Why TV/VCR uses 2pin not 3pin power cord?

[juihau]

2 pin power not grounded

Hi, this question has been on my mind for a long time.

Why most TV/VCR uses 2 pin (Live & Neural) and not 3 pin plug?

I understand the cabinent of TV is mostly plastic and those exposed parts (A/V connections, screws) are connected to the secondary (cold) gnd. There is a converter transformer separating between the primary (hot) and secondary (cold) gnd.

Suppose we use the 3pin plug, is it possible to connect the secondary gnd to the Earth pin?

For VCRs/DVD players, most of the back cabinent casing is metal. Does it make sense use a 3pin instead a 2pin plug (same as with ATX power supply for PCs)?

Thanks to anyone who can offer some explanation on this.

 

[kiprono]

why two pin plug in tv earthing

VCR/DVDs use dc power and there there is no risk of electric shock even on the metalic casing. The ac input is passed through a rectifier and dc-dc converters.
In the TV, there are no exposed live parts an users are warned no to open the unit.

 

[FvM]

shock from cable tv cord

Without discussing the pro and con of using protective isolation with a two terminal mains connector generally , it particularly makes sense for TV, video (and also audio) equipment. Otherwise you get nasty ground loops producing hum and moving bars in TV picture. A TV installation has usually at least one earth connection through antenna cable, connected possibly to a distant ground potential or a cable TV system. Any additional connection brings on ground loop interferences.

 

[FANT]

mild shock from tv

This is because the electric "live"parts have a double insulation to the user, in case of an accident at least one of the insulations will survive and will possible to detect the broken part.
This is to use the tool or TV in places where there is no grund connection ( it happens quite often ) and to garantee the same degree of safety.

Mandi

 

[zlatkoMM]

how to make 2 pin & 3 pin power cord

Somethimes has ac voltage on antena jack wich is no good but just turn net jack.

 

[FANT]

is the tv antenna earth isolated from the mains

The AC antenna voltage is due to the capacitors used on the EMI/RFI filters on the power supply.
The current is lower than 1 mA, and is not dangerous.
In a 230 Vac system you should find about 115 Vac from the antenna connector to ground.

Mandi

 

[juihau]

leakage current tv antenna

Thanks everyone for the explanation. But it links to another related question.

Power supplies for TV is using smps which is the same as ATX power supplies for PC. TV uses 2 pin but all PC power supplies are using 3pin in which the metal casing is connected to earth pin.

Isn't 2 pin sufficient to provide double insulation for PC? Is it because due to different safety standards for TV and PC power supplies?

 

[laktronics]

dvd player mild shock

Hi,
No doubt the question is more involved than it appears. Most likely the need to keep the TV circuits floating arises out of the need to protect the equipment against lightning and high voltage coupling to the Antenna. With a grounded TV circuit, the Antenna can act like a lightning arrester and can damage the equipment. The reduction of safety to humans due to non provision of safety earth is compensated by use of the insulating plastic casing and keeping the circuits isolated from mains using isolation transformer.

Also short radiated disturbances due to lack of shielding for TV and audio circuits may be tolerated but for Computers and Digital circuits even a loss of one bit can not be tolerated and therefore in such circuits metallic casing and adequate shielding are functionally required.

There could be other reasons as well, as FVM has already mentioned the effect of ground loops on cable services.

Regards,
Laktronics

 

[juihau]

grounding using 2 pin to 3 pin adaptor

Thank you laktronics, I have a clearer picture now.

 

[pravardhan]

using tv power cord as pc power cord

Um... Now I have a question...

My friend bought a Phillips DVD player few years back. Myself & him went together & bought the player. After connecting the player & then switching ON, i just touched the body of the player & i felt a mild shock, which could be felt when a PC is not grounded. It's actually a 2 pin.

If you people say its just a transformer, then i shouldn't have received a shock! I think the all companies are not using transformers because of cost & bulk (height, weight & width). I think this could be the problem???

The same thing happened to a ADSL power adapter of UT Startcom, while i just touched the output pin!

Bye,
PRAVARDHAN

 

[juihau]

non earthed tv power cords

Hi PRAVARDHAN,

I'm not exactly sure but I think most power supply portion of TVs (smaller inches) are using flyback topology which consist of a converter transformer to step down to the required dc voltages for Backlight inverters, Audio supply, Video ICs.

For the mild shock, it is due to the Y Caps that are used to reduce common mode noise at the input supply lines between the lines and ground. Safety regulations IEC60065 impose limits on capacitor ratings and the maximum ground current to minimize shock hazard. If I'm not wrong, the max leakage current should be < 0.7mA.

Despite so, even if manufacturers meet this spec, a small current of 200uA - 300uA might create a mild shock. Hence, it is a challenge for TV makers to reduce this leakage current to as small as possible at the same time to pass the EMC (conducted emission) regulation.

Regards
juihau

 

[pravardhan]

vcr casing voltage shock

Hi juihau,

The mild shock problem was not with that piece only. I and my friend went back to the dealer for exchange & he said that if we wait till the evening, the technician will come & change it to 3 pin from 2 pin!

Um... I don't know how these companies can manufacture products without checking it properly!

Bye,
PRAVARDHAN