Filament lamp
A small incandescent bulb in a screw holder, like the flashlight bulbs in school circuit kits. Aliases: bulb, light bulb, incandescent lamp, MES bulb.
A coil of tungsten wire glows white hot in a glass bulb. It looks like the simplest part in the box, and it is the one most circuits lessons are built on: series and parallel, brightness, and what a fuse is for. It is also not a resistor, which is the part of the lesson the simulator can show.
Pins
| Pin | What it does |
|---|---|
1 |
One terminal of the holder. |
2 |
The other. A lamp has no polarity. |
Properties
model is the rating printed on the bulb: 2.5V 0.3A (the flashlight bulb, the
default), 6V 60mA, 6V 0.3A or 12V 0.1A.
While it runs
The filament glows with the light it is giving: a dull red when it barely glows, orange, then yellow-white at its rated voltage. A burnt-out lamp shows a broken filament and stays dark.
What the model gets right
Tungsten's resistance climbs with its temperature. The filament runs at about 2500 K at its rating, and its resistance goes as the temperature to the power 1.2, so cold it is about a twelfth of its hot resistance: a 2.5 V 0.3 A bulb is 8.3 ohm lit and about 0.7 ohm cold, which is what a meter on an unlit bulb reads.
Inrush. Switched on cold, a lamp draws about ten times its rated current for the first few milliseconds, until the filament heats.
Warm-up and fade. The filament's heat capacity gives it a time constant of 40 to 80 ms at its rating, depending on the bulb, so it glows up and fades over tens of milliseconds, where an LED is instant. Flick it on and off quickly and it never reaches full brightness.
It is not ohmic. The filament radiates its heat away as T^4, so on half its
voltage it runs cooler, its resistance is lower, and it draws about two thirds of
its current, not half. Its light goes as the voltage to the power 3.4: half the
voltage is a tenth of the light. Two lamps in series on one lamp's voltage glow a
dull orange; two in parallel glow fully and draw twice the current.
It burns out. Past about twice its rated voltage the filament melts. A 2.5 V bulb on 6 V flashes once and is gone, and stays open until the circuit is rebuilt.
The rated temperature and the warm-up time constants are assumptions in the range
small lamps show; the T^4 and T^1.2 laws are the textbook ones, and together
they give the empirical I ~ V^0.55 rule for tungsten lamps.
What it does not model
The heat lost down the leads and through the gas (all of it leaves as light and infrared here), so a lamp cools a little slower than a real one. The slow death of a lamp run at one and a half times its voltage: here it lives forever below about twice its rating. Color is drawn from the brightness, not computed.
Common mistakes
Running a lamp straight off an Arduino pin. A pin can supply about 20 mA; even the 6 V 60 mA bulb wants three times that, and the inrush is ten times more. Switch it with a transistor or a MOSFET.
Expecting the resistance a meter reads on a cold bulb to tell you the running current. It is a twelfth of the hot resistance.
See it in action
Lamp inrush switches a 6 V lamp with a MOSFET and measures the surge every millisecond. Open it at /templates.