I've researched the forum before posting, but it seems my question is a bit specific and not answered before. Please excuse me if I'm wrong.
So what we have here is a chinese scooter alternator, which gives 70W max. The alternator is a half wave stator, half of the power is converted to DC through a regulator to charge the battery, the other half is also regulated by an extra regulator I put there and drives a 45W LED headlight. So far so good. But the problem is LED flickering due to the nature of the regulator/rectifier.
So I'm trying to decide what smoothing capacitor to add after the regulator. I noticed by trial and error that the flickering stops completely after I put a 10000uF electrolytic cap in there. I also after that, calculated that a minimum 4700uF cap is needed for an approximately 5% ripple voltage, for the current drawn (2V ripple at around 14VDC).
But the main problem, which I think everyone misses (or I am wrong and I'm trying to figure which of the two stands here) is the CONSTANT current draw (Irms) for the smoothing capacitor, in order to maintain a smooth output.
I've run a simulation with a full bridge rectifier and it seems a 10000uF, if inrush current is left out, it draws a constant current Irms of around 9A!!!!
So, unless I'm missing something here, the alternator which can only give 70W max (so 5.8A at 12V), will be constantly overloaded and eventually will burn out.
Of course half wave stator will give half of the power and the available power will be half of 70W, but I have no problem with that, I can fix it to run full wave by removing the center tap. My problem is the constant current drawn from the smoothing cap.
Could anyone please help me on that and tell me if my calculations/simulation are wrong or if indeed that is the case here? And if it is so, what could I do in that case?
Thank you all very much in advance!
Best Answer
A smoothing cap is loaded only when the rectifier voltage output is higher than the capacitor voltage. This gives you current spikes at the top of the rectified sine, and no current at all for the other times.
You are right the rectifier needs to be able to handle the current spikes, or it will burn out.
A better design used another single diode, and put a substantially smaller smooothing cap behind it, just big enough for powering the headlight.
BTW: A 45W LED headlight is much brighter than other people's normal headlights, and most likely, than allowed. The scooter has no license and you will run into insurance problems if you are involved into an accident.