Right Stuff Wrong Stuff

XS650 (XS1B) FET Regulator

I like Tirill (mechanical) regulator but it is too old for today's transportation condition. So I designed the FET regulator for my XS1B.


The circuit diagram is similar to my GX250. GX250's regulator operates at minus side of field coil But this regulator operates at plus side of field coil. Thus the circuit diagram is symmetrical to GX250.

I harvested in my junk box. 2SJ113 (100V-10A) is enough for this application. you can use equal FET instead of 2SJ113.

GX250 needed "C1" and "R5" for moderate speed switching. But my XS1B may not need "C1" and "R5". I didn't verify them But I added them...


I checked the behavior by the prototype. And decided "R3" and "R5". Target Voltage is 14.5V.


I changed only inner parts. Because I like an outward of Tirill (mechanical) regulator. Inner parts will be fixed by the glue after road test.

The corroded terminal and protection tube will be exchanged too.


Left hand one is FET regulator. Right hand one is Tirill (mechanical) regulator. You (and I) can't distinguish them.

Target voltage of regulator is 14.5V. But the voltage of battery is different. At 1000rpm voltage is 14.0V. Over 1200rpm (with head light OFF) voltage is controlled into 14.7V. Over 1200rpm (with head light ON) voltage is controlled into 15.0V.

Why the voltage of battery is 15.0V ? It is not an error of regulator. It is voltage drop of power line. Detection wire of regulator (brown wire) is connected to halfway at power line.

In Japan we must keep head light "ON". I feel that 15.0V is a little high for my battery.

If I have "Right Stuff" I must reduce the voltage drop of power line. But I who have "Wrong Stuff" changed the target voltage of regulator...

I changed the value of "R3" from 150ohm to 120ohm. Target voltage of regulator was changed to 14.2V. Battery voltage is 14.4V with head light "OFF". And battery voltage is 14.7V with head light "ON".

14.7V is not so bad. I checked the voltage by multimeter. Battery voltage was controlled into 14.6V-14.8V. It was great improvement from old regulator.