My last project (motorcycle LED tailights) was a great success thanks to ideas posted on this website. Now to the current project:
I am adding a computer case fan to one of my KTM radiators to get some cooling when I'm in a tight spot with no forward movement. Others have done this with good results, but they are using 12vdc battery power.
These machines have a split system with a common ground point part way through the windings. Most of the windings above the common point output AC that goes to the regulator/rectifier and exits as regulated AC for lighting loads. The other windings output AC to the regulator/rectifier that exits as DC for battery charging.
This DC is only about half an amp, but is sufficient for the intended purpose. My goal is to power the 12vdc fan from the main regulated AC power so as to not sacrifice battery charging. The fan would run anytime the engine is running which is fine by me. If I can't solve this, I'll install a thermostatic switch and use DC like everyone else.
There is 10.5 volts AC (RMS I believe on my fluke 73 multimeter set for ac) at the headlight with the engine idling at 1800rpm (redline is 10,000). With a 4 amp bridge rectifier connected here, the output is 8.6 VDC. This does run the fan, but at too low a speed. I thought I needed a filtering capacitor on the DC side, so I hooked up a 35V 10,000uF electrolytic. I was very surprised to see the DC voltage jump to 17.5 volts.
This of course is too high (great airflow though). Can someone please explain what is happening, and possibly suggest a better capacitor value?