Halogen Blinky Test

Dropping the ordinary flashlight bulb into the drawer where it belonged revealed what I think is a halogen flashlight bulb, so I rebuilt the blinky test setup:

Halogen flashlight bulb test setup
Halogen flashlight bulb test setup

This time I used a BUZ71A MOSFET (13 A, 100 mΩ RDS) driven with a 10 V gate pulse to make sure it acted like a switch instead of a current sink.

The first attempt looked … odd:

Halogen 3V - no cap - 4ms 1A-div
Halogen 3V – no cap – 4ms 1A-div

The gate pulse is yellow, the drain voltage is magenta, the bulb current is cyan at 1 A/div, and the timebase ticks along at 2 ms/div.

Moving the magenta trace to the supply voltage on the other side of the bulb produces even more weirdness:

Halogen 3V - no cap - Vsupply - 4ms 1A-div
Halogen 3V – no cap – Vsupply – 4ms 1A-div

Apparently, slugging a 3 A bench supply with a 3 A pulse lasting only 4 ms causes distress of the output tract.

Kludging a hulking 22 mF (yes, 22000 µF) cap across the power supply provides enough local storage to make things work properly:

Halogen 3V - 22000µF - Vsupply - 4ms 1A-div
Halogen 3V – 22000µF – Vsupply – 4ms 1A-div

With the cap in place, the drain terminal looks less unruly:

Halogen 3V - 4ms 1A-div
Halogen 3V – 4ms 1A-div

The drain voltage starts at about 600 mV with the 3 A pulse, a bit more than you’d expect from the alleged 100 mΩ drain-source resistance, but those numbers are generally aspirational and the test setup leaves a lot to be desired.

A 10 ms pulse produces a distinct flash, rather than a dull orange blip (timebase now at 10 ms/div):

Halogen 3V - 22000µF - 10ms 1A-div
Halogen 3V – 22000µF – 10ms 1A-div

A 30 ms pulse reaches full brightness as the filament settles at normal operating temperature:

Halogen 3V - 22000µF - 30ms 1A-div
Halogen 3V – 22000µF – 30ms 1A-div

A 20 ms flash might suffice for decorative purposes, in which case each pulse requires 90 mW·s = 3 V × 1.5 A × 20 ms of energy. Running it all day requires 7.8 kW·s = 2.2 W·h, so it’s even less appealing than that old skool tungsten bulb.

Which is, of course, why LED flashlight bulbs are a thing.

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 …

Vacuum Tube Lights: Urethane Coated Plate Cap

With a generous dollop of JB Plastic Bonder left over from a set of Bafang brake sensor magnets, I tried coating the ersatz plate cap of a triode tube:

Triode - urethane coated plate cap
Triode – urethane coated plate cap

That’s the result after leaving it hanging upside-down while it cured to push all the drips to the top.

For comparison, the uncoated cap back in the day:

Triode - plate cap plug
Triode – plate cap plug

Seeing as how the urethane is an adhesive, not a coating, I’d say it looks about as bad as expected.

As with all 3D printed things, one must embrace imperfections and striations, rather than endlessly strive for perfection.

Now, if I had a resin printer …

Discrete LM3909: Green and Blue vs. Dead Alkalines

These two discrete LM3909 circuits recently stopped blinking:

LM3909 AA alkaline - Green and Blue
LM3909 AA alkaline – Green and Blue

The green LED (on the left) took six months to wear its pair of not-dead-yet AA alkalines from 2.7 V down to nearly zero.

The blue LED in the radome took two months to go from 1.0 V (!) to nearly zero. It didn’t start very bright and went decidedly dim along the way, but the LM3909 circuitry still managed to jam a few microamps through the LED.

In both cases, one of the cells was reverse-charged by a few hundred millivolts, although neither leaked.

Both got another set of not-quite-dead AA cells and they’re back in action.

Satco PAR30 LED Spotlight Teardown

One of those LED spotlights may have barely outlasted its worthless warranty, but not by much, and has been languishing on the back of the bench with “Flickers hot” scrawled on its side.

The metal base didn’t respond to twisting, so I slit the threads with a cutoff wheel:

Satco PAR30 - thread slit
Satco PAR30 – thread slit

Applying the screwdriver removed the base to reveal a silicone rubber casting:

Satco PAR30 - thread silicone
Satco PAR30 – thread silicone

The small wire emerging near the edge of the plastic case seems to be the neutral contact to the shell, with a poor enough joint to suggest it might have been why the lamp flickered when it got hot.

Some brute force snapped the silicone off at the bottom of the plastic case and broke the two wires bringing AC to the PCB:

Satco PAR30 - thread silicone base
Satco PAR30 – thread silicone base

Digging around inside produced a debris field of silicone crumbs, broken resistors, torn caps, and various other components, with zero progress toward removing the shell:

Satco PAR30 - silicone extraction
Satco PAR30 – silicone extraction

A little lathe work converted a chunk of PVC pipe into a crude mandrel supporting the mangled case:

Satco PAR30 - base cutting setup
Satco PAR30 – base cutting setup

A few millimeters of sissy cuts released a silicone O-ring sealing the shell against the reflector:

Satco PAR30 - O-ring seal
Satco PAR30 – O-ring seal

Continuing the cuts eventually revealed the three screws holding the shell to the reflector and the two wires powering the LED:

Satco PAR30 - reflector separated
Satco PAR30 – reflector separated

Chopping off the screws with a diagonal cutter freed the shell and revealed the top of the PCB:

Satco PAR30 - electronics top
Satco PAR30 – electronics top

It really does have a surprising number of components!

Those three screws connected the LED panel / heatsink to the shell through the back of the double-walled reflector. More brute force peeled the outer shell away and released the panel:

Satco PAR30 - lens assembly
Satco PAR30 – lens assembly

Each of the 5050 packages contains a pair of white LEDs with 5.2 V forward drop for the pair, at the very low test current. They’re all in series, so you’re looking at well over 60 V total forward drop:

Satco PAR30 - LED panel detail
Satco PAR30 – LED panel detail

Note that the wiring, which nobody will ever see, follows the electrical color code of white = common and gray = hot.

Perhaps I should turn the lens into an interesting art object

Discrete LM3909: Blue LED

Once again, the discrete LM3909 circuitry can blink a blue LED while running a pair of alkaline cells all the way down to about 1 V, with one cell ending at 0.2 V and the other at 0.8 V. They started out discharged to 1.2 V each during their useful life, then blinked for a month; it’s as good a use for dead cells as I can think of.

With another pair of not-dead-yet cells providing 2.4 V, it started up again:

Blue LM3909 2.4V alkaline - 042
Blue LM3909 2.4V alkaline – 042

That’s a frame from a short video taken in subdued light, just to show it really does work.

Discrete LM3909 Blue LED: Off at 1.0 V

The blue LED inside the radome got fainter as the alkaline AA cells faded away, but remained visible in a dark room until the discrete LM3909 circuitry stopped oscillating with the battery at 1.0 V. One of the cells had flatlined, with the other supplying what little current was needed.

The circuitry restarted with a pair of weak alkalines applying 2.4 V across the bus bars:

LM3909 Blue - 2.4 V alkaline
LM3909 Blue – 2.4 V alkaline

The LED waveform shows it needs about 2 V:

LM3909 Blue - 2.4 V alkaline
LM3909 Blue – 2.4 V alkaline

It’s barely visible in normal room light and strikingly bright at night.