Why a
TV Chassis

If you are in anyway involved with electronics you would have likely been confronted with the arduous task of having to repair a TV. During this exposure it is often found the chassis on a TV "bites". You measure the chassis to ground and find unexplained voltages exist on the chassis, what's more, it measures both an AC and DC characteristic.

The second common problem is an accidental short to proper ground (Earth) and have had the Earth Leakage Breaker trip out. This happens either when wanting to solder something or measuring with an oscilloscope. Hopefully no damage to the power supply rectifier diodes is done when this happens.

Here is an explanation as to why these above things happen.

The above picture shows how this voltage appears on the chassis. Most TVs do not have an isolating power supply like computers. The mains is rectified and fed directly to the circuitry that has it's negative connected to the chassis.

What confuses a lot of people is that Neutral on the mains is also 0v i.e. it does not move with respect to ground (apart from a small voltage that exists as a result of current through the Neutral wire which has some resistance). It is directly connected to Ground at the supply transformer and is deliberately split for the earth leakage system.   With this in mind (that Neutral and Ground are effectively the same potential) it must be accepted that Live swings from positive to negative by +/- 325volts (230 volts * 1.414 to get peak voltage).

It can therefor be seen from the drawing that when the Live swings negative it 'drags' the chassis with it to -325volts. When it crosses over and swings positive the chassis is held at 0 volts by the appropriate diode. This is what forces most meters to "measure" approx 150VAC but also, as it only ever swings negative, would also measure about -85 volts DC.

There is also a resistor to the chassis from the Neutral. What this does is to ensure the chassis is brought to ground potential when the TV is turned off (as explained earlier the Neutral is effectively ground) and by bringing it to ground the isolating capacitors in the antenna connection are all discharged (hopefully) so that the antenna connection does not "sting" any poor unsuspecting housewife (or some half-knowledged twit who then claims the TV is dangerous!) while trying to reconnect the TV to the video recorder!

If there is a need to use an oscilloscope for any measurements it is recommended to use a mains isolating transformer that will allow the chassis to be brought to ground potential (via the 'scope usually). This is, however, not always at hand and for a single repair may not be cost effective. An alternative is to "soft earth" the instrument. This allows the scope to work at rather strange voltages without bringing them to ground but also to ensure the chassis of the 'scope is brought back to ground potential once the voltages have been removed. Please note the chassis of the oscilloscope takes on the same potential as the TV and must in no way be brought to ground or even touched, it too will "bite" while connected to the TV.

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© 14.08.00 / 04.01.03