Thursday, April 7, 2016

Mains Current Limiting Bulbs

When working on a mains electricity powered device, you will need to safely power up the unit by limiting mains current.

This is achieved here by placing a non linear load in series with the unit.

The resistance of the incandescent globes increases as the current through them increases, so under normal conditions (where the device under test does not draw a large current) the globes will not light up.

This graph indicates voltage drop across the globe with respect to current.

Under a fault condition, the device under test will draw a large current, and the globes will light up. This reduces the voltage drop across the device under test, preventing further damage to the device.

Maximum current (at short circuit) can be calculated:
For one 100W bulb
I = P/V = 100W/240V
I = 0.4167A

The unit can be constructed from several batten fixtures.
4* 100W incandescent globes is enough to provide idle current for a very large amplifier.
If a lower current limit is required, simply unscrew a few globes.

Current limiting bulbs installed on test bench.

Example of fault in device under test
(here provided by a short circuit).
Alternatively, this device can be constructed as a rack unit.
Switches are wired such that each bulb can be taken out of circuit in order to provide the correct current limit.
A meter displays output voltage and current.
Bulbs are mounted in lamp-stand fittings.

Please note, this method cannot be employed on some "universal" switch mode power supplies, as the supply may enter a mode designed for a different voltage mains supply (ie; a 240V supply might think it's receiving 110V). I have found this only rarely to be problematic.