tube prototyping safety

[Harry Bissell]

The question is what is the voltage as well as the current of the power
supply.

Most human beings have a resistance of about 10K ohms. So in general
power supplies of less than 48VDC are considered relatively safe from a
"shock" hazard point of view.

Most higher voltage supplies can deliver enough current to be a hazard.

The isolation transformer does not serve only to limit current (which is
still enough to be a hazard even in a "small" transformer). They allow
the circuit to operate independantly of the incoming main power. So if
you were to have a connection (your hand??) from the circuit hot or
neutral to the main power (earth) ground, little or no current would
flow.

There is another class of power supply that is called and "off-line
switcher". This does not use a primary transformer at the mains. It
charges a capacitor and delivers a pulse waveform to a small high
frequnecy transformer, which provides the "isolation". These can be more
dangerous to an experimenter as they have a 200-300VDC main cap charged
dorectly from the mains. This CAN hurt.

Another danger in all supplies is the current capability or stored
energy capacity fo the supply. Big transformers can deliver big currents
to a short circuit. And big caps can deliver a lot of stored energy...
the dangers here are fire, things melting, and arc damage.
An arc from a large cap can spray quite a bit of hot metal around (think
arc welder...).
Capacitors can also "blow up" from damage or abuse (backwards... too
much ripple current... etc) and that can be a hazard. The best bet here
is SAFETY GLASSES.

The right transformer is not too big, or not too small. You are in as
much danger from overloading a small transformer (heat fire short from
primary to secondary kaboom...) as you are from a large transformer.

A really good idea is to use a GFI (ground fault interrupter) protected
outlet for your "unit under test" power. This will sense a small current
imbalance (like 10mA) and shut off the
power for you. Sometimes they can false trip... but more often than not
if they do there is a reason... and a good one. Find out why it tripped
and fix it !!!

Yes... as a rule a bigger transformer / cap / power supply is more
dangerous than a smaller one. But a big supply that does not "blow up"
due to overloading is actually safer that an overloaded smaller supply
(which does blow up).

This has been said before but needs repeating... A Variac is an
Autotransformer, which offers NO ISOLATION from primary to secondary. If
you need to use one, use it to drive the primary of an isolation
transformer... and then use that to drive your circuit.

Hope this helps...
:^) Harry

Plinio Barraza wrote:

> Hi all, happy new millenium:
>
> I hope this posts on synth-diy, though I think
> it wont.
>
> A while back, some one was talking about current
> capability of power sources.  I would like to kn
> ow what determines this.  Part of it will be the
> capacitors for sure, but I wonder about the tran
> sformer.  Do larger transformers have the abil
> ity of providing more current than smaller ones?
>
> If this is so, then would a smaller isolation
> transformer be safer as far as isolation?  I
> hope this does not sound to naive...
>
> Thanks in advance
>
> Plinio
>
> ---------------------------------------------
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