Live chassis  - Page 1
Forum section: electronics / beginners

author: electronics man
author_rank: Frequent Contributor
timestamp: February 16, 2014, 01:42:17 pm
content: In his most recent video dave talked about live chassis in TVs what was the point of them and why couldn't they just earth the chassis

author: nihilism
author_rank: Regular Contributor
timestamp: February 16, 2014, 02:18:28 pm
content: The reason these TV's are known as "live chassis" is because they do not have an isolated power supply - there is no transformer between the primary and secondary (if you can call it that) of the power supply.Basically some of the supply rails are just rectified straight from the mains then filtered and regulated, hence no isolation.This makes parts of the main circuit board at mains potential. They put isolation transformers on external connections to prevent the mains from entering the outside world, like the antenna socket and headphone jack. Dave touched the coax from the tuner before the isolation transformer, which was at mains potential.

author: SeanB
author_rank: Super Contributor
timestamp: February 16, 2014, 02:25:48 pm
content: Live chassis was to reduce cost of manufacture. If you were to have a grounded chassis in early TV sets and radios you would need a very heavy and very expensive mains transformer, just to provide the power. In a TV set this would likely double the mass of the set along with needing a bigger cabinet along with a lot of steel magnetic shielding. In a radio it would have doubled mass and cabinet size. Having a live chassis meant both a lighter set and a lower cost one, and those outputs that needed to be accessible ( antenna and headphones) can have isolation using small low cost capacitors and output transformers, that would be either there in the first place, or would have a lower voltage rating only. All operation controls are designed to have enough insulation to withstand the mains voltage along with a safety factor, which in many cases is provided by the design in any case.

author: electronics man
author_rank: Frequent Contributor
timestamp: February 16, 2014, 03:01:18 pm
content: then why did they stop doing them like that in the early 90s

author: PA0PBZ
author_rank: Super Contributor
timestamp: February 16, 2014, 03:05:42 pm
content: Quote from: electronics man on February 16, 2014, 03:01:18 pmThey invented switching power supplies

author: electronics man
author_rank: Frequent Contributor
timestamp: February 16, 2014, 03:07:12 pm
content: ah that makes sense never thought of that

author: commongrounder
author_rank: Frequent Contributor
timestamp: February 16, 2014, 03:21:15 pm
content: Those TV designers were very clever then. They used the horizontal drive circuit as a type of switching power supply by adding extra windings on the horizontal output transformer. Those outputs were rectified and filtered for use in the various circuits. That gave some isolation as well, so the only hot part needed to be the (opto-coupled) horizontal drive circuit itself and the mains powered supply for it. They still used little isolation transformers for the antenna in and headphone out to meet safety standards for leakage currents.

author: PA0PBZ
author_rank: Super Contributor
timestamp: February 16, 2014, 04:13:49 pm
content: Quote from: electronics man on February 16, 2014, 03:07:12 pmIt is only half the truth, SMPS started to arrive when the tubes were swapped for semiconductors so the total power needed went down. I think the 90's is a bit late, it must have been end 70's, begin 80's.

author: vk6zgo
author_rank: Super Contributor
timestamp: February 17, 2014, 07:32:57 am
content: Quote from: SeanB on February 16, 2014, 02:25:48 pmThe extra weight & size of transformer supplies is quite minimal.I have carried both Australian & UK valve type TVs (the latter presumably transformerless),& they feel about the same weight.Much of the weight of a 23" & larger TV is in the tube,& metal chassis,anyway.TV sets & Radios in Australia used transformer supplies almost exclusively,up until the advent of switchmode supplies.The advantages of transformer supplies are :-Standard 6.3v filament tubes (no need for weird series string values)Full-wave rectification (lower value filter caps & chokes.)Very much less stringent insulation requirements.Very heavy transformers?---you can carry the transformer from a 4 valve Mantel set in one hand!The transformer for a valve TV set is about 1.25 times as big in all dimensions.Heavy magnetic shielding?--nope!

author: electronics man
author_rank: Frequent Contributor
timestamp: February 17, 2014, 11:21:47 pm
content: what i still dont get is why they conect the chassis to any thing i understand if you have an earth conection, why couldnt they just isolate the chassis from any thing live.

author: nihilism
author_rank: Regular Contributor
timestamp: February 18, 2014, 01:32:55 pm
content: The term "chassis" does not refer to the cabinet etc, it generally refers to the main pcb assembly.

author: Psi
author_rank: Super Contributor
timestamp: February 18, 2014, 01:52:01 pm
content: Live chassie refers to the metal mounting frame under the plastic back shell.Ive seen multiple old TVs where the entire metal mounting assembly for all the boards is live.It stems from the fact that rectifying the mains directly results in DC that can never have a common earth reference.As the AC waveform changes polarity current flows from mains ACTIVE/LIVE through a bridge diode to BOTH + and -.So when designing a TVs earth/case grounding without an isolation transformer the designer has two options.- Connect no TV electronics to the case at all. Then they can earth it.- Use the case as a common ground for the TV electronics, in which case they cannot earth it because that would short the mains.Since the designer kinda wants to screw many PCBs to the metal frame and use that as a common TV ground for convenience most designers went for option 2.Otherwise they would need to run grounding wires between each board.

author: vk6zgo
author_rank: Super Contributor
timestamp: February 19, 2014, 12:24:33 am
content: Quote from: Psi on February 18, 2014, 01:52:01 pm*Funny----there were very fewAustralianvalve era TVs that didn't use power transformers.There were many in the UK,though!Most SMPS in colour TV sets confine the "rectified Mains" section to a small area of the PCB,with all the other stuff isolated by the SMPS ,so there is no direct connection between the chassis metalwork & the Mains---it was not needed!Some,in fact had a completely separate,shielded SMPS.Maybe things have gone backwards since I last worked on TV sets!P.S. Sorry about that! I need to put my specs on!!I thought you were in Oz---obviously,the Electronics scene was different in NZ.