Constant Current Regulator

[turner7A8]

I've recently purchased a Magnavox 15" LCD-TV {15MF400T/37} which requires 12 volt, 3.5 Amp, DC input.
I want to use this TV in my RV - on a Standard 12 volt, Deep Cycle, Vehicle Electrical System - where voltage can vary from 10 - 15 volts depending on the state of discharge or recharge, etc.

Will it be necessary to regulate the current input (to the TV) with a constant current regulator, and/or voltage regulator ??
Or can I just plug it Straight IN ??
Thanks,
turner7A8
PS- The TV came with a 120 V. AC to 12 V., DC, 4.16 AMP converter and there's no documentation regarding vehicle usage and Magnavox Support says there's no accessory for 12 volt DC to DC operation.

 

[VEBill]

It might be revealing to measure the open circuit voltage (when not yet connected to the TV) provided by the included AC power supply. I suspect you'll find it floats quite a bit higher than 12 volts.

See if the OEM offers any optional straight-thru power cords for connection to a cirgarette lighter socket.

 

[Skogsgurra]

First thing: The current consumption (3.5 A) is just that. It is what the TV will consume when connected to a 12 V source. It may consume less, but probably not more.

A TV that is specified to run off 12 V DC will usually accept a wide voltage range. A low 9 or even 8 V is not a problem and the high voltage usually exceeds 12 V by a fair amount. So I see no problem connecting the TV directly to the car 12 V system. That is probably what it was designed for in the first place.

There are some precautions you can take. One of them is to unplug the TV when working on the car's electrical system. So called "load dumps" can do things to the device. But they only occur if you disconnect the battery while running the motor. Not very likely. So the risk is minimal.

VERY IMPORTANT! Check polarity TWICE before you connect! And make absolutely sure that you understand what the polarity shall be before you smoke test.



Gunnar Englund

http://www.gke.org

 

[VEBill]

Minor point:

I thought that 'load dump' meant when a major LOAD is DUMPed (turned off). I didn't think it was related to disconnecting the battery while the engine was running (unlikely in the extreme).

 

[TurboXS]

I agree with skogsgurra. In automotive terms "load dump" is the voltage spike that occurs when the battery is disconnected from the alternator circuit when the engine is running.

 

[waross]

It's easy to design a set to work on one DC voltage and then use different power supplies to accomodate different voltages and frequencies around the world. If the TV does not specifically say it is suitable for vehicle use, it is probably not. The TVs that are suitable for vehicle use are often marketed as such.
respectfully

 

[OperaHouse]

I was just working on a color LDC monitor that ran on 12V. I just purchased this without a power supply and had to configure something. I assume these are both very similar with an internal switching regulator that is very forgiving. This unit worked easily from 10-15V, CRT type TVs with power derived from the horizontal ar less forgiving. It was rated at 3A, but actually consumed less than 1.5A in any screen condition. Used an older HP 18V 1.7A supply and just changed the zener going to the opto to reduce the voltage. In any vehicle application I like to put a small inductor in series with the positive, a diode to protect the polarity, and at least a 1,000uf cap to filter out any nastys.

 

[itsmoked]

A load dump is the removal of any large load... Not just the battery though the battery is generally the worst of the load dumps.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[TurboXS]

Load dump in automotive applications

http://www.st.com/stonline/products/literature/an/10493.pdf

 

[itsmoked]

Load Dump transients also occur when heavy loads are switched off although their magnitude and duration will be lower. These transients are capable of destroying semiconductors on the first 'fault event'.

An abrupt reduction in the alternator load causes a positive voltage transient called "load dump." In a load dump transient the line voltage rises to 20V or 30V in a few microseconds, then decays exponentially with a time constant of about 100 microseconds. Much higher peak voltages and longer decay times have also been reported.
The worst case load dump is caused by disconnecting a low battery from the alternator circuit while the alternator is running. Normally, this would happen intermittently when the battery terminal connections are defective.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[bmarshian]

There are "universal" step-up and step-down DC-DC adapters on the market that will plug into the standard car cigarette lighter (or airline DC outlet). Generally the specs say these are quite forgiving in the acceptable range around the nominal 12V input--the output V is fixed or selected by the user depending on the power requirements of the TV and the max Watt rating of the adapter. They come with a variety of tips that are supposed to fit most TVs or whatever type of device the adapter is designed for (laptops, etc.)--my main concern would be the polarity of the tip. Assuming that one of the tips actually fit my TV, I'd want to check this out before I turned the set on. If you're at all concerned about efficiency (battery drain, etc.)--this seems better than using an inverter to convert DC to AC, then using the AC adapter to convert back to DC. Does the description of that process absurd to you too?

 

[itsmoked]

No that sounds fine.

Usually products have a symbol on them now that shows tip+ or tip-.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com