Due to the extreme non-linearity of a tungsten filament, lamps do not make good loads for circuits that may be very sensitive to load impedance.
In certain lamps, a cold filament may be in the order of milliohms, which can be a near short on the DUT. After it heats up a bit the resistance moves into the ohmic range and the energy can then have a more reasonable load to work into.
Oftentimes the DUT cannot provide enough surge amperage to overcome the cold resistance of the incandescent lamp.
For example, a 100-watt, 120-volt lamp has a resistance of 144 ohms when lit, but the cold resistance is much lower (about 9.5 ohms).
Twelve volt automotive lamps, such as brake llamps have lower initial resistance.
A variable resistor as the load can be a better help in impedance matching the DUT, and makes calculation of power easier.
In some cases though, an incandescent bulb will try to automatically find the impedance sweet spot, providing the DUT can provide the current to overcome the cold resistance and if the lamp is suitably sized to the DUT output.
In the final analysis, an incandescent lamp can provide "rough eyeball" indication of power output without DSO's true RMS multimeters etc. so are not to be ignored.
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