Changing PC keyboard LED indicators to neon indicators - need advice for circuit - Page 1
Forum section: electronics / projects

author: seanspotatobusiness
author_rank: Regular Contributor
timestamp: December 21, 2017, 10:32:48 am
content: I have a plan to replace the LEDs on my keyboard with neon indicator lamps. I will use a laptop CCFL inverter to obtain the AC needed. I was hoping someone would advise whether the following design would work. I have read that the frequency of a CCFL inverter is something like 40-80 kHz and I'm not sure an opto-coupler triac will function at that frequency. The neon indicator lamps need something like 80 V to strike and then drop to 60 V. I expect to use around 1 mA (I will use the inverter's brightness adjust pin to reduce the current to the minimum).The MOC3083 has Critical Rate of Rise of Off–State Voltage 600 (min)-1500 (average) V/µs - is that relevant to the frequency it can operate at?

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: December 21, 2017, 11:13:10 am
content: CCFL inverter will be far too much voltage - you may be able to under-run it though. An EL inverter is closer to the voltage you want.

author: seanspotatobusiness
author_rank: Regular Contributor
timestamp: December 21, 2017, 11:26:12 am
content: . I think the voltage drops to achieve the current output that the inverter is trying to push.

author: floobydust
author_rank: Super Contributor
timestamp: December 21, 2017, 01:33:23 pm
content: I think there will be a problem with the neon lamps staying on.TheOn-semi MOC3083opto-triac at 40kHz-80kHz, I don't have experience with but it might not work. These are usually mains (low) frequency parts.How fast the (internal) zero-cross detection is, the off-state capacitance, this is not on the datasheet. I'm guessing there will be enough current to keep the neon lamps glowing or the opto will refuse to turn on. Especially with a square wave for power.Best to breadboard that or maybe other people here have run these Optos at high frequencies?

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: December 21, 2017, 01:54:30 pm
content: You can probably light a neon with a simple flyback converter with a small ~10mH inductor - MPSA42 HV transistor, E to 0V, C to inductor, other end of inductor to +5-12V, and Base driven via ~1K from a few khz squrewave, e.g. from a 555. Use the 555 reset signal to enable from the keyboard LED signals.

author: seanspotatobusiness
author_rank: Regular Contributor
timestamp: December 21, 2017, 11:26:03 pm
content: There is an opto-triac part calledIL420 and it seems to accept higher than normal frequencies(Critical rate of rise of voltageat current commutation = 7 V/?s).I don't understand much of what mikeselectricstuff just said but I will do some reading on the train tomorrow.

author: Cyberdragon
author_rank: Super Contributor
timestamp: December 22, 2017, 06:06:22 am
content: A 555 for each lamp? I think it can be simpler.A two transistor oscillator (some form of astable or something) with one side going to another beefy transistor switching a bus to ground. Each lamp circuit would have a coil going to the positive rail on one side and to the lamp and a transistor going to the oscillating ground bus through a diode on the other. When the LED signal comes into the enable transistor, it closes the circuit from the lamp and coil to the main drive transistor and the diode stops the other polarity from back feeding through the enable transistors.Note: normal isolators would drive the enable transistors, as no components would be switching AC. Or maybe just use the isolators in place of the enable transistors, if they could handle the frequency.