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6 volts DC for 2 inch TV

I have a 2 inch Sony Watchman tv that runs on 4 AA batteries. I want to use it in the car during a trip this spring.

Is it better to buy a "power supply" that plugs into the cigarette lighter and generates 6 volts, OR

to run it with 4 batteries, measure the amperage, calculate the effective resistance of the TV, and then make my own power supply that plugs into the cigarette lighter, that would consist of a plug, a jack, a wire and a fixed resistor. The resistance of the resistor would be the same or nearly the same as the calculated different of the tv.

My experience is that fully charged AA batteries are usually 1.56 volts x 4 = 6.3 volts, which is exactly half of the resting voltage of fully charged car batteries, 12.6 volts. I won't be watching tv when the car is running, but maybe I would sometimes run the car to charge the car battery, or even to warm the car** Then, iirc and I can measure it to be sure, the voltage goes up as high as 13.2, half of which is 6.6. Also, the label on the back of the tv says that it uses, on average, 1.6 watts, but I will measure its current current use and not rely on that.

Is that too high for a tv that is meant to run on 6.3? If so, I'll be glad to make the resistor bigger and thus the voltage lower.

It also has provision for turning off the picture and only getting the sound. I guess it would use less current then and I would calculate a higher resistor to be needed. Maybe I should just use a resistor substitution box and see at what resistance the tv works best? Or should I buy a device made for this purpose. I have a spare cell phone charger, but I don't think it wil put out a full 1.6 watts.)

Thanks

**On my best car trip, I slept in the car for 7 or 8 nights in a row. It was very comfortable but it was a much bigger car, so I'm not sure I can do even one or two nights this time, but I want to get some use out of my 2 inch tv, which I got as a gift sort of, and which will be obsolete in 36 months or so. For tht reason alone, I'd like to sleep in the car rather than a motel. Or probably more likely, in a tent in a state park, in late winter or early spring, in the South, where I guess it will be warm enough. If I stay in a tent, I'll make a longer wire.

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Reply to
mm
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Resistance! I don't know where I got "different".

By "fully charged", I don't mean chargeable batteries, I just mean new, non-rechargeable AA batteries. I plan to add a power-in jack via a 3 conductor wire, and arrange it so that the batteries are disconnected from the circuit at one end when the power plug is plugged into the tv's new power jack. There is currently no provision in the tv for charging batteries, and I don't want to spend the money on 4 new rechargeable batteries, and I don't want to risk applying a charging voltage to dying non-rechargeable batteries. I just want to powr the tv from the cigarette lighter.

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Reply to
mm

Hi...

Just thinking out loud; wouldn't it be so much easier and so much more practical to just pick up a nimh/nicad charger that works off of 110 or 12 volts, along with 8 batteries, and use one set while charging the other?

Take care.

Ken

Reply to
Ken Weitzel

mm wrote in news: snipped-for-privacy@4ax.com:

Sounds like you have a good grasp of using a resistor to drop the voltage, and the fact that if the car's battery voltage varies, so will the voltage dropped by the resistor.

Better way: Use a voltage regulator IC to put out a constant 6 or 6.3 volts, even if the battery voltage varies (down to 10 or up to 15). Or, easier, buy a power adapter that does essentially the same thing.

Reply to
Jim Land

Are you familiar with load dump? ISO says that voltage can be 270 volts. SGS Thompson defines it at 80 to 100 volts. Your design will make load dump irrelevant. Cheaper inverters hope you never learn why they are cheaper - while forgetting to include load dump protection.

Reply to
w_tom

That is a crude method - far better to use a voltage regulator. The output voltage may go too high under certain conditions - like standby - if you use just a resistor as that voltage is load dependant.

Like all these things it's probably cheaper to buy a ready made one.

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    Dave Plowman        dave@davenoise.co.uk           London SW
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Reply to
Dave Plowman (News)

Either should be an effective way to kill the tv, though the resistor method will do it double quick. A resistor dropper is most inappropriate, a regulator will not generally be able to handle load dump transients, and is liable to pass them to the tv, ensuring its well toasted. Stick with running it off batteries.

NT

Reply to
meow2222

On Thu, 14 Dec 2006 23:05:16 -0500, mm Has Frothed:

Yes.

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Reply to
Meat Plow

"w_tom" wrote in news:1166169455.692246.111910 @f1g2000cwa.googlegroups.com:

About Load Dumps in vehicles:

"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. "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'."

Reply to
Jim Land

On Thu, 14 Dec 2006 23:05:16 -0500, mm put finger to keyboard and composed:

If you are looking to do this on the cheap, then I'd modify a mobile phone car adapter/charger. But then I have dozens of them.

- Franc Zabkar

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Reply to
Franc Zabkar

Given the number of semiconductors in use in the average vehicle - and have been since the first transistor radio - it seems to be a problem that has been solved...

