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Last Updated: August 08, 2011

FET type motorcycle voltage regulator

This is another voltage regulator for motorcycles that uses Field Effect Transistor for better charging efficiency of the system, although still using half wave bridge system on charging the battery, the control to which the system monitors the battery condition. What makes it better is that is a MOSFET controlled device rather than the crude SCR shunt type that is on most bikes until recently and also is a 50A rated device.
MUCH better voltage regulation and runs cooler too due to more efficient devices and control circuitry.The SCR shunt type consumes more energy in the Regulator itself than the bike is using and dumps a ton of current into the heatsink (feel yours & just see how hot those things run!!!! - don't touch it - you'll burn yourself - seriously!) The problem is exacerbated because their efficiency goes even lower when they get HOT so it's a vicious circle. Heat is the number 1 killer of these devices.
Incidentally its a misconception that shunt type work harder with increased load i.e. higher-wattage lights, etc - actually, the higher the load on the output, the less work the shunt regulator does in dumping that excess energy and will actually run cooler!!

The FET has extremely low resistance in conducting state and this results in a lower dissipated power from the device while conducting load current, as opposed to the SCR which shunts the maximum current across a significant volt drop, resulting in a higher dissipated power - and resulting temperature, much more so than the FET device. 

The picture shows a typical FET type voltage regulator for motorcycles, as opposed to the SHUNT type to where an SCR is used to control the windings to generate more than enough on the system, here an FET is used tag as Q5 in the diagram, IC2a is the control circuitry of the battery charge and discharge. D1 and D2 is the rectifier that converts the AC voltage of the stator to DC that charges the battery.

Let us analyze more on the circuit with block by block representation of the circuit itself. 
The BLUE and GREEN block covers the monitoring circuit, and charging control of the battery, half of the IC2A lm393 is the area of monitoring the battery voltage, via W1 trimpot,  will be adjusted so that Q5 fet will off at 14.4 to15 volts for battery charging. W2 on the other hand will be adjusted at 10.5 volts so that no further discharge action will take place when the motorcycle is stored and or the motorcycle battery is being used for other purpose with engine off.

The RED block is for your lighting system, the two SCR T1 and T2 controls the light brightness. both charging and lighting system uses the stator winding together as opposed to normal shunting to where the charging uses the whole stator winding wherein the lighting system uses the half of it.

 SCR T1 connected on other end of L1, T2 on the other end of L2, when the engine speed is 3000r/min, adjust R1 to turn T1, L1 voltage at 15V or so; adjust R2, the T2 turn, L2 voltage stable at around 15V. When the engine is at low speed, T2 is not conducting, T1 conducts, L1 + L2 remained relatively in high voltage, in actual use, the engine speed is 3000r/min, the light is still bright. 

NOTE: the system uses stator winding not earth grounded at the other end or it is isolated from ground to where a SHUNT regulator commonly uses from my previous post.

Disclaimer: If ever you wanted to build this system, i shall not be held responsible if something fails or something gone wrong.

UPDATED 8-16-2013 (added part list)

Part list

Resistor --all 1/4watts

R1 100 ohms
r2 1k 
R3 390 ohms
R4 150 ohms
R5 18k
R6 470 ohms
R7 390 ohms
R8 10k
R9 130k
R10 120k
R11 130k
R12 128k (5 band resistor.)
R13 268k (5 band resistor)
R14 1.5k
R15 1k
R16 1k


C1 & C2 10uf / 50v electrolytic


D1, D2, D9 MUR3060 (fast diode)
D3 1n4001
D4 FEP16 (16A fast diode)
D5 33v /500mw zener 
D6 18v / 500mw zener
D7 5.1v / 500mw zener
D8 MUR1560 (fast diode)

SCR BT151-500
Q1 2n5401
Q2, Q3 2n5551
Q4 J118 (mosfet)
Q5 IRF9430 P-ch mosfet


W1, W2 50k

IC1 LM7808T

IC2 LM393


Julian Maruli said...

If I want to build this system but using 3 phase full wave regulator (full DC, I mean all electricity control by motorcycle battery).. what kind the modification I need?