Re: 12v TV


"Woody" <harrogate3@xxxxxxxxxxxxxxxxx> wrote in message news:xtgSl.247174$oh6.86667@xxxxxxxxxxxxxxxx
"gazz" <gazz@kampen~getlostspammers~wagen.co.uk> wrote in message news:gvbajr$81l$1@xxxxxxxxxxxxxxxxxxxxxxxxxxxxx
"Tom" <nospam@xxxxxxxx> wrote in message news:LvKdnUIv37uTc4XXnZ2dnUVZ8v2dnZ2d@xxxxxxxxx

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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