Trying to convert a mains powered string of LED's

Hi, I purchased a set of Xmas lights believing them to be 'low voltage'.  My plan was to run them from the caravan 12 volt DC.  However, it appears that they run from the 230 volt AC mains.  This is something I am not familiar with.

I have attached my own wiring diagram of the circuit and would like to know if anyone can tell me how to switch the four strings of LED's individually.  I have an inverter that I can use as the current draw will be minimal.

I imaging that the LED's are wired as opposing pairs so that both halves of the mains sine wave are used.  Presumably, this prevents the set being run from 12 volts.

Any help will be welcomed.

Alan ...

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newalfi (author) 3 years ago

Well, the idea I had proved to be impracticable - it simply involved bypassing the controller completely and replacing it with 4 x toggle switches.

Jack A Lopez pointed out that the controller is probably controlling the phasing of the AC and the zero crossing etc to turn off the SCR's.

So, despite the inconvenience of having to press the button 8 times ;o) I have decided to go with verence's original suggestion of switching the original unit into the 'All On' mode then switching the individual lines as and when required. This was also confirmed by Jack A Lopez.

With this in mind, I shall move the whole thing into a separate project box. I need to buy some suitable and matching switches and so will not get around to it for a few days. Thank you all for your help and advice ... I shall post some pictures of the project as it develops.


Alan ...

newalfi (author) 3 years ago

Hi iceng,

Yes, I followed the article further and it seems that your man has simply opened up the box and drawn a circuit (which may well be wrong!). Then he seems to be claiming that it's a cicuit that is easy to make - as though it was his original design :o)

I have double checked my diagram (above) and the only thing that is wong is the connections to the switch.

I will draw up a suggested circuit today and post it. Perhaps you would be kind enough to comment.


Alan ...

newalfi (author) 3 years ago

Hi again,

I have found another circuit diagram showing a similar set of lights but only using two strings of LED's. This circuit has only one rectifying diode (half wave?) and the IC has a different number but seems to do exactly the same job.


As Jack A Lopez suggest, a simple SPST switch in each of the four strings is probably all that is required. The controller IC only seems to trigger the SCR's and so that seems to be all that I would need to do ;o).

Perhaps a small R/C off the bridge rectifier would give me a suitable voltage.

If you agree, can you give me suggested values ... would the existing 10uF-16v/2Meg setup be suitable or is it possible that the controller IC modifies this even further?

Thanks again,

Alan ...

iceng newalfi3 years ago

Caution following that circuit ( yea I'm a broken record ).

But even the least able schematic reader

will notice the wire meant to go to the common cathode end of the two LED strings

IS WRONGLY wired in between two LEDs.

Makes you wonder what else is wrong ( I think intentionally ) with the diagram !

I think I own a few of these same light sets. I took one apart once, and the wiring looked like you describe: 100 white LEDs total, in four strings, each string consisting of 25 LEDs in a series, with each 25-LED string switched by a SCR controlled by a microcontroller.

I think re-wiring this set to run from a 12 volt DC supply, is going to be a non-trivial task.

Consider that the forward voltage drop across one of these 25-LED strings is approximately 25*3.6 V = 90 V. I mean, how do you get your 12 VDC supply to do that?

Mpilch suggests rewiring all your LEDs, from 4 25-in-series strings, to 25 4-in-series strings. However, I think doing that would be a huge amount of wiring work. Plus I do not immediately see a way to keep all the microcontroller driven blinky-blinky effects, with this new topology.

I am going to humbly suggest running these lights from an inexpensive inverter. I don't know for sure how well this would work, but since I happen to possess both a cheap inverter, and this style of AC-powered Xmas lights, I would be willing to test this for you.

Just reply saying you want me to test this out, and I, Jack A Lopez, will upload unto this forum a picture of a 12-volt battery, powering a cheap inverter, powering a set of these AC-powered Xmas lights. I will also try to make some measurements on power flows, if you are interested in that.

Or if you happen to own a cheap 12-volt inverter, maybe you could find the courage to try this magic trick yourself.

If you choose to accept this mission, or whatever. ;-)

newalfi (author)  Jack A Lopez3 years ago

Hi Jack A Lopez,

I have actually had the lights working through an inverter :o), please see my follow-up above. It's the switching that puzzles me.

12 volts I can handle (model trains etc.) but 230 volts AC is a little out of my comfort zone.

Thanks again,

Alan ...

So you want to "switch the four strings of LED's individually"?

I mean would it suffice just to put a single pole single throw (SPST) switch in series with each string?

Or by "switch" do you mean something more complicated? Like maybe you wanted to trigger the SCRs using your own signals, like from a microprocessor of your own.

I suspect, but have not yet proven, this light set is controlling current to the LED strings entirely by way of the SCR timing.

