting hot, as the OP described, then it is either a very
poorly designed SMPS, or it is faulty.

I would suggest that any manufacturer making something to work
from 12V must assume that it will at some point be used on a
battery supply, ergo it must be safe to run off at least 13.2V if
not 13.8V (which is, by default, the output voltage to which most
nominal 12V supplies are set.) I would however agree that the 12V
supply of a caravan - unless it is taken directly from the
battery - can be quite dirty with some horrible spikes, so some
sort of protection is a good idea. A small choke in series with a
following capacitor in parallel will remove the spikes but as the
battery is in effect a huge capacitor, direction connection is
the best bet.

If you want to spend money then a 12V isolating switching
regulator is the best bet. Those by Alfatronix (of Poole) are not
only well made but also carry a lifetime warranty, so if it goes
wrong they swap if f.o.c. They are very widely used in the
mobile radio industry to run radios in buses, trains, trams,
etc - and they run COLD.


--
Woody

harrogate three at ntlworld dot com


.

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