[allroadoz]

I have an LCD mounted on my headrest which has been wired into the cigarette lighter. Red to positive Black/Ground to negative.
TV specs are 12 volt 7.5 watts.
When the car is off it does not work.
When the car ignition is 'on' (sometimes called accessories) it will work.
When the car engine is running it does not work.

I am waiting for a friend to bring a voltmeter around but I was under the impression that a cigarette lighter gave a constant 12 volts.

Anyone have any ideas why this might be happening?

 

[PetrolDave]

I have an LCD mounted on my headrest which has been wired into the cigarette lighter. Red to positive Black/Ground to negative.
TV specs are 12 volt 7.5 watts.
When the car is off it does not work.
When the car ignition is 'on' (sometimes called accessories) it will work.
When the car engine is running it does not work.

I am waiting for a friend to bring a voltmeter around but I was under the impression that a cigarette lighter gave a constant 12 volts.

Anyone have any ideas why this might be happening?

First there's no such thing as constant 12 volts in a car - ANY car. The voltage depends on the state of charge of the battery, the temperature and whether the engine is running or not.

When the car is off whether you get any volts at the cigarette lighter depends on the car designer, some makes (like VAG) do but some (like GM) don't. If there is any voltage is will be whatever the battery voltage is, maybe just over 12 volts.

When the ignition is on but the engine isn't running the cigarette lighter voltage will be whatever the battery voltage is, maybe just over 12 volts again.

When the engine is running the cigarette lighter voltage will be whatever the alternator voltage is, maybe between 13 and 14 volts (even higher on a cold day with a battery not in good condition).

 

[allroadoz]

Thanks for clarifying that Dave, so do you think that jump to 13 or 14 volts would be causing my TV's to not turn on?

 

[PetrolDave]

It's possible - some electronics (badly designed) is very sensitive to supply voltage.

If you know a friendly engineer who has access to a bench supply with a variable volltage it would be worth getting him/her to check it out.

 

[allroadoz]

Ok Dave, I know someone who works in car electronics so I'll try and get his help.

Anyone know if there is any way to add something that will smooth out the voltage?

 

[MCNJS4]

Normally, any electrionics designed for use in a car are able to operate over the range of voltage normally seen in a car...so its surprising your LCD is so sensitive to voltage.

But if you want to keep a constant 12 volts (assuming your supply remains at or above 12 volts), use a voltage regulator.

 

[allroadoz]

The LCD screens I bought were definitely on the lower end of the LCD price spectrum, so that may explain why they may not be able to handle the variations.

I'll definitely have a look for a regulator, thanks for the advice

 

[MCNJS4]

Yeah, a basic 7812 regulator and a small heatsink (suitable for a TO-220 case) are all you need.

 

[bjarne]

Yeah, a basic 7812 regulator and a small heatsink (suitable for a TO-220 case) are all you need.

Unfortunately, that will not work too well, as the 7812 requires 14.6 V to guarantee regulation, and probably at least 13 V to do any regulation at all. You would expect 12 Volt or slightly less if you have equipment on and the engine not running, clearly less than that as you start the engine. With the engine running and charging it will be about 14 Volt. This is all with a good battery, if the battery is older, you could see bigger variations.

So if you want constant 12 Volt you will need a switching regulator meant for this application.

Bjarne

 

[MCNJS4]

The way I read his issue, the LCD wouldn't work when the car was running (probably providing on the order of 14 volts)...but would work when the car was not running (providing closer to 12 volts).

I certainly am in no position to doubt that the 7812 does not work ideally at only around 14 volts...but for the $2 it'll cost him to try this, it may be a worthwhile experiment.