Help with AC & LEDs

[CarlosT]

Hello I am making advertising lightboxes that are powered by a motorbike alternator AC like the headlights and tail lights. The bikes are typically old and in a bad state (i live in Cambodia). I use a bridge rectifier to switch AC to DC. Which works fine to power the 5630 LED strip lights (about 1.5 meters - 2 amps) WHEN the bike is moving. The problem is when its idling the LEDs have a horrible flicker. I first tried adding a 10000uf capacitor and that didn't help then i added a voltage regulator to keep 12v but that also didn't help. Finally I made a circuit with a bridge rectifier, a 16v 5f super capacitor bank and a voltage regulator. This works like a dream to kill the flickering however it makes the bike's headlight very dim.

The problem as I understand (bearing in mind i know very little about electronics) is that the super cap is drawing all the available power 4+ amps by my multimeter. But the LEDs only need about 1.8 amps. How can I make the circuit pull only the power it needs. I thought once the cap was full it would pull less amps but that does not seem to be the case it just sits on 4.4amps. Is there some kind of current limiter I can add to the circuit? I considered using a bridge rated at 2amps but i suppose that would just burn out? Would a resistor in parallel help?

 

[CarlosT]

Just to clarify with the LEDs connected via the tail lights the headlights do not dim at all. However with the LEDs and the capacitor connected they dim, and with just the capacitor they dim too.

 

[MikeMl]

Post the circuit that shows how the capacitor is wired relative to the regulator and loads.

 

[CarlosT]

Hi Mike, here is a drawing...

 

[MikeMl]

Are you sure that the LED strips have internal current-limiting, and are designed to be driven with 12VDC constant-voltage?

 

[CarlosT]

Yes they are definitely designed for 12V they are perfect off a 12v battery. But they can even run off the alternator direct which is 14v i believe I haven't had one blow yet but they flicker on idle. But the capacitor is pulling 4+ amps even when the LEDs aren't in the circuit.

 

[CarlosT]

I like the idea of using a supercap because they don't die and charge up very quickly (3 seconds). But they are quite expensive. Another possible solution I have come up with is this (file attached)...

It uses a bridge rectifier, 2 relays and the smallest motorbike battery i can find. In the illustration the grey wires activate the switch between the brown and the red wires and the yellow wires are ground. I have tested this on a wall socket 12v power supply and a motorbike battery and it works as intended (i think). The idea is that the alternator will both power the the LEDs and charge the battery and when the alternator is not putting out enough current the LEDs will pull from the battery.

This only lights up and charges when the motorcycle lights are on as its attached to the tail light of the tuktuk carriage. I do not want a solution that has to be attached to the motorbike in any way it is too timely to install.

Can anyone see a problem with the attached solution? Would it work as I expect? Would I need a diode to stop the battery discharging to the dim tail lights at idle?

 

[MikeMl]

I think I know what is making the headlights dim, but I dont have the time to post it right now. I'll get back to this later this weekend...

 

[CarlosT]

Nice one. Thanks Mike

 

[CarlosT]

I figured out the problem was the adjustable voltage booster. It seemed it had a sweet spot of around 11.2 volts where the amps would be around 1.5 anything above that it jumped up to 4.4 amps.

 

[CarlosT]

Actually I still have a problem The headlights are still going a bit dim without the voltage booster, not as bad as before though. I believe this is because of the bridge capacitor has a voltage drop of around 2V. Can anyone recommend me a bridge rectifier that has a lower forward voltage drop. I read that if they use Schottky diodes they have a lower drop.

 

[cct314]

I believe that there are several problems happening here:

- The operating voltage of your LEDs is very close to 12 volts. Blue/white LEDs like to be powered from 3.6-4.2 volts, but if you drop below that on a per-LED basis, they get very dim, very quickly: There's a very sharp voltage "knee" below which they go from being "normal" in brightness (say, 12 volts) to being dim/off (11 volts) - just to through out some totally arbitrary numbers. If you have an adjustable power supply, you can try this for yourself and see the problem.

- The voltage regulators on motorobikes are often abysmal and "approximate" at best. All they need to do is throw something in the range of 12-16 volts to (more or less) keep the lights on and the battery charged and that voltage goes all over the place with engine RPM. Some seem to be shunt type where others appear to be phase-angle to produce an "average" voltage. The current that you are seeing in the supercaps is probably very likely "spiky" - that is, very nonsinusoidal, even more than one would expect from the clipped top of an AC waveform.

The solution to your problem could one of two ways.

- Use LEDs that operate at full brightness from a lower voltage (say, 9 volts) and drop a bit with a resistor. This will give a bit a leeway in voltage variation with less brightness change. Since you already own these lights and want to use them, you probably won't do this!

- A better way is to use a buck-boost regulator to set the voltage at the desired level. These are electronic devices that will take, say, 10-16 volts but always give you 13 volt (or whatever you set). They can be found on EvilBay IF you look closely: There are boost regulators (go from low-to-high) and buck regulators (high to low) but what you want is *specifically* a buck-boost, which is built differently from the other two! If its specifications don't specifically say "buck-boost" then it's not what you want. (You could kludge and get a boost converter to take it up to 16-20 volts and then a buck converter to bring it back down to 12-13 - but that *is* a kludge!)

Using four Schottky diodes will probably give you another volt or so, but it probably won't help that much.

Best of luck!