Motorcycle MOSFET Full Wave Shunt Regulator Circuit

The following post of a full wave motorcycle shunt regulator circuit was requested by Mr.Michael. Let's learn  the circuit functioning in details.

How a Shunt Regulator Works

Shunt regulator is a device which is used for regulating voltage to some fixed levels by means of shunting. Normally the process of shunting is done by grounding the excess voltage, just as zener diodes do in electronic circuits.

However one bad aspect with such regulators is the generation of unnecessary heat. The reason for heat generation is the principle of its operation where the excess voltage is short circuited to ground.

The above practice may be implemented by simpler and cheaper means, but cannot be considered efficient and advanced. The system is based on destroying or killing energy instead of eliminating or inhibiting it.

The circuit of a motorcycle shunt regulator discussed in this article takes a completely different approach and restricts the in-flow of excess voltage instead of "killing" energy and thus stops the generation of unnecessary heat.

Circuit Operation

The circuit functioning may be understood as under:

When the mobike is started, voltage enters across the P-channel mosfet source/drain pins due to the gate trigger that becomes available via R1.

The moment the high voltage reaches R3, which happens to be the sensing input of the opamp, pin#3 of the IC senses an increased voltage.

As per the set reference at puin#2, the instantaneously reacts to the situation and the result puts the output of the IC to a high logic level.

The immediate high logic pulse restricts the negative base trigger of the mosfet, switching it OFF at that particular instant.

The moment T1 switches OFF, voltage at the junction of R3/R4 reverts to the original condition, that is the voltage here now drops below the reference level......this instantly activates the opamp output with a low logic signal which in turn switches ON T1 back into action.

The process repeats at a very rapid speed, keeping the output voltage marked with +/- at a constant level determined by the setting of R2/Z1 and R3/R4.

The above principle utilizes voltage inhibition technique of the excess voltage instead of shunting it to ground, thus saves precious power and also helps to control global warming in some way.

Parts List

R1, BR2 = 10Amp bridge rectifier

R1 = 1K
D1 = 1N4007
C1 = 100uF/25V
IC1 = IC741
T1 = mosfet J162

R2/Z1, R3/R4 = as explained in this article

Shunting Excess Power to Ground is Recommended in Alternators

When it comes to alternators, the best way to restrict or limit excess voltage is to short the excess power or shunt the excess power to ground. This eliminates the rising current in the armature and protects the winding from heating up.

A voltage regulator using this method can be witnessed in the following examples:

Video Clip below shows an opamp based shunt regulator circuit, and its testing procedure

Parts List

R1, R2, R3 = 10K
R4 = 10K preset
Z1, Z2 = 3V zener 1/4 watt
C1 = 10uF/25V
T1 = TIP142 (on large heatsink)
IC1 = 741
D1 = 6A4 diode
D2 = 1N4148
Bridge rectifier = standard motorcycle bridge rectifier

How to Set up the Circuit

For a 12V system, apply a 18V from a DC power supply from the T1 side, and adjust R4 to precisely set 14.4V across the output terminals.

An even simpler motorcycle shunt regulator using the shunt regulator IC TL431 can be witnessed below, the 3k3 resistor can eb tweaked to chnage the output voltage to the most favorable level.

For single phase alternators, the 6 diode bridge rectifier could be replaced with a 4 diode bridge rectifier as shown in the following diagram:

Feedback and Update from an Avid reader Mr. Leonard Fons

I've come up with a bit more that needs to be considered.
I'm using a MOSFET (IXFK44N50P) for the clipper and series regulators.  Never did much with FETs because when they first came out, the least little static charge would blow them out in a heartbeat.  So this is actually my first attempt to use them. 

I assumed that, like junction transistors, the more power they handle, the more power needed to drive them.  NOT TRUE.  In looking again at the datasheet, I see that Gate current is plus or minus 10 nano Amps. 

That is ten trillionth of an amp.  They do not require a TIP142 to drive them.  A one watt, high gain darlington will do the job very nicely.  And the entire circuit will fit on one board.  I still need another regulator housing for the rectifier.  But I'm about ready to put this all together and try it out.

Of course, I will try it out before I actually mount it into the housing, but I don't expect to make any modifications. 

Realizing that these FETs use almost no gate current at all makes quite a difference.  I'll find out just haw accurate my theory is for the current to ground when clipped at 60 volts, rather than shunting all current to ground. 

A when I pot it into the I have to insure that the FETs have no gap to the housing.  That was another issue with one of the others.  A sixteenth inch space between the components and the housing,

With that gap filled with epoxy, it's not very efficient at dissipating the heat.  By the time the housing starts getting warm, you'd burn your fingers on the components.  One change I may make is the series diode in the monitor line.  A green LED located where I can see it while riding will let me know if it's charging.