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Motorcycle Regulator Rectifier Testing

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How to test a regulator rectifier?

Regulator rectifiers are plug and play in most applications. But how to test a regulator rectifier? This guide aims to help identify when is the right time to buy a new one?

Regulator rectifiers can fail in either direction…

They can allow too much current to pass through and over charge the battery.

Or they can restrict the flow current and not charge the battery at all.

Lamberts Bikes, Regulator Rectifier, Failed Diode
Blown diode

Firstly, let’s look at some key principles for testing and diagnosing regulator rectifier faults…



Basic principle of regulator rectifier diagnostics

Typically, if the battery is under charging, then the rectifier side of the unit has failed.

Whereas, if the bike is over charging, then the regulator side of the unit has failed.

However, modern units are combined in a single sealed unit. This means that if one fails the whole unit needs to be replaced.

Voltage test

To diagnose regulator faults, a voltage test is often sufficient. Using a multimeter, carry out the following test…

  1. Set your multimeter to volts DC
  2. Measure the output DC voltage at different engine rpm

Voltage test results

The ideal voltage range of the DC output should be between 12.3Volts and 14.5Volts

Any lower and the regulator rectifier will not be able to charge the battery.

Any higher, then there is a risk of damaging components.


Bench testing the rectifier

Rectifier diodes are connected directly between the AC inputs and the DC output pins. Therefore, the diodes can easily be tested with a multimeter.

Diode test mode

Most modern multi-meters come with a diode test function. This measures the forward bias voltage of a diode (i.e. the volt drop as current passes through in the right direction).

  1. Set your multimeter to diode test
  2. Place the red probe (+) on the anode the diode
  3. Place the black probe (-) on the cathode the diode
  4. Measure the forward bias voltage across the diode

Diode test results

The forward bias voltage measured is typically around 0.5V. But it can also be as high as 0.7V or as low as 0.2V.

Also, the forward bias voltage is dependent on the quality of the diodes. Any higher than 0.7V or any lower than 0.2V; then there is a good chance that the diode has either failed, or it is well on its way to failing.



Bridge rectifier test

An animation of a bridge rectifier:

Diodebridge-eng

The bridge rectifier animation highlighted, shows a simple bridge rectifier circuit that is used in motorcycle regulator rectifiers.

Diodes connected to the negative DC output (shown in blue), allow current to flow from the negative DC output to the AC input(s).

Whereas, diodes connected to the positive DC output (shown in red), allow current to flow from the AC input(s) to the positive DC output.


To measure the forward bias voltage of the diodes connected the negative DC output, follow these instructions:

  1. Using a multimeter. Connect the positive red probe to the negative DC output.
  2. Then connect the negative black lead to each of the AC inputs.
  3. Note down the measurements for each of the AC inputs.

To measure the forward bias voltage of the diodes connected the positive DC output, follow these instructions:

  1. Using a multimeter. Connect the negative black probe to the positive DC output.
  2. Then connect the positive red lead to each of the AC inputs.
  3. Note down the measurements for each of the AC inputs.

Bridge rectifier test results

From the recorded readings it should be clear if there are failed diodes. If your unable to get any ford bias voltage reading, then it is likely that, that particular the diode has failed.

Remember, diodes do not part fail. They either work or they don’t!

However, but be wary about any drawing any immediate conclusions…


Have wave rectifier test results

Half wave rectifiers only have one set of diodes! They can be on either on the positive or negative DC output, but never both! The results from a rectifier bench test would show that on side of the diodes have failed…

Lamberts Bikes, Bridge Rectifier DC Ripple, Half Wave Rectified

Alas, that may not be the case! Always check the specifications of the regulator rectifier before jumping to any immediate conclusions. To learn more, view “how do a regulator rectifiers work?”


Rectifier bench test video​

This is a fantastic video tutorial by an external source; Roadstercycle of YouTube, originally published July 2013.

This video shows the method described above in action:


Learn more…

To learn even more about your motorbikes power circuit, view “how do a regulator rectifiers work?”

Want to learn more about regulator rectifiers? Don’t miss our motorcycle regulator rectifier data library.



Download wiring diagrams for FREE!

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