tube prototyping safety

[Harry Bissell]

You can "float" the scope by deliberately disconnecting the ground pin...
but then the chassis is at circuit potential. And the big honkin' case
will be a capacitive load...

Battery Powered scopes are even safer... but again the case goes live so
watch out !!!

If you're concerned about safety (and rich I might add...) consider
getting a
Tektronix 5200 active differential probe for your scope. They cost about
$450 but
have 1/50 and 1/500 attenuation, allowing you to measure voltages up to
1000V directly
with no danger of "ground connection".

If you are less well hung... an op-amp based diffamp using high value
input resistors can
work also. I've used a TL081 with 1 megohm input resistors and 10K
feedback and bias... (so 1/100 gain).  Power it with batteries for the
ultimate in isolation... or use a transformer isolated power supply... but
remember that the transformer has to be able to support the difference
between the measured voltage and ground. Some of the Signal Transformer
"flattran" series has excellent isolation because primary and secondary
are on opposite sides of the core, on separate bobbins. Note that the
isolation isn't as good at high frequencies either... but its OK.

I use 2 - 499K 1% resistors in series for the "1Meg" inputs. Use 1  watt
parts... not for wattage but for voltage (creep) distance. Mount these
resistors up in the air so high voltage can't arc along the PCB surface.
Slots cut in the board material are a really good idea. Air is a very
good, free insulator.

I've measured voltages of 700 VDC (slewing at 500V/uS - 2.5K pulse rate)
without noticible difference between the Tek probe and the "cheapo"
circuit. The Tek probe is way safer. But doing differential measurement of
high voltage is a really good idea. Measuring what I measure wtih a
"ground clip" is suicide !!!

:^) Harry (Lethal Voltage II) Bissell

Ingo Debus wrote:

> jhaible wrote:
> > But what are you doing while prototyping, when you're
> > working on the open circuit ? My thought would be *not*
> > to attach protective earth anywhere and keep the high
> > DC voltage floating, so you'd only run into danger when
> > you're touching both the high voltage and the signal gnd
> > at the same time. (With parts of the circuit connected to
> > PE you'd be in danger just touching *one* point of the circuit.)
>
> I'd say, as long as there's mains voltage anywhere in your
> device/circuit (i.e. mains transformer included), keep the PE
> connection. But if you have a seperate DC high voltage supply in a
> closed box (with PE) it's safer to have a floating supply. But, as
> Magnus already wrote, as soon as you hook up a scope, it's no longer
> floating anyway.
>
> The separate closed DC supply might be a good idea anyway, because you
> cannot get in touch with high voltage AC anymore. Not that DC isn't
> dangerous, but AFAIK AC at mains frequency is more dangerous.
>
> Ingo