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    Dave Plowman        dave@davenoise.co.uk           London SW
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Reply to
Dave Plowman (News)

Do the new ones put out 1.6 watts, which is what the back of the tv says it uses on average? I have a spare new one.

Do even the old ones put out 1.6 watts?

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Reply to
mm

Yes. However a resistive dropper does not solve it, nor does a simple volt reg. A 12v vehicle system load dump onto a 6v tv would be a setup waiting to fail.

If the OP is incautious enough to do this, it should survive longer if you never switch heavy loads on or off while the tv's connected. The heaviest of course being the starter motor.

NT

Reply to
meow2222

Yes. The problem is solved because first ISO and SGS Thompson defined the numbers. Then electronics is designed to withstand those numbers without damage. Do you think mobile equipment costs more just because the manufacturers conspire to be greedy?

One designing for electronics must do what is standard for auto compatible electronics.

Reply to
w_tom

I've got a few home built bits on my car with standard voltage regs with no high voltage protection - they mostly are designed for peaks of around

40 volts, IIRC. The chances are the TV has internal voltage regulation too.

As I said how did a crude early germanium transistor radio survive, then? But the quote said it was caused by the alternator. Which isn't charging during cranking.

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    Dave Plowman        dave@davenoise.co.uk           London SW
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Reply to
Dave Plowman (News)

It costs more? Can't say I'd noticed.

So just what are these 'standard' precautions that aren't essential elsewhere?

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    Dave Plowman        dave@davenoise.co.uk           London SW
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Reply to
Dave Plowman (News)

Problem and solutions were defined in that previous post - ISO and SGS Thompson products for automotive applications. Consult SAE J1455 and ISO 7637-1. Learn about National Semiconductor's harsh environment semiconductors for automotive applications such as the LM5005. Learn about General Semicondictors 6KA series products used in load dump protection circuits. Read Linear Technologies design notes and their LT3437. So much is so readily available that I am surprised one should have to ask.

They often did not which is why load dump was discovered. Same failure also occurred to alternators with failed germanium diodes. The Detroit manufacturer even overnight delivered those replacement alternators. Load dump created by a manufacturing design defect created germium semiconductor failure.

Reply to
w_tom

mm ha escrito:

Well, 1.6 Watts at 6V is nothing more than 267 mA. (1.6V / 6V =3D 0.267 A) I would be greatly surprised to find a cellphone charger not able to produce that amount of current. Check the mA rating of the DC charger you want to modify, if it provides a *regulated* output of 6V at 300 mA or more, you will be fine.

You want to find a charger who outputs nothing more than 6V, and 300 mA or more. The TV will take the amount of power (consumption in mA) it needs, meaning that a 500 mA power source will not damage it, because the TV will only draw 267 mA from the 500 mA power source. Just be careful of the voltage produced by the charger, because that=B4s a critical paramater and it shouldn=B4t be higher than 6.6V or lower than

5=2E4V under any circumstances.

To close, the idea of the dropping resistor is an awful one. Why? Because the 1.6 Watts consumption stated in the back label of the TV probably is an average value, or a maximum one obtained only under worst case conditions. The consumption power of a TV, radio, or amplifier, is really a variable one. It will vary depending on how high the speaker volume is, and even the screen content. For instance, a white image will demand more power than a mainly dark one. So you can=B4t really obtain a fixed voltage just using a fixed dropping resistor if your load is a variable one. Even worse, as other posters had previously stated, the DC voltage inside a car can go anywere from

12V to 14V or more. So now you have two uncontrolable variable parameters: a) The voltage source, and b) The power consumption of the TV.

Don=B4t risk frying your TV. It=B4s more worth having it fuly operational than the cost of a proper DC regulated power supply.

Reply to
lsmartino

A voltage regulator IC, or a modified REGULATED charger (modifying it might be more of a hassle than it first appears) should do fine. If it were me, I would simply use a series dropping resistor followed by a shunt filter cap and a shunt 6.3 volt zener. The zener will probably have to be mounted on a small heat sink - but so would a voltage regulator IC. Pretty simple.

Bill

Reply to
Bill Jeffrey

On Fri, 15 Dec 2006 21:09:04 -0500, mm put finger to keyboard and composed:

The majority of mine are switchmode PSUs based on an MC34063 IC (I also have some linear supplies).

The application circuit on page 7 of the MC34063 datasheet shows a

5V/500mA design example:
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In a typical car charger you will need to recalculate R1 and R2 to get the desired output voltage.

FWIW I've adapted one of these chargers to supply 7V to a 12V fan in a DVD player.

If you are worried about the load dump problem being discussed elsewhere in this thread, then you might like to place a transorb or transil (transient voltage suppression diode) across the 6V output. However, I suspect that a SMPS of this type is largely immune to supply transients (due to the series inductances). You may still want to add the diode for overvoltage protection in the event that the IC fails.

- Franc Zabkar

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Reply to
Franc Zabkar

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