What size inverter for 19" LCD tv

[Glenn_5132962]

What size inverter would I need to run a 19" lcd tv from my caravan battery & roughly how much current would this draw from the battery?

I want to put a 12v caravan plug on the inverter but my 12v sockets are limited to 10amps.

Thanks.

 

[xtrailman]

Depends on your TV rating. I use a 150 watt version, which has extra capacity.

Cost

 

[emmerson2]

I got a 300amp inverter from Maplins for

 

[J.Lo]

I use a 300 watt inverter on a 19" tv and it runs for more than 12 hours on a 85 amp 6 years old battery. Feed the inverter straight from the battery, not forgetting an inline fuse and a mains lead out to the tv. J.Lo

 

[GrahamDerby]

J.lo i have withe very van run a seperate wire dirct to the battery with a fuze to tun a 12v TV as the wiring in the van is too thin to take the power. The TV lasts 3 times longer that way.

 

[ProfJohnL]

Hello Glen,

Inverters are usually sold by the power rating they can handle so they will be rated in Watts (W).

The size of inverter you need is determined by the wattage of the device you want to run. Somewhere on your TV there will be a data plate/sticker that should give you the information you need. You will need to get the information from your TV, but as an example;

There is a relatively simple relationship between the three main factors of electrical power, Watts, Volts and Amps The power is the Wattage, the Current is measured in Amps, and the Potential is measure in Volts

Watts = Volts x Amps or W=VxA.

I have a 15" TV and the data plate tells me it uses about 200mA (0.2A) of 230V mains power. To convert that to Watts I multiply 230Vac by 0.3 to get 46W.

So to run my set I need an inverter that will handle 46W or a bit more. The typical smallest inverters are about 75W. Other typical sizes are 150w 300w 450W 600w 1000w 3000W.

In theory Inverters will only convert enough power to run the attached appliance, so if you used a 300w inverter to run my 46W TV, it would only pull 46W from the 12V supply, But nothing is perfect, so they will tend to pull a bit more just to make the inverter work.

The inverter will need about 4A (Amps) of 12V current to power my 46W TV and the inverter will need about 1A just to make it work so that is about a 5A draw from the 12v Battery.

Now the down sides to this.

Our domestic mains supplies produce a nominal 230V ac. This is in a sine wave form which is kind to electronics. The majority of lowcost inverters produce what is called a "modified sine wave" which roughly resembles a true sine wave, but has many rough edeges, which can cause problems for some electronics.

You can get pure sine wave inverters ( I have one that I bought many years ago from Maplin electronics) and these are fine, but they are less efficient, so will use more power to run the inverter for a given output.

Most inverters will need a healthy battery to run, typically one that produces 11.5V under load. (that is why emmerson points out it is best to use them on a running engine) The difficulty with caravans is that the battery is under a constant drain and the terminal voltage will drop quite quickly. Most inverters have a safety cut out which operates in the battery voltage falls to far.

As J.Lo suggests the inverter needs to be connected directly to the battery. The reason is that low voltage wiring needs to be able to carry a lot of current if it is to supply the inverter. Unfortunately high currents and long wires are not a good combination, because they heat up and waste power. They also cause voltage drops, so even though the battery may be above 11.5V, the voltage available to the inverter at its terminals may be significantly less than 11.5, so it shuts down. - Keep the 12V wires a s thick and short as possible. - It is easier and better to use a standard mains extension to get power from the inverter to the appliance rather than long 12v wires.

 

[xtrailman]

i run my inverter directly from the 10 amp caravan supply socket, adjacent the tv ariel.

 

[ProfJohnL]

As Ray says, you may get away with the 12V socket near the TV, but if you try to use an appliance that needs more power - a large laptop (60 to 90W) then the limitations of the caravan wiring may become more apparent.

Briefly, to keep voltage drops and cable losses to a minimum, keep 12V wiring as short as possible, keep the inverter as close to eth battery as possible, and if extensions are necessary, it is better to use them on the inverter's 230V out put rather than the inverter's 12V input.

If you want a more detailed explanation read on:-

The science is that all conductors offer some resistance to the movement of a current through them. Double the length of a conductor and the resistance doubles, so to minimise cable losses the cables should be as short as possible. Also, as the current flow is restricted, it develops a potential difference (Voltage loss) between the ends of the conductor so you never get out the same voltage out that you put in except when no power is being used.

The maths is:-

Cable losses (cable heating) in W= the cable Resistance x the square of the Current.

So short wires are one strategy, but the other and more effective strategy is to reduce the current passing through the wire.

Current = Watts divided by Volts, Thus my 46W TV only draws 0.2A at 230 V but needs 4A at 12V plus 1A to run the inverter so that is 5A

If I were to use the same wire and distance, and for the sake of the argument it has a resistance of 0.1Ohm then the losses at 12V will be 5 x 5 x 0.1 = 2.5W and at 230V will be 0.2 x 0.2 x 0.1 = 0.004W or 625 times less!

The other important factor here is the inverters min V requirement, The voltage drop along the cable is given by;

Volts = Current x Resistance

So for the 12V line system it will be 5A x 0.1Ohm = 0.5V. By comparison for the 230V system it will be 0.2 x 0.1 = 0.02V.

Although the 12v systems voltage drop is only 0.5V, if your inverter needs 11.5V then the battery needs to be a minimum of 12V to keep the inverter and TV running.

One of the symptoms of a 12v wiring voltage drop problem, is the inverter cycling between shutting down and the restarting. Simply, when the inverter shuts down it draws a much-reduced current, so the voltage drop along the cable also falls allowing the voltage at the inverter to rise. It may increase enough to tell the inverter to switch on, but as soon as it draws the current to feed to the load, the voltage drop due to current along the cable sets in and the invert shuts down again.

Connecting the inverter as close to the battery as possible means the cable losses are minimised and the inverter will work for longer as the battery can sink further.

The same reasoning

 

[xtrailman]

I run 95 watts approx off my inverter, using the built in 10 amp socket, the invertor has a built in cut off should the voltage fall below approx 10 volt.

Its not complicated.

 

[ProfJohnL]

I run 95 watts approx off my inverter, using the built in 10 amp socket, the invertor has a built in cut off should the voltage fall below approx 10 volt.

Its not complicated.

With respect Ray,

Your experience is not the same as mine. I have suffered inverter problems due to voltage drops when using manufacturers installed wiring.

I have given Glenn information and possible solutions if he does have voltage problems.

 

[donb]

I run a 19inch flat screen with a unit from amperor,

 

[xtrailman]

johnl

with respect, i am giving advise based on my own usage.

And am a experience electrician.

Bailey have installed a 10 amp dedicated 12v socket in my caravan, backed with a 10 amp fuse. To suggest using it will cause a problem is nonsense.

 

[ProfJohnL]

Hello Ray,

I am in no way calling your experience or qualifications into question, nor am I saying that a 10A supply wire WILL cause problems, I wrote MAY but given that I have encountered wire losses that caused an inverter to switch off and on due to the inherent resistance of the installed wiring, the probability is there.

If someone has similar experiences, then the advice I have given is still valid, especially if the supply battery is not in the best of condition and already partially discharged, or where as in the case I had to sort out where the wire route went