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prateeksikka

TELEVISION

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You asked this in another area....

Mine has the third prong for grounding. So, I cannot give you a reason why they do not have them ;D

In circuits which use the third prong for grounding, this is a safety feature so that the end user will not get zapped in case the mains return line becomes disconnected and the case becomes live. Many appliances are double insulated and do not need this protection.

MP

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hi MP and others!
I HAVE HEARD PEOPLE say that amateurs should not try to repair atleast the TV or CRT because at some points there is very high voltage inside even more than the supply of 220V.how is it possible and where are these points so that i take care when i repair my TV IN future.
plz reply
prateek

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Hi Prateek,
See? You don't know that a TV has a flyback transformer to drive the horizontal deflection of the beam and to generate the extremely high voltage to make the beam. Some TVs also work when powered by the 12V fom a car battery. They don't have or need 220VAC.

Therefore I recommend that you stay away from the dangerous inside of a TV until you learn what's in there.

Basic things like these are explained fairly well at www.howstuffworks.com .

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ONE OF MY TEACHERS ONCE TOLD ME THAT YOU CAN HAVE VOLTAGES ABOVE THE SUPPLY IN AC CIRCUITS BUT NOT IN D.C.

Allow a DC supply to power an oscillator called the horizontal one in a TV.
The oscillator passes its AC output to a transformer called the flyback one in a TV.
The high voltage AC output of the transformer is rectified and cap filtered into high voltage DC. Sometimes a voltage doubler or tripler is used as a rectifier/filter cap.

EVEN WITHOUT TRANSFORMER.IS IT POSSIBLE?

A voltage multiplier circuit can double, triple or multiply the peak voltage of AC many times without a transformer, but at a fairly low output current.

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Dear audioguru

The High voltage of the CRT is not dangerous for human.
The only thing happen is that it gives you a little shock, because when the FyBack Transformer amplifies the Volltage, It reduces current
(Power on both sides of a transformer are equal)

The only dangrous section of a TV, is the power section before the Switching tranformer of power supply, and this part is marked with a WHITE line around it, the other parts of a TV do not hurt you because it is isolated with SMPS Transformer.
The only thing sometimes happen is that some Parts shock you in such way that you learn never try to repaire your TV Unless you are a expert.

Shahriar


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Hi Shahriar,
TV high voltage isn't just static. It is about 25,000V at about 100W. It is regulated so while it is zapping you, it boosts its power. If the shock doesn't kill you, when it throws you you might get hurt badly.

Many TVs are also dangerous to work on because they don't have power isolation transformers. One side of the mains is directly connected to the chassis. If it is plugged in wrong and you are grounded then you will be fried!

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hi shahriar and audioguru!
can i generalize the statement that all AC CIRCUITS are dangerous because the voltage across the connected components can be even higher than the supply as according to kirchoffs law the nrt voltage is the phasor sum of voltages across the components.for e.g in an RLC circuit i have a net supply of 400V ac(assumed)
across R--------400V
across L---------1000V
ACROSS C------- -1000V
HENCE NET 400V AM I GOING IN THE RIGHT DIRECTION????

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Hi Prateek,
You can have voltages higher than the supply voltage in tuned LC circuits but most stepup transformers are not tuned by a matching capacitor equaling their inductance. They stepup the AC voltage due to magnetic coupling between their windings and the ratio of turns of the windings.

I have 120VAC/60Hz for my mains. I found a fan with a 220VAC/50Hz rating. It would barely run slowly on my mains and sometimes wouldn't start. I tried various capacitors and found that a 6.8uF cap in series with it made 185VAC across it on my mains and it ran fairly well. I put a high-value resistor across the cap to discharge it when the circuit was turned off because the cap also had about 185VAC across it when the fan was running with it.

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Guest Kasamiko

Hi Shahriar,
TV high voltage isn't just static. It is about 25,000V at about 100W. It is regulated so while it is zapping you, it boosts its power. If the shock doesn't kill you, when it throws you you might get hurt badly.

Many TVs are also dangerous to work on because they don't have power isolation transformers. One side of the mains is directly connected to the chassis. If it is plugged in wrong and you are grounded then you will be fried!


With set as big as 29 inches the voltage going to CRT reach as high as 30,000 volts!! and it is very dangerous to person suffering some heart problem..
There is also a danger of touching another voltage source like the mains input..
Mostl latest television I serviced now are all line isolated..Using a switched mode power supply, chopper..But touching some metal parts like the RCA jacks, antenna terminals and your standing on a cement floor barefooted and you will also received a nasty shock!!
What I did is to connect all exposed metal parts to Earth/Ground..using large short piece of wire..

rhonn ;) ;)

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Hi Rhonn,
What is more dangerous, working inside a TV with a 46" CRT or trying to lift it?! ;D
I saw one (Mitsubishi) with that huge CRT and it was so sensitive to the earth's magnetic field that it had a switch on the back for the direction it was facing.
Years ago TV techies worked with one hand in their pocket. When the working hand frequently got zapped, the juice didn't go through their heart.
You work with bare feet? I see my feet only at "b" time: beach, bath and bed. ;D

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The voltage determines the current which is determined by how much you are sweating. It takes many volts to make a noticeable current to flow in your body. Maybe very high voltage burns a hole in your skin to make a nice conductive path inside your wet guts.

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AUDIOGURU!
A FEW mA current is sufficient to kill a person .if i take a voltage source of 3V,short circuit it,a large,huge current will flow,even if i touch the wire at this point it wont even make me feel the current while a roomheater which has a few mA or hardlyA current can tear apart a person.ur comments?
prateek

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Guest Kasamiko

hi audioguru!

even if we get a shock!what is that which killsa person?
high voltage or high current?
prateek

It's the CURRENT that kills!!

rhonn ;) ;)

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How little current flows in you if your skin measures 200K ohms and you have only 3V? 15uA. You don't feel it.

Put a 9V battery on your nice wet, conductive tongue. You can taste the current and I think it feels warm. I haven't measured the current.
Try it again after eating something very salty. Keep a glass of cold water nearby because you might see steam, and you know what steam means is happening. :o

Put 25,000V on your 200K skin and 125mA will try to flow, and if you are sweaty or if the spark burns through your skin a lot more current will try to flow. The current will be limited by the power rating of the HV supply.

Current kills but you need a lot of voltage to make enough current flow in your high resistance skin.

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When the high voltage spark burns through your skin and travels much easier inside you, it will probably take only a few uA to kill you.

29mA seems like a lot of current through a person, my country's limit is much lower. Since your skin has a high resistance, the current probably spreads out over it.
I think the current limit is for AC mains voltage that shakes you and throws you away if shocked. High voltage DC would make your muscles contract and you can't let go, getting a much longer shock with more burning causing higher current.

He, he. Maybe your body has a circuit breaker. The current stops when your blood boils away!

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hi audioguru!
can i safely conclude that it is neither voltage,nor current but high power kills a person.high voltage and high current alone cant do anything .it is the power(voltage*current) which kills the person.
comments from all?
prateek

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It is not as simple as saying it is power that kills:
1) AC or DC makes a big difference.
2) The skin's resistance (wet or dry) or whether the spark has burned into you makes a huge difference.
3) Where on your body the current flows makes a huge difference. You can pass a high current through a single arm or leg all day long. You won't die until it falls off! But pass a tiny DC current through your heart between both arms or an arm on one side and a leg on the other and your heart stops. Then you'll be very dead very soon.

You also can't accurately determine how much voltage is lethal since the body's resistance is so variable. How many people did they kill to find out how much current is lethal? Try it! Maybe they used a de-fib thingy to get the hearts going again. ;D ;D

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