What Does A Rectifier Do On A Motorcycle?



Your motorcycle’s electrical system comprises various components, and the rectifier plays an essential role. In this brief article, I’ll explain what a rectifier does and why it must work correctly.

What does a motorcycle rectifier do?

In more modern motorcycle electrical systems, the rectifier is usually combined into one unit with the regulator. Here, we will concentrate on just what the rectifier does and look at regulators in a separate article.

Your motorcycle rectifier converts (alternating current) AC into direct current (DC). You have probably seen these terms, but only some understand what they mean. To understand how a rectifier works, we need to take a quick look at AC/DC theory – no, not the band!


Alternating current

An alternating current is produced by your motorcycle stator or an alternator on a car. The rotating cycle of the alternator produces a voltage that fluctuates from a positive peak to a negative peak. We can draw the voltage fluctuation as a sign wave.

Sinosouidal Wave

The electrical system on your motorcycle operates on direct current, and the rectifier converts the oscillating voltage from the stator to a steady DC voltage.

Direct current

Direct current provides a steady positive or negative voltage. For example, the voltage from your motorcycle battery is around 12 volts DC.

Direct current diagram

Why does your motorcycle need DC power?

Electrical systems generally either work on AC or DC voltages, although items such as lamps can use either. Modern motorcycles now use DC for the whole electrical system, but in the past, particularly on 6-volt systems, manufacturers would connect the lights to the AC output.

Read: How many volts does a motorcycle battery have?

However, I digress. Your battery needs direct current for charging, plus electronic systems such as your fuel computer require DC. 

How does a rectifier convert AC to DC?

Two primary methods of converting AC to DC are half-wave and full-wave.

Current rectification diagram

Half-wave rectifier

A half-wave rectifier is a cheap and simple solution that uses a one-way diode. This allows the positive or negative portion of the sine wave through and discards the rest.

Although simple and cheap, it is inefficient, as you lose half of the alternator’s power.

Full-wave rectifier

As the name suggests, a full-wave rectifier allows both the positive and negative current through, but this is more complicated as one of the cycles must be inverted. The simplest method is to use two one-way diodes. 

The benefit, of course, is a more efficient system.


As you can imagine, the voltage output from the rectifier isn’t very smooth, so some filtering is required to give a more stable DC voltage.

Depending on the application, the filter can be a simple circuit that removes only the largest variations or a series of filters to achieve an almost perfect DC voltage.

Symptoms of a faulty motorcycle rectifier

Now we understand what your rectifier does; what are the symptoms when your motorcycle rectifier fails?

You won’t notice anything at first when your rectifier fails. Your bike will continue to run as long as the battery has enough charge.

Read: How long does it take to charge a motorcycle battery?

As the battery voltage drops, your lights will become dimmer, and electronics will play up. Eventually, your battery won’t have enough power to start the engine.

How to test a rectifier

If you can still start your bike, the quickest check is at the battery terminals. This test doesn’t test the rectifier individually but the whole charging system. If this test passes, then your rectifier is working.

Battery terminal test

  • Set your multimeter to a DC voltage range of at least 20V.
  • Connect the multimeter probes to the battery.
  • Start the engine.
  • Your multimeter should read a little over 13 volts.
  • Now rev the engine a little, and the voltage should increase to between 13.2 and 14.8 volts.

Rectifier terminal test

The rectifier test is a little more involved but easy for a home mechanic. Inside the rectifier are two diode circuits, one for positive voltage and the other for negative voltage. 

You will use the battery output from your multimeter to put a voltage on one side of the diode and measure the voltage output from that diode. You must repeat the test four times to check both the positive and negative sides.

  • Disconnect all the rectifier cables. There should be five in total
    • Two output cables, usually red and black
    • Three stator input cables, often yellow
  • Switch your multimeter (MM) to the diode test
  • Connect the MM positive probe to the red output cable
  • Touch the MM negative probe to each of the stator cables in turn
  • The reading should be zero, and no tone from the MM
  • Now connect the MM negative probe to the red output cable and use the MM positive probe on each stator input
  • You should hear a tone from the MM and see a voltage of around 0.5 or 0.6 volts.
  • Connect the MM positive probe to the black cable and touch each stator cable with the MM negative probe.
  • You should hear a tone from the MM and see a voltage of around 0.5 or 0.6 volts.
  • Connect the MM negative probe to the black cable and touch each stator cable in turn
  • The reading should be zero, and no tone from the MM

You have just put a voltage on one side of each diode circuit and test if it allows the voltage through. If this passes, your rectifier is likely to be good, and any fault you have is likely something else.

Can you repair a rectifier?

Whether you can repair a rectifier depends on several things;

  • Your technical ability
  • If the rectifier is sealed or not
  • Are the parts available
  • Is a wiring diagram available

If the rectifier is available as a spare part, then fitting a new or used rectifier is the simplest option.

Repairing the rectifier will require tools such as a soldering iron, the ability to read circuit diagrams, and dismantling the rectifier. Remember that the rectifier and regulator are combined on most motorcycles, so the unit’s internals may be more complicated than you expect.

It is possible to repair, but it may only be worth it if spare rectifier units are unavailable.

Image Credits

By Farzad_f58 (talk) (Uploads) – Farzad_f58 (talk) (Uploads), CC BY 3.0, https://en.wikipedia.org/w/index.php?curid=38463453
MikeRunCC BY-SA 4.0, via Wikimedia Commons
B. Jankuloski (vectorization). PNG original uploaded by Guam from en.wiki, where the author was MadokaCC BY-SA 3.0, via Wikimedia Commons