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Motorcycle SCR Shunt Regulator Circuit

Motorcycle SCR Shunt Regulator Circuit

The circuit presented here is a Rectifier plus Regulator for a 3-Phase charging system of Motorcycles. The rectifier is full-wave and the regulator is shunt-type regulator.

By: Abu Hafss 

Motorcycle Battery Charging System

A motorcycle's charging system is different from that on cars. The voltage alternator or generator on cars are electro-magnet type which are quite easy to regulate. Whereas, the generators on motorcycles are permanent magnet type.

The voltage output of an alternator is directly proportional to the RPM i.e. at high RPM the alternator will produce high voltages more than 50V hence, a regulator becomes essential to protect the entire electrical system and the battery too.

Some small bikes and 3-wheelers which do not run at high speeds, only have 6 diodes (D6-D11) to perform full-wave rectification. They don't need regulation but those diodes are high ampere rated and dissipate a lot of heat during operation.

In bikes with proper regulated charging systems, normally shunt-type regulation is used. This is done by shorting out the alternator's windings for one cycle of the AC waveform. An SCR or sometimes a transistor is used as shunting device in each phase.

 Circuit Diagram

Rectifier plus Regulator for a 3-Phase charging system of Motorcycles

Circuit Operation

The network C1, R1, R2, ZD1, D1 and D2 forms the voltage detection circuit, and it is designed to trigger at about 14.4 volts. As soon as charging system passes this threshold voltage, T1 starts conducting.

This sends current to each gate of the three SCRs S1, S2 and S3, via current limiting resistors R3, R5 and R7. D3, D4 and D5 are important to isolate the gates from each other. R4, R6 and R8 help in draining any possible leakage from T1. S1, S2 & S3 should be heat-sinked and isolated from each other using mica insulator, if using common heat-sink.

For the rectifier, there are three options:

a) Six automotive diodes

b) One 3-phase rectifier

c) Two bridge rectifiers

All must be rated at least 15A and heat-sinked.

The automotive diodes are two types positive body or negative body hence, should be used accordingly. But they might be little difficult to contact to heat-sink.

Using Two Bridge Rectifiers

If using two bridge rectifiers, they may be used as shown.


Using Two Bridge Rectifiers


Bridge Rectifier



Automotive diodes

Automotive diodes


3-phase rectifier

3-phase rectifier

Bridge Rectifier


About the Author

I am an electronic engineer (dipIETE ), hobbyist, inventor, schematic/PCB designer, manufacturer. I am also the founder of the website: https://www.homemade-circuits.com/, where I love sharing my innovative circuit ideas and tutorials. If you have any circuit related query, you may interact through comments, I'll be most happy to help!

6 thoughts on “Motorcycle SCR Shunt Regulator Circuit”

  1. Howdy, Friend! Interested to Learn Circuit Designing? Let's Start Discussing below!
  2. Hello Swagatam, i need some circuit for home automation specially ensuring security of every home of a community. Which can be handled from single point. which may include IR, Motion etc. with central alert point. Can u help me out.

    • Hello Hadi, I'll surely do it, but I would require a more detailed explanation from you regarding the project with specifications, it will help the designing easier for me.

  3. Hi Sir,

    What modification should be made if I want to use mosfets instead of SCRs, I have read somewhere that it is more efficient in using mosfet instead of scr because of mosfet on-resistance is very low.


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