Developed by the Graffiti Research Lab a division of the Eyebeam R&D OpenLab , LED Throwies are an inexpensive way to add color to any ferromagnetic surface in your neighborhood. A Throwie consists of a lithium battery, a 10mm diffused LED and a rare-earth magnet taped together. Throw it up high and in quantity to impress your friends and city officials.
Guys from Evil Mad Science Laboratories provided some experiments with LED Throwies.
“The throwie (and I use the term loosely, referring to devices with or without magnets) is a darned useful thing. But the first time that I was introduced to the concept, I asked what a lot of other electronics-oriented people do: “where’s the resistor?”
Turns out that it’s a fair question. LEDs aren’t normally driven directly from a battery. And yet (1) people were making throwies with every imaginable color of LED and (2) they apparently weren’t exploding. So what’s the deal?
Turns out that the deal depends on what kind of LED you’ve got, and a few more things after that. The “standard” battery for this sort of thing is a CR2032 3V lithium coin cell. Almost any kind of an LED can be found in one of those little keychain flashlights, but throwies usually are made with 10 mm diffused-lens LEDs in assorted colors.
Most garden-variety LEDs are rated to operate safely at 20 mA or perhaps a little bit higher. Blue, white, and most modern green LEDs have a typical forward voltage of about 3.5 V when run at 20 mA, and typical red or yellow LEDs have a forward voltage of about 1.7 V when run at 20 mA.
This is actually straightforward for the 3.5 V LEDs. It means that when you put that 3 V coin cell across a blue or white LED, the current will stay safely below 20 mA, because the coin cell does not provide enough voltage to exceed that.
And while this does provide an upper bound, it doesn’t actually say much about what goes on once you hook up the LED. The current through the LED depends exponentially on the voltage across it, and the voltage across the battery depends on the current due to its internal resistance, and a number of other potential factors come into play. If you actually measure it, you might find that the current across the LED looks like this: Read more…
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