24 VAC driver for 5 mm, self-flashing LEDs - Page 1
Forum section: electronics / projects

author: calzap
author_rank: Frequent Contributor
timestamp: July 27, 2020, 06:52:50 pm
content: Question came up in this threadhttps://www.eevblog.com/forum/projects/indicator-led-for-valve-controller/msg3100273/#msg3100273as to whether the timing IC in self-flashing LEDs could tolerate rectified, filtered 24 VAC (with a current-limiting resistor, of course). See the pic below. The LED would see about 35 VDC with a 5 V ripple (at 60 Hz).I had previously tested a few self-flashing LEDs using unfiltered 24 VAC (with a limiting resistor and reverse protection diode) and discovered it didn't work. It didn't kill the LEDs either.So, I finally got around to testing 16 different types of self-flashing LEDs with the driver pictured below. All were obtained on eBay from sellers in the U.S. or China. All were 5 mm, round-top with a clear and colorless capsule. Voltage drops varied from 2 to 3.6 V, and max allowable currents were either 20 or 30 mA. The specs provided by the sellers varied from virtually none to extensive. Only two stated that that the LED could be driven by 3 VDC without a resistor. A few intimated that limiting resistors were required by offering to sell them. The flashing style varied: some were single colors or white with regular flashing, some were flickerers, some went through a spectrum of colors slowly or rapidly ... quite smorgasbord of lights.I checked the supply was 24 VAC within 0.1 volt. All were tested for 12 hours. Good news is all performed flawlessly and survived. I only tested one of each type, so it was hardly a statistically valid QC exercise. Some are rated for a 100,000 hour life ... I'm not about to test that! However, I feel comfortable using the driver for my own projects. The components are small enough to be encased in heat shrink tubing, which is convenient.Mike in California

author: Zero999
author_rank: Super Contributor
timestamp: July 27, 2020, 10:23:09 pm
content: Have you tries connecting an oscilloscope across the LED. Does the voltage change when it turns off?I've always wondered what sort of IC is inside those flashing LEDs. It would be interesting, if someone could decapsulate one and do a die shot.

author: calzap
author_rank: Frequent Contributor
timestamp: July 27, 2020, 11:53:22 pm
content: I haven't put scope leads across an operating self-flasher. I would expect the dominant thing to see would be the ripple in the rectified DC supply ... but I'll try it sometime. I have put a multimeter in series with the regular flashing types (not flickerers or color-shift types) to observe current. Most flash at about 1 Hz, and the oscillating current draw is obvious.Mike in California

author: Zero999
author_rank: Super Contributor
timestamp: July 29, 2020, 10:06:37 pm
content: Quote from: calzap on July 27, 2020, 11:53:22 pmI wonder is they go open circuit, during the off period. If so, what's the maximum voltage it can withstand, when it's off? Alternatively, does it act like a zener diode, when off?

author: calzap
author_rank: Frequent Contributor
timestamp: July 30, 2020, 12:19:29 am
content: Good question. Slowly re-assembling my lab after some construction. Will get the scope unpacked soon and take a look.Mike in California