[R-390] Inrush Current Limiters

[Tim Shoppa]

N4BUQ asks:
> Does anyone know the failure mode of an Inrush Current
> Limiter (ICL)?

The inrush-limiter failures I've seen were due to a short elsewhere
in the circuit.

Normally when "warmed up" the current limiter only dissipates
a few watts and has a voltage drop of a few AC volts across it.
But when the circuit that the limiter is protecting goes dead-
short, the E-squared-over-R losses shoot up really really
fast (remember R for the inrush limiter drops as it warms
up) and the limiter ends up dissipating hundreds of watts
for a few seconds and pretty much incinerates itself. If
the wires fall off in the incineration process, it can go open
like you worry about below.

>  The reason I ask is where I want to install
> this one is on the neutral side (it's slightly more
> convenient); however, if the ICL fails in an open mode,
> then it could act like a blown fuse and if it's on the
> neutral side, then the radio would be dead, but there
> would still be power up to the ICL (same reason it's
> not a good idea to install a fuse on the neutral side).
> If it fails in a closed mode, then it really wouldn't matter.
> I'm just trying to decide if I really want to install it on
> the less convenient hot side or just go with the
> neutral leg.  Any comments?

In retrofitting old radios, I've always installed the
ICL in a dangling position between two pre-existing terminals.
When the limiter incinerates itself, you might want
to think where the dangling leads will end up. But in
the incinerated condition, one dangling lead will be hot (or
at least transformer shorted primary impedance to hot) and
the other will be neutral (or at least transformer shorted
primary impedance to neutral), and this will be true whether or
not it's on the hot side or the cold side. So I don't think
it matters much. Anything in the AC power section would
be treated as "hot" anyway (nobody really trusts that
those knuckle-dragging electricians never get them
mixed up, do they? I've got lots of light sockets
in my house with hot on the screw thread... goddamn
knuckle-dragging electricians! How did they ever 
pass the inspection???)

Tim.