24VAC inverter for Christmas lights. - Page 1
Forum section: electronics / projects

author: aargee
author_rank: Frequent Contributor
timestamp: November 21, 2011, 05:14:20 am
content: For a long time now I've put up a range of outdoor Christmas lights. It all started when the kids were little... <insert dream video sequence here>anyway...Most of my gear runs from 24VAC (or has been converted to). In the past I have removed the russian roulette of power boards and plugpacks down to one 240-24V 150VA Transformer running the lot - now mostly all commercial LED strings.I have come across a quantity of Gates Cyclon SLA batteries and want to run the lot from solar. The DC side is under control and the lights don't like running from DC. So I have to produce 24VAC.The inefficient way would be to use the batteries to run an off the shelf 240V inverter, and plug the current system into that. I'm thinking of running a H-Bridge to produce a square wave 24V at 50Hz, I'm sure the little Chinese light strings would be OK with a square wave, maybe with a bit of capacitance to reduce the sharp wave edges...I'm just looking for a design to start from, anyone have any clues/pointers?TIA- Rob.

author: IanB
author_rank: Super Contributor
timestamp: November 21, 2011, 06:31:56 am
content: It's curious the lights don't like 24 V DC. Have you tested some to make sure? The reason being that most power supplies that take an AC input rectify it to DC before they do anything else with it, unless they have a traditional transformer inside them which is uncommon these days.

author: Simon
author_rank: Unknown
timestamp: November 21, 2011, 07:01:16 am
content: yes surely leds run off DC, if you really need an AC generator H-bridge circuits are quite easy, you can use a micro-controller quite simply

author: hacklordsniper
author_rank: Frequent Contributor
timestamp: November 21, 2011, 07:14:26 am
content: Quote from: Simon on November 21, 2011, 07:01:16 amEven a 555 if a micro is not available

author: Simon
author_rank: Unknown
timestamp: November 21, 2011, 07:17:29 am
content: err, I think the 555 has shoot through, that might cause some big shoot through on your H-bridge, but the power transistors will get a lot hotter

author: hacklordsniper
author_rank: Frequent Contributor
timestamp: November 21, 2011, 07:45:22 am
content: Quote from: Simon on November 21, 2011, 07:17:29 amErr i made a few kw inverters in past with it

author: aargee
author_rank: Frequent Contributor
timestamp: November 21, 2011, 07:48:43 am
content: Yep, the little controllers use AC, the chip part uses rectified to DC but then drives (I assume) SCR type devices running from the 24VAC that fire the LEDs. Obviously the LEDs only light on the positive cycle, and the SCRs need the zero cross to turn off. Running from DC just turns all the LEDs on until power is removed, the design looks like a left over from the lightbulb version.I'm thinking of a FET H Bridge driven by a small PIC, just so I can program a little dead time and avoid shoot through? Though the simplicity of the 555 sounds attractive!- Rob.p.s. the AC also helps with keeping the corrosion down on any wiring that gets exposed to the weather.

author: hacklordsniper
author_rank: Frequent Contributor
timestamp: November 21, 2011, 10:46:17 am
content: Quote from: aargee on November 21, 2011, 07:48:43 amYou could possibly replace the whole controller with custom variant. You can easily do that again with micro or as i like to say "If you cant do it with 555, you cant do it". But whichever variant you will decide it should be pretty easy.

author: Balaur
author_rank: Unknown
timestamp: November 21, 2011, 11:00:05 am
content: As a side-note, I warmly recommend aPIC-based LED chasserThe firmware has a lot of very nice sequences. I build one as a gift for a good friend's kid and I plan to build a multi-colored one for next year Christmas.Cheers,Dan

author: ejeffrey
author_rank: Super Contributor
timestamp: November 21, 2011, 11:31:57 am
content: An integrated H-bridge will likely contain automatic dead-time to prevent shoot through. You would just feed the square wave from a 555 (or any other source) to the direction pin.What is the purpose of the SCR trigger? Is it just a dimmer, or is their a microcontroller that is triggering some flashing sequence and then relying on the negative phase of the AC to turn it off? You should make sure it will perform acceptably from a square wave drive. If the SCR is triggered by a threshold on the AC waveform like a dimmer usually is, it may not do what you expect from the square wave.

author: Simon
author_rank: Unknown
timestamp: November 21, 2011, 12:35:56 pm
content: Quote from: hacklordsniper on November 21, 2011, 07:45:22 amwell I stand corrected then

author: Zad
author_rank: Super Contributor
timestamp: November 21, 2011, 05:00:52 pm
content: Just don't try to receive AM radio for several km

author: Simon
author_rank: Unknown
timestamp: November 21, 2011, 05:21:04 pm
content: Quote from: Zad on November 21, 2011, 05:00:52 pmuh yea tyou will get sued sent to prison and all sorts of things (allegedly)

author: Zero999
author_rank: Super Contributor
timestamp: November 21, 2011, 06:32:08 pm
content: What's the current rating of the lights? Does it matter if it's a square wave?It's possible to build a self oscillating h-bridge but there will be some shoot so it's only suitable for low currents.Here's a design for an astable designed for 24V 50Hz and a load of 500mA. It does draw some brief 6A current spikes but it shouldn't be a problem as LTSpice says the RMS dissipation in the transistors is only 123mW. If it's a problem, an inductor and zener can be used to help to reduce the spikes.By the way, notice how R3 is set to 1k51? That's because LTSpice won't simulate it otherwise but in real life R3 and be 1k5.

author: ivan747
author_rank: Super Contributor
timestamp: November 21, 2011, 07:56:04 pm
content: Quote from: ejeffrey on November 21, 2011, 11:31:57 amYes, dimmers rely on the sineness of a sine wave to trigger and SCR at the appropriate level. In that case, you should make an H-bridge (preferably use an IC H-bridge) and toggle it at a certain rate with a PWM pulse representing a sine wave. Then apply some filter on the output of the H-bridge. Just a suggestion, I haven't tested it.

author: aargee
author_rank: Frequent Contributor
timestamp: November 25, 2011, 10:59:44 pm
content: Thanks for all your replies, I'm going to build a small H-Bridge and run one set of lights to see if it's feasible.. then on to a full blown version.- Rob.