PCB Assembly Services - Screaming Circuits: And now, Back to Our Regularly Scheduled "Fun with LEDs" Programming


And now, Back to Our Regularly Scheduled "Fun with LEDs" Programming

Fun with LEDs, from the Screaming Makers

LEDs are one of our favorite subjects here in the Screaming Circuits blog. Between inconsistent marking and... Well, it's mostly about inconsistent marking. In fact (or maybe in opinion), LEDs are one of the largest sources of pain in the electronics design world. [Ref. our blog article here.]

One trick from the olden days sometimes used by experienced technicians is to simply to place opaque tape over the LEDs. Then it doesn't matter if they are backwards or not. Unfortunately, many people insist on being able to see them, so let’s try to get one working.

An LED is a one way device that lights up when current is going through it in the forward direction and doesn’t let the current flow backward.

Briefly (and there’s no numbers here on purpose), it needs a little bit of forward voltage to get started, lights up and gets brighter while the voltage keeps rising, and eventually will be destroyed if the voltage keeps rising. It does nothing while the reverse voltage keeps rising, until it gets destroyed.

If not protected with a current limiting resistor, once it gets a little voltage forward the current takes off almost straight up until it is destroyed. It is briefly quite bright. When reverse voltage is applied, nothing happens while V keeps rising, for much longer than forward voltage until the reverse voltage gets high enough for a reverse current to take off almost straight up. It is briefly quite bright again, so there’s that.

LED head on
To recap:

  • With forward voltage, too much current destroys LEDs.
  • With reverse voltage they aren’t destroyed right away, they just don’t turn on. If you keep pushing it, you will find a way to blow it.

The forward biased LED will work forever as long as the current doesn’t take off. There are two ways to prevent that: ensure that the power supply won’t provide enough current to exceed the LED max current rating, or the ensure that the current meets enough resistance for a given voltage. Handily, there’s a simple formula where the ratio of the voltage and resistance tells us what the current will be.

Let’s light up an LED, and easy on the math, since we can look up the current. The one thing we know for sure is not to let too much current in. How much is too much? It’s in the data sheet, of course.

The first data sheet for a random LED I looked at helpfully said ABSOLUTE MAXIMUM RATINGS then parameter, forward current, value 30, unit milliamps. So, 30 mA, preferably less.

Then, pick a battery for power. Which one should we use? Too make it fun, and destructive, let’s not use a resistor for this design, just wires. Battery, wires, LED. “The light that burns twice as bright burns half as long - and you have burned so very, very brightly..” so to speak.

There is a little internal resistance inside every battery, it’s what keeps if from discharging instantly. I looked some of them up, and guess with which one the LED will last for hours? Remember the maximum current allowed is 30 mA.

A. 9V Alkaline, 1 or 2 ohms
B. 1.25 AA NIMH, 30 milliohms
C. 12V Lead Acid, .001 ohms
D. 3V CR 2032 button cell, 15 ohms
E. None of the above

If this was a test the correct answer would be E., but since it’s not, lets go with the one that will destroy it the slowest. Back soon.

[To be continued]


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