Power supply musings

[terry michaels]

Message text written by Buck Buchanan
>Hi all,

To this day I've never truely understood *why* the they built so much hot
chassis stuff?  It just seems like it's asking for it!  I would think the
danger of having a floating chassis (that *could* be hot in a failure) is
safer than having a guaranteed dead!
ly chassis if the AC line plug is just turned around the other way.  Didn't
everyday appliances used to kill people occasionally?

Can anyone expand on the original rationality behind the hot chassis? 
There's nobody using it today....right?

Thanks much!

Buck
<

Hi Buck:

The hot chassis stuff was done strictly to be cheap and save manufacturing
cost.  It was commonly done with table radios in the '50s and '60s.  The
goal was to eliminate the power transformer and the cost thereof.  This
design used a vacuum tube rectifier which derived 160 volts DC directly
from the 110 vold AC line to power the radio circuits.  This was done with
a half wave rectifier connected to one side of the AC line, and the chassis
was connected to the other AC line.  Most radios  typically used 5 vacuum
tubes with the heaters in series, running directly off the 110v AC line. 
This arrangement depended on a two prong polarized plug to connect the
chassis to the neutral side of the line.  Safety was assured by putting the
potentially hot chassis in a plastic case, or in a metal case with
insulating washers, etc. between the two.  The problem with this is when
someone miswires a wall outlet with hot and neutral reversed, putting the
chassis at 110VAC relative to anything earth grounded.  Or they replace the
plug on the radio with a non polarized plug.  All it then takes is a break
in the plastic case, or a missing insulating washer to develop a deadly
situation when someone touches the radio and a sink, water pipe etc. at the
same time.  The available current would be limited by the fuse or circuit
breaker in the house, typically 15 amps.

I have not seen any recent vintage equipment made this way.  Maybe the
manufacturers figured out that putting a $5.00 transformer in their product
was cheaper than a $500,000.00 product liability judgement for an
accidental electrocution.

Terry Michaels