Homebrew PCBs

On Thu, 25 Feb 2016 07:08:17 +0100, you wrote:

>You know just enough to be dangerous.
>None of your statements are correct.
>
>Cheater cords are not dangerous because they defeat the isolation between
>the TV and power outlet - there is no isolation between the TV and power
>outlet. They are dangerous for other reasons (chiefly miswiring).

It used to be that to get power to a radio (the all American 5) one
side of the AC line was connected directly to the chassis, and the
other side was rectified to produce power.  The plastic knobs and case
were used to protect the user from the AC line.  The particular
difficulty with this design is that the original cable could be
plugged in backwards.  The radio would work, but the chassis was now
sitting at full line potential.

Any television so wired (and there were some) would suffer from the
same difficulty.  I once modified a small color TV to be a monitor.
Unfortunately, it had the same kind of power supply and required an
isolation transformer to be used safely because the chassis was now
directly exposed.  

In measuring equipment, typically oscilloscopes, the chassis is
connected to the AC ground pin.  Thus the probe ground is equivalent
to the safety ground at the wall.  This prohibits using the scope in a
"floating" mode where the ground/chassis would be at an elevated
potential.  

Some people simply cut the ground pin on the scope power cord in an
effort to permit this kind of measurement.  That results in the
metallic chassis of the scope being at elevated potentials.  There's
also the problem when the device being measured has a fault that can
elevate its chassis above ground.  There are some scopes where the
chassis is isolated and the scope is battery powered.  These are
specifically made for such kinds of measurements (Tektronix 200
series).

The isolation transformer would help run the device in this situation
and prevent possible difficulties.  In fact, there is a piece of test
equipment that combines an isolation transformer with a variac, and
has metering to detect leakage to ground in the device plugged in.
This is not a common piece of equipment, though.

As a note, the back of the TV set used to have the power cable
attached directly.  Remove the back of the TV and the power cable was
disconnected.  A cheater cord was this power cable that did not have a
back attached.  

Harvey

>
>Autotransformers and variacs are not generally referred to as widowmakers
>any more than all tree limbs are referred to as widowmakers. The proposed
>use here is safe and in compliance with the relevant standards.
>
>You are dealing with lethal voltages at the output, no matter if it is
>ground referenced or not.
>
>
>
>ST
>
>On Thu, Feb 25, 2016 at 6:30 AM, rolohar@... [Homebrew_PCBs] <
>Homebrew_PCBs@yahoogroups.com> wrote:
>
>>
>>
>> YOU ALWAYS NEED "SAFETY" WHEN WORKING WITH POWER LINE ELECTRICITY!
>>
>> TV "cheater cords" once used in repairing TV sets were known as "Suicide
>> Cords" because the defeated the
>> isolation between the TV set and the power outlet.
>>
>> Autotransformers and Variacs are often referred to as "Widow Makers"
>> because they also do not isolate the
>> user from being exposed to the full potential of the voltage
>> source.......be it 120 or 220 or whatever.
>>
>> Mark My Words!
>>
>> Get a step-up or step-down transformer that has the primary winding
>> completely isolated from the
>> secondary winding and works on the concept of INDUCTION rather that direct
>> hard wired connection
>> to whatever power source you are using.
>>
>> Regards,
>>
>> RFH
>>
>>