I mean one sort of interesting thing about the LED light sets I have, is that it appears they were made to run from either 220 VAC mains, or 110 VAC mains. Actually that's just based on the shape of the plug. My set came with a European style plug, plus a little shape-changer adapter for to change it to a US-style plug.

I live in the former US, so that means I have tested my light set with 110 VAC mains, and recently also with 110 VAC from a cheap modified-sinewave style inverter. I haven't tested it with 220 VAC, or an ac supply with an RMS voltage in between 220 and 110, or even less than 110.

I mean what I suspect, but have not yet proven, is the (average) current to the LED strings is being regulated somehow, and this device is very robust with respect to its input supply voltage.

I also suspect the input has to be AC, just because triacs kind of need AC. Bad things will happen if a triac gets stuck in the "on" state, and that will happen if the current does not go to zero every now and then, like it does in an a AC waveform.

Final note: I am naively assuming that the light set I have is the exact-same-damn-thing as the one you have, but some visual identification might help. Attached are some pictures of my light set, including a close-up of the plugs and controller box. Let me know if it looks similar to the one you've got.

newalfi (author)  Jack A Lopez3 years ago

Hi Jack A Lopez,

Please see the follow up above.

My light set appears very similar to the one in your picture - except, of course, that mine's black and has a different plug on it :o)

Alan ...

I think I understand your vision now.

You want dim-able mood lighting, instead of 7 modes of epilepsy-inducing, flashy-flashy, disco-rave party, lighting.

Actually I think this is a noble goal.

The best, and most sexy, way to do this would be to replace the existing microcontroller with one programmed to do what you want it to do. Maybe even give it two buttons: one to up-arrow (increase) the level of illumination, and the other button to down-arrow (decrease) it.

Of course doing it that way would require almost complete understanding of how the original circuit works. It's doing something to sense the timing of the mains power waveform, like detecting zero-crossings, or something like that. It does this so it can get its timing control of the SCRs correct, to turn them on for just the right duration per cycle. I also suspect it might be sensing the current in each string somehow, for to regulate this, because this is typical for circuits driving LEDs.

So the little plastic box is doing a lot.

But programming your own microcontroller can be tricky, so the quick, and not-so-pretty, way to do this is to just put some toggle switches, or I think Verence was suggesting relays, in series with each of the four LED strings.

Final note is that both of those approaches still require some form of AC power, just because SCRs require AC power, for reasons mentioned previosly. So either way, running it from a 12 volt battery still requires the use of an inverter.

verence3 years ago

Tricky. The AC gets rectified by the 4 1N4007 diodes. So, you will have to handle 400V (unregulated, double half wave) DC! I'd guess the 150k resistor and capacitor produce the operation voltage for the logic chip. The 2M resistor provides a kind of zero crossing sensor that allows the logic to switch on the SCRs at the right moment (phase). You could try to replace the SCRs with relays. But I think this will fry the LEDs as there are not enough LEDs to stand the 400V. Just driving the LED strands with 12 V will not work. Apart from replacing the logic completely (doable, but far from being trivial or safe) there is one way I can think of: Am I correct that the push button allows to select different blinking modes? Is there a mode all 4 strands always on? If so, switch into this mode and put relays between the SCRs and the LED chains.

newalfi (author) 3 years ago

Thank you all for your replies ... maybe I wasn't specific enough in my question. What I am trying to do is not simply to run a set of sparkly flashing lights from 12 volts but to be able to bypass the flasher completely and turn on each string of lights independently.

I am trying to achieve four levels of mood lighting with those lights chosen being permanently on (not flashing).

I was thinking that I may be able to send the voltage that comes out of the R-C circuit following the Bridge rectifier through a switch to light each string.

Any further advice will, of course, be welcome.

Alan ...

iceng3 years ago

Don't forget there is a synch line watching the AC LINE which is necessary for timing and The Main Thing on straight DC the SCRs will Turn_ON only ONCE and Stay_ON because that's how SCRs work....

What colour are the LEDs, and how many are there in a chain ?

iceng3 years ago

The 1N4007 diodes form a DC Bridge with the positive and negative now marked.

Current through the LEDs and the SCR flows from the PLUS to the MINUS and makes more sense as shown for Lights_3 and Lights_4 where the SCR gate current returns to the NEGATIVE rail.


iceng iceng3 years ago

Most SCR circuits are not DC because once an SCR is turned ON it will stay in conduction until the current drops to almost zero ( called holding current ).

In your circuit the full wave bridge drops to zero every half cycle to accomplish the SCR + LED natural turn off.


Each LED in the string is your basic 2.5V to 3V LED. All you need to do is rewire them into strings of 4 LED in series. You can then wire several of those strings in parallel. With the right resistor on each strig you can easily connect them to a 12V battrey.