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Topic # 139046 25-Jan-2014 22:01 Send private message

Friend has a crappy little 24" TV in the back of their people mover for the kids to watch off a USB stick random animated shows that seem to keep children quiet.

It has a 12v input on it, and it will work off the car battery directly when the car is not running, but when the car is started and charging it shuts down. The inverter they have been using to power it thru the mains input has blown up when a PS3 was tried on it, and I thought a laptop power in the car adapter with a 12v output would do the trick to get the TV working again. The thing is do they actually regulate down as well when charging? They are not that expensive to get off trademe but I dont want to suggest it and have the same thing happen.

I want to get my inverter back off them ;)





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  Reply # 974386 25-Jan-2014 23:06 Send private message

I would have thought just running it direct off the cigarette lighter will work.  
Most 12v appliances such as modems/routers will happily accept up to 14v - a fully charged 12v battery is close to 14v.  
 




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  Reply # 974387 25-Jan-2014 23:07 Send private message

Oh jaycar might sell a 12v regulator - probably in kitset though




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  Reply # 974390 25-Jan-2014 23:09 Send private message

raytaylor: I would have thought just running it direct off the cigarette lighter will work.  
Most 12v appliances such as modems/routers will happily accept up to 14v - a fully charged 12v battery is close to 14v.  
 


It doesnt, it shuts down and wont power up again till it has been unplugged for a while.

It will work at idle with the headlights on, but give it any gas and once its past about 13v (only had analog multimeter) it turns off.

I think one of my universal laptop power bricks may have a lighter plug input on it, will dig it out and do some testing tomorrow.

Im sure this must be a common enough problem that there should be a cheap easy solution for it. 9v is easy to find, 15v is also easy to find.




Richard rich.ms

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  Reply # 974404 25-Jan-2014 23:33 Send private message

Sounds like an over-voltage protection.  
 
I also just had a thought - there are multivoltage regulators avaliable for 12v cigarette lighter plugs that allow you to select the output voltage. They use a switchmode chip which would allow you to regulate the voltage to 12v. 
 
The trick will be finding one that has enough amps output to meet the needs of the tv because they usually top out around 500ma and you might need 1 or 2 amps.





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  Reply # 974406 25-Jan-2014 23:38 Send private message

Yeah thats why I was thinking of the laptop one. Seems some of the older eee pc's took 12v so there may be 12 on the one I have somewhere in the shed. I stopped using it because it had changable ends that were always dropping off so I bought a proper dell adapter without changable tips and a switch. Just hope I still have all the ends for it ;)




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  Reply # 974407 25-Jan-2014 23:39 Send private message

I am thinking of one of these 
http://jaycar.co.nz/productView.asp?ID=MP3324&w=MP3472&form=KEYWORD 

But i am sure I have seen a selectable 3v to 12v one




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  Reply # 974409 25-Jan-2014 23:40 Send private message





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  Reply # 974645 26-Jan-2014 15:32 Send private message

raytaylor: Found it 
http://search.dicksmith.co.nz/search#w=Dick%20Smith%20DC%20Adaptor%201.5-12VDC%201.5A

Im off to town now. goodnight.


Yikes...
I doubt that 1.5 amp will be enough for a 24" TV, even LED backlit. Smoke may come out of it.
Those cigarette lighter plug in voltage adapters use 79xx regulator chips which top out at 1.5 amps (x 12v = 18 watts).
There are PSUs used in vehicles with 24V systems, so that higher power use 12v devices can be used (ie a 12v car stereo). Jaycar have some listed, but I note that the specs state 20V minimum input, ie http://www.jaycar.co.nz/productView.asp?ID=MP3063
A (another) 150 watt modified sine wave inverter to power the TV from 230v is probably the easiest way to go. Some modified sine-wave converters (as opposed to full sine wave inverters) cause noise on audio/video. A cheap one I bought from Jaycar seems okay, http://www.jaycar.co.nz/productView.asp?ID=MI5102&w=inverter&form=KEYWORD

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  Reply # 974763 26-Jan-2014 19:18 Send private message

I'd use an adjustable laptop power adapter. They are rated for high power and are regulated.




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  Reply # 974789 26-Jan-2014 19:58 Send private message

I'd use a 7812 regulator and if necessary use this to drive a power transistor, if you need more the than 1.5 amps. The 78XX or 79XX series make it  simple and easy to build reliable power supplies.. I've made a quite a few of these over the years.  A good heat sink is needed for the 78XX or 79XX or for the power transistor if that is used.




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  Reply # 974858 26-Jan-2014 22:13 Send private message

The LM7812 needs (from memory) 2V headroom for proper regulation, so 14V input. When the car is turned off the output will be about 2V below the battery voltage. And if you use a power transistor to boost output current you need another almost 1V headroom.

Use a laptop power supply, it is a switch mode power supply which will regulate irrespective of the input voltage and minimal heat dissipated.




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  Reply # 974882 27-Jan-2014 01:06 Send private message

Laptop power supplies wont work though, if the voltage is raised too high (which I think is causing it to shut off) such as when the car engine starts and its raised to 14v, then a laptop supply at 15v to 22v is surely going to be too much?

Unless you were to step up the voltage up to 15v then it could be switched or regulated back down to 12v




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  Reply # 974904 27-Jan-2014 08:35 Send private message

It is called a buck-boost regulator, can both boost the voltage and buck the voltage. It is really easy to do, I use them in almost every product I design.

I do not know if this is quality but it is an example: http://www.trademe.co.nz/computers/laptops/power-adaptors/universal/auction-688726457.htm. If the output is not 12V at 14V input then you should return it.

A cheap solution is add a silicone diode in series with the car power. At light load it will have a voltage drop of 0.7V, at heavy current 1V. So the power to the TV will be just over 13V max, and about 11V min when the car is off. I'd expect the TV to still work at 11V.

But whatever you do, the first thing you need to know is the maximum current the TV will draw. Either the current or power will be printed on the TV rating label.




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