CRT handling Was: Re: It's ALIVE!

From: Lawrence Walker <lwalker_at_mail.interlog.com>
Date: Tue Feb 23 20:58:17 1999

On 23 Feb 99 at 0:46, Tony Duell wrote:


SNIP
> TVs can be dangerous - more dangerous than monitors, actually. But the
major
> hazard is not the CRT, and it's not the EHT. It's the mains, and supplies
> produced from it.
>
> A TV set can have most of the electronics directly connected to the mains
> using what's called a 'transformerless PSU'. The only external connection
> is the aerial, and that's easy to capacitively isolate. The chassis of a
> TV might be connected to one side of the mains (which could be the live
> side) or it might be at half mains voltage. Either way, it's an easy way
> to get killed.
>
> A monitor almost always (there is one common exception [1]) has an isolated
> PSU. The chassis is at earth potential. There is still mains on the
> primary side of the PSU, but that's all. You can work on the rest of the
> monitor if you take care and not get killed. But make sure you know what
> you are doing before starting out. Read the service manual, ask here,
> read the manual again.
>

 I have worked on various appliances ,electrical devices and/or wiring
most of my life, close to 50 years, and I have been "bitten" on occasion.
 Of course, except for stoves and dryers, the voltage here is 110 v. I can
understand the respect given to 220v.(3wire) and have never worked on
anything at that voltage without shutting down the circuit. I have also been
very cautious about working on anything in damp conditions like in a
washroom or wet basement. I realise that it is the total amperage that kills
when you become ground. Most household circuits here are 15-20 amps.
Having been "bitten" to no ill effects other than by my reaction to the shock,
what are the parameters which would kill you on these circuits. e.g. rubber
soled shoes vs leather, etc.

> Of course if you are working on a live chassis unit, or on the primary
> side of a PSU, then you should use an isolating transformer. And if
> you're working on anything that might be dangerous, don't work alone.
> Have somebody with you who knows how to turn off the power if anything
> goes wrong. However, in my experience, it's not a good idea to have a
> parent or a girl/boyfriend in the same room. They tend to cut the power
> if anything at all unusual happens, which gets very annoying fast.
>

 I used isolating transformers in electronics lab (more than 15 years ago) and
had them explained at that time. I've forgotten their functioning. IIRC they
are an ordinary transformer, altered to provide "isolation" from ground (via
your body). I have several large transformers. How difficult would it be to
build one from these.

> [1] The live chassis monitor is the US (110V) version of the original
> TRS-80 model 1 monitor. It's a converted live-chassis RCA portable TV,
> and it has an opto-isolator on the video input. Most of the internal
> circuitry is directly connected to the power line. This is one monitor to
> treat with great respect.
>
> The UK (240V) version is a lot nicer. It uses the same TV chassis, but it
> includes a double-wound isolating transformer to step the 240V down to
> 110V. And thus the circuitry is isolated from the power line by that
> transformer. There is no opto-isolator.
>
>
> -tony
>
ciao larry
lwalker_at_interlog.com
Received on Tue Feb 23 1999 - 20:58:17 GMT