Dim Bulb Schematic

[Marcus]

Different tools for different jobs:

You use the variac to ramp up the power on anything that hasn't been powered for a while - usually if I'm doing this on an unknown device i would monitor the current draw, and would spot a potential short cct anyway.

For e.g.: my old Tek564 storage scope if it hasn't been fired up for a while, but was working fine the last time i used it.


If I'm powering up a (hopefully) repaired device that shouldn't need a slow ramp up, but wanted to limit the power input in case it decides to self destruct, i may use a series lamp.

For e.g.: one of the immersun repairs - i was certain it would power up fine, but wanted to control the power in case it had an issue when it started diverting (although i did need to use a high wattage lamp for that one due to the large amount of a.c. capacitance meaning it draws 1/2 amp at idle).

[Stig]

An incandescent bulb has a very high inrush current until it warms up -the filament has a positive temperature coefficient, i.e. lower resistance when cold. I can see the use for detecting a short circuit without blowing the fuse and it's resistance will indeed reduce the current flow but more so once it's warmed up than at the start.