Rectifier / Regulator

Rectifier/Regulator

The regulator/rectifier is a very standard part found on many motorcycles.  It takes a 3-phase AC input and converts it to a charging voltage of nominally 13.8 VDC.

If you are seeing diagnostic trouble codes in K-Scan like P0563 (System voltage too high) testing the rectifier/regulator would be prudent.

I have been experimenting with an aftermarket OTK M261 MOSFET rectifier/regulator after an original equipment (OE) unit failed in one of my bikes.   MOSFET rectifier/regulators exhibit some theoretical benefits including more efficient, cooler operation and a more constant output voltage.  Although it seems to be working fine, the output voltage is a bit higher than I would like to see (so can not yet recommend it).  It seems to make a constant 15.0 volts DC regardless of rpm.   OTK's specification sheet says 14.5 +/- 0.3 VDC.  It exhibits a very small AC content (as measured with a true-RMS meter) of about 0.4V.  Conversely, the OE thyristor-based rectifier/regulators are more variable with rpm - approximately 12.6 to 13.1 VDC, with a higher AC reading of around 1.5 volts.

Filter Capacitor (Early Models)

The filter capacitor is mounted behind the number plate.  It is closely related to the rectifier/regulator because there is no battery in the system.

The output of the rectifier/regulator is pulsating DC.  In most vehicles' electrical systems, a lead-acid battery smooths these pulsations into a low-ripple DC output.  Because the TR280i has no battery, the capacitor is necessary.  I measured its capacitance at approximately 23,000 uF.  Electrolytic capacitors exhibit a large tolerance, and 22,000 uF is a standard value.  A proper replacement capacitor would be rated to withstand sufficient ripple current (which makes these capacitors fairly large physically).  The capacitor weighs 162 grams.

On bikes with the Easy-Start system, the filter capacitor is smaller (10,000 uF) and mounted inside the Easy-Start box.  Furthermore, this capacitor is only connected to the rectifier/regulator when the fan is energized via the chassis relay.   

Testing Large Electrolytic Capacitors

Most multimeters will not read a capacitor value greater than 9999 uF (if they can measure capacitors at all).

But if you connect the capacitor to a digital multimeter on the ohms setting, the resistance will appear small initially, and gradually increase as the capacitor charges.  You can then short the capacitor leads together (discharging it) and repeat the observation. 

Depending on the particular DMM, its ohms setting will charge the capacitor to something on the order of 350 to 500 millivolts.  Once the capacitor is charged, switch the multimeter to read volts (or better yet, millivolts).  The capacitor should hold that charge for a long time (perhaps hours) but you will be able it see it slowly discharge due to the drain imposed by the multimeter.

Passing these tests does not guarantee a good capacitor, but it's better than nothing and about all the DIY mechanic can do.

Note that an electrolytic capacitor is a polarized device.  Operating it at its rated voltage backwards, will destroy it.  However, when testing will a multimeter, polarity is not a concern.  This is because the voltage is so low, no damage will occur.