What does the Rectifier do on a Motorcycle?

What does the Rectifier do on a Motorcycle?

The rectifier on a motorcycle sounds like an overly-technical device, but it's really a lot more basic than it sounds. At its simplest, a rectifier performs two main functions.

The first is that it converts ac voltage into dc voltage, which is the voltage a bike needs to function properly. A rectifier's second function is to regulate this voltage so that it stays at a safe level. That's all there is to it!

But you probably want a bit more info than that, so keep reading and we'll run through everything there is to know about rectifiers.

What is a Regulator/Rectifier?

dirtbike in the sun

A rectifier is an electronic device that converts ac power into dc power. Motorcycles typically have an alternator that generates ac voltage, but the electrical system of a bike requires dc voltage to function properly.

The rectifier also keeps this dc power output within a specific range. If the voltage exceeds safe levels, this excess power runs the risk of damaging the battery or other electrical components.

Types of Motorcycle Regulators/Rectifiers

There are two main types of rectifiers that are fitted into most motorcycles. Each requires a different type of voltage regulator rectifier.

  1. Permanent Magnet Rotor Alternator (PMR): This type has permanent magnets that revolve around the engine. Some of these have two output wires (single phase), but most have three (three phase).
  2.  Field Control Type (FCT): Instead of fixed magnets, this type has a ‘field’ or ‘exciter’ coil. Once supplied with power from the regulator, this coil becomes magnetized.

How does a rectifier work in a motorcycle?

rider facing camera

So how does the rectifier make sure things are functioning correctly? Rectifiers have a sensing mechanism to measure the output voltage, and it references this with a predetermined reference voltage. The rectifier will make sure the voltage never gets too far from this predetermined point. So it's constantly working to keep the voltage stable.

Why is my rectifier going bad?

Here are some tell-tale signs that your rectifier might be on the outs:

  1. Diode Burnout: With a diode rectifier, individual diodes can fail because of overheating, excessive current, or aging. A single failed diode can disrupt the rectification process and lead to erratic or no power output.
  2. Shunt Regulator Burnout: A Shunt Regulator is a small electronic component that clamps a power supply voltage at a fixed level. If it fails, the headlight may become very bright and the lamp might blow. Your battery might have also boiled dry.
  3. Total failure: With total failure, it doesn't necessarily mean that every part of the unit has died. It might be because the internal connections of these parts have failed. This could be from a manufacturing defect, overheating, or metal fatigue from too much continuous heating and cooling.
  4. Failed Rectifier/regulator: If the regulator is burned out then try checking the battery connections. If they've become loose or corroded then the regulator has nowhere to route the output.

What else could be affecting the rectifier?

If all else fails then it might be time to perform a rectifier test. You'll need a digital multimeter with diode testing capabilities, so if you don't have one then it might be time to pick one up.

Keep in mind that not all rectifiers are the same. Performing a test will vary slightly depending on the type you're running with. For example, here are the ideal forward bias measurements for the main brands:

  • Yamaha regulator rectifier test: Should be about 0.48 volts to 0.52 volts, and the positive terminal diodes should be about 0.12 volts.
  • Honda regulator rectifier test: About 0.48 volts to 0.52 volts
  • Polaris regulator rectifier test: It should be about 0.48 volts to 0.52 volts, and the positive terminal diodes should be about 0.12 volts.


 guy working on bike

How does the rectifier convert the AC to DC?

So exactly how does the rectifier convert the AC to DC current? The process is achieved through diodes, which allow current to pass in one direction but not the other.

During the rectifying process, the diodes switch off and block the current from flowing in the opposite direction. The result is a pulsating DC output, which has a positive voltage with some ripple.

How do I know if I am having trouble with my rectifier?

There are several signs to know if you are having trouble with your rectifier. One of these is output voltage fluctuations. The voltage should ideally be smooth and constant, so if you notice significant fluctuations or instability, it could be a sign of rectifier malfunction.

Another sign is excessive ripple voltage. While some ripple is expected in rectified DC output, an unusually high ripple voltage can indicate a problem with the rectifier. This may be from failed or faulty diodes in the rectifier bridge or issues with the smoothing capacitor.

Other problem signs are overheating, no output or low voltage output, and the circuit's protection mechanisms constantly triggering.

How do I know if my motorcycle rectifier is bad?

The first way to know if a motorcycle rectifier is bad is to perform a visual test. Look for signs of damage and abnormalities. Watch out for burned or discolored components, bulging capacitors, loose connections, or any other physical signs of malfunction.

Testing the voltage measurement is a good sign of problems with a rectifier. Using a multimeter or voltmeter, test the output voltage to see whether it reads at the expected rate. If the reading is lower than expected, it might indicate a faulty rectifier.

Will a motorcycle run without a rectifier?

So will a motorcycle even run without a rectifier? Typically it won't, especially if it's equipped with an electrical system that relies on a battery and requires DC voltage to operate.

Motorcycles generally need a dc voltage to function correctly. Without a functioning rectifier, the battery won't receive the proper charging voltage, leading to a discharged or weak battery over time.

How do I know if my bike rectifier is working?

How to know if a bike rectifier is working usually means testing the voltage output using a multimeter or voltmeter. If the reading is lower than expected, it usually indicates that a rectifier is not functioning correctly.

You can also perform less formal checks such as a visual inspection. Look for any signs of damage or overheating. Check for burned or discolored components, melted wires, or any other physical indications of a problem.

Try checking the rectifier's temperature while it's in operation. If it's hot to the touch, that could be from an excessive current or power dissipation.

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