Incandescent Blinky Test

A flashlight bulb emerged from the clutter, which prompted me to ask if it might make an interesting blinky. Spoiler: probably not.

The bulb had “2.4 V 0.7 A” stamped on its shell, so the test setup looked like this:

Flashlight bulb test setup
Flashlight bulb test setup

A list seems helpful:

  • Solder wires to bulb in lieu of a socket
  • Bench supply at 2.4 V
  • Grossly abused 2N3904 NPN transistor as a switch
  • Function generator pulsing the base
  • Scope voltage probes on base (yellow) and collector (magenta)
  • Tek current probe on bench supply lead (cyan, 500 mA/div)

The function generator has a 50 Ω output, so depend on it to limit the base current just like it was a resistor. The output voltage is symmetric around 0 V, so apply an offset of half the peak-to-peak signal to get a positive-going pulse:

Flashlight bulb test - function gen setup
Flashlight bulb test – function gen setup

A 150 ms pulse gives the bulb just barely enough energy to light as a little orange blip, with the collector voltage dropping as the filament heats up and its resistance increases:

Tungsten 2.4V 700mA - 150ms
Tungsten 2.4V 700mA – 150ms

Given 350 ms to heat up, the bulb produces a nice white-hot flash:

Tungsten 2.4V 700mA - 350ms
Tungsten 2.4V 700mA – 350ms

The poor transistor acts as a 600 mA constant current sink, which isn’t surprising given its 300 mA absolute maximum current rating.

Homework: figure the base drive and current gain

Protip: don’t do that to a cherished transistor

The bulb resistance starts out at 0.5 Ω and rises to 2.5 Ω when the filament glows white-hot at the end of the pulse.

Something like 250 ms produces a noticeable blink, requiring 360 mW·s = 2.4 V × 600 mA × 250 ms from the power supply. Blinking once every ten seconds all day means 8640 pulses for a total energy of 864 mW·hr; call it 1 W·hr.

A pair of (fresh) AA alkaline cells provide 7.5 W·hr for maybe a week of blinkiness.

A not-dead-yet 18650 lithium cell might offer 15 W·hr, but running the bulb from 3.7-ish V, rather than 3-ish V, increases the energy per pulse by 20% and decreases the run time correspondingly.

Surely not worth the effort …

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