Edmund H. Ramm wrote:
In Andrew VK3BFA writes:

On Aug 18, 1:38 am, (Edmund H. Ramm) wrote:
In Andrew VK3BFA writes:


Are physics reversed down under? Up here in the old world a light
bulb's filament (tungsten) resistance increases with temperature.


Er - yes, thats correct.So whats the problem?

There will only be negligible inrush current limiting. For that you'll
need an NTC resistor like an Epcos NTC22. All you get from a light bulb
is a voltage reduction -mains voltage minus the voltage drop across the
bulb- but only AFTER the inrush current you intended to prevent has already
flown.

That lightbulb heats up pretty quickly, especially if there is a short
in the radio.

Just as a comparison:

a 250 watt bulb has a cold resistance of around 3 Ohms.

A 60 watt bulb, 18 Ohms

A 40 Watt bulb, 25 Ohms

I don't have a 25 Watt bulb handy, so I'm not sure of it's cold
resistance. But the other bulbs in that neighborhood are at around 14
times the resistance hot vs cold, I'll assume it is around 60 ohms.


Sorry for the math here, but R = V squared/P

That would equal around 58 Ohms hot resistance for the 250 watt bulb

240 Ohms for the 60 watt bulb

360 Ohms for the 40 watt bulb.

830 Ohms for a 25 watt bulb


I usually use a 25 watt bulb, so discounting any inductive effects, I'll
be looking at a maximum start current of 240 watts if the radio is a
dead short, and that for only a very short period of time.


A light bulb in series with an inrush current limiter might be suitable
for smoke testing equipment after prolonged disuse, though.

There isn't anything wrong with that approach, but it is probably overkill.

- 73 de Mike N3LI -