12V/240V Televisions.

I have already explained it to you in some detail at least twice in the last year

No, you have never explained why a switch mode supply/charger is so sensitive to harmonics on the input side when the first thing it does is to chop the input AC up into a square wave.

Your view seems to be contrary to those who actually do design them.

John
 
I am really sorry guys - I did not mean to start a war over inverters!!!!!!
I just wanted a simple answer to a simple question - but maybe I am wrong. I thought that truckers and the like who park up overnight in layby's miles from anywhere just plugged their TV into the cigar lighter and hey presto.
So, looking at some of the suggestions online it appears that they are supplied with just this sort of plug as well as a facility for mains when on shorepower - or am I totally going down the wrong track. /forums/images/graemlins/confused.gif

bob
 
Us too, shore power, inverter, or 12 volt off batteries, no problems with any.

Am interested in any outcome of the thread as our current 12v/230v TV is 10 years old (a Sony CRT) - am looking for a LCD 15-17 inchish (multi-system, HDTV compatible from a major manufacturer) so interested if anything turns up - in this part of the world these seem to be currently not listed by the major manufacturers.

John
 
[ QUOTE ]
No, you have never explained why a switch mode supply/charger is so sensitive to harmonics on the input side when the first thing it does is to chop the input AC up into a square wave.

[/ QUOTE ] No, that's a big part of your misunderstanding. If the chopper was right at the input it would spew RFI right across the spectrum. It would make it impossible to listen to the radio or telly in your own home and half way down the street. If is just not done that way. First thing is a filter which filters out the incoming hash and prevents the chopper from radiating back out of the equipment. There are other issues as well, but they are beyond the filter and hopefully you will now see that these design overviews are fine to get a general feel of how things work, but you can't rely on them when it comes to operating things way outside the design parameters.

[ QUOTE ]
Your view seems to be contrary to those who actually do design them.

[/ QUOTE ] I used to design switch mode supplies - albeit quite a few years ago. The components and technology have moved on considerably but the basic techniques and principles haven't changed much.
 
Don't run your shiny new telly off of a square or quasi-sine inverter, John /forums/images/graemlins/smile.gif
 
if this point is valid then I think the article on invertors in the current MBM is extremely misleading..............

would yo care to pick some specific holes in their observations and contribute those to the mag?
 
Come on, they first full wave rectify it (so very harmonicy straight away) and minor filtering into rough DC (so your telling me that diodes and smoothing capacitor don't like harmonics [aghast smilie here]), so can chop that up into a square wave by transistor THEN major filtering. So filtering to stop all the stuff you are talking about (that is not being able to hear anything on radio or TV) is AFTER it is already chopped into a square wave by the power supply itself - the harmonics from any inverter supplied ac on the input side have no influence on that at all.

Enuff from me, I am more interested in someone with some sense identifying a nice LCD TV for me.

John
 
I'd be happy to do that but being in Spain I seldom get magazines. Is it convenient for you to scan it and email it to me, say in pdf format? If so, I'll PM you my email address.
 
No, rectification usually comes after a filter. And there may be several filters. A filter stops the electrical noise from the high frequency (tens of kilohertz) chopper from getting back into the mains power lead and into the 'house' wiring. Either the same or a different filter is also present to prevent RFI which might be on the line from noisy equipment such as motors, etc., from interfering with the equipment. To put it another way, the filter stops nasties from getting into the telly and stops nasties from the telly's inverter from getting out of the telly.

[ QUOTE ]
your telling me that diodes and smoothing capacitor don't like harmonics [aghast smilie here

[/ QUOTE ] Indeed. A shunt C for smoothing the 50Hz fundamental looks like a much lower impedance at the 3rd, 5th and 9th harmonics. Impedance of a capacitor is = 1/2*pi*f*C so you see it starts to draw much higher currents at higher frequencies. If you look up the design features of power electolytics you will see that the loss is a major issue and the maximum ripple current is always specified. If you exceed it you will blow up the capacitor (which becomes very hot, first). It really isn't possible to predict with any reliability what will happen with a square or quasi-sine supply. You would need both the circuit diagram and parts list and ideally the design notes and test data as well. As I keep saying, it might work and be harmless. It might work and shorten the life of the product by over-stressing components. It might not work as well as intended. It might blow up the product within seconds or minutes. There is no way to tell and absolutely no come-back on the supplier if it was spec'd for a 230V sine wave input.

Frankly, whatever you do with your own stuff is your own affair but you do represent yourself in this forum as something of an expert and on this particular subject you are misleading people. I really think you ought to be less dogmatic about your beliefs because you must surely agree that someone somewhere might have designed a product that will be damaged by a square or quasi-sine wave? And someone reading this thread might have one of those products.
 
I really, really hate to admit this, but I've only understood about six sentences in this thread! /forums/images/graemlins/frown.gif /forums/images/graemlins/blush.gif /forums/images/graemlins/crazy.gif
 
you must surely agree that someone somewhere might have designed a product that will be damaged by a square or quasi-sine wave?

As you claim to have designed a few I have no doubt that you are correct in that claim /forums/images/graemlins/smile.gif.

John
 
I had the same problem but kindof in reverse? I will admit that I used to think it almost heresy to have either a TV or a laptop onboard for all the reasons discussed on this thread, until that is I managed to take a week out cruising at a time when it was still essential to keep in touch with the 'office' via email. The problem was solved by taking my wireless enabled laptop along which allowed me to get email when alongside, usually via the Squaremile network. Having taken the plunge and taken a laptop to sea, I then began looking at all the other good stuff it could do for me. Tidetables were first, and I found an excellent free programme here http://www.arachnoid.com/JTides/index.html As a chartplotter I found another excellent and free programme, AIS enabled as well, in SeaClear2. I dont remember what first led me to think of using the laptop as a TV as well, probably again work related, as I need to keep abreast of the news. The solution I found, and relevant to this thread was http://www.ebuyer.com/customer/products/...duct_uid=101969 Its an MSI MegaSky 580 USB digital TV tuner in the form of a thumb drive. It came with a mini external antenna and conventional antenna adaptor as well. In short it cost less than thirty quid icluding postage and is brilliant. Even using the supplied mini antenna placed manually on the cabin roof I get most freeview channels with excellent clarity on my laptops 15.4 inch LCD. From my mooring by Whale Island, due to the proximity of Brittany ferrys I think, I can normaly only get Sky news, the History channel and a few others. But hey ho, If I use a normal portable home antenna, I get everything as before, up to 80 channels? As to the power source this was my major limitation at first as I was dependant on shore power for most of the time due to the laptops limited battery life. I resolved this by purchasing a 300W Invertor from Maplins which was on offer for again about thirty quid. Yes, its a modified sine wave, but I will proudly admit to being a boater on a budget. You can find it here. http://www.maplin.co.uk/Module.aspx?ModuleNo=48723&criteria=Invertor&doy=9m10. In trials it has powered my laptop in TV mode for 7 hours continuous use. Before I get a ragging, I dont watch TV for 7 hours whilst aboard, normaly an hour a day max to get the news, the weather and such. So far the Invertor has worked fine, neither the boat or laptop have exploded to date, indeed it doesnt even seem to struggle. I use an 80Ah Leisure battery. One final point. When I tried this system plugging the Invertor into a cigarette lighter type socket it complained a lot then automatically shut itself down - its designed to do this. When I connected it direct to the battery, using the crocodile clip connectors, it worked just fine. Im puzzled, and am NOT an expert - perhaps somebody who is could tell me why?

Mark
 
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