TWN Contessa - Electrics

page updated February 22, 2011 by Dave

Norris (Bosch) Regulator

Mechanical Regulator on TWN

On removing the metal cap on the regulator, the internals looked to be in quite good condition considering their age. I needed to make sure everything checked out electrically though so I sketched the diagram below for reference and took ohms readings of the current coil (0.3Ω), voltage coil (50Ω), and shunt resistor (8Ω). I also illustrated the Field and Starter windings.

Mechanical Regulator Wiring on TWN

I'm more used to seeing the more modern transistor regulators, so I had to get a refresher course on how the mechanical version works. I found an excellent document: "Understanding And Maintaining The BMW /2 Electrical System" here (.pdf 3.16MB). The regulator section is closely relevant to the TWN - see the diagram below pulled from the document. It compares quite nicely with my diagram above.

Following is an excerpt from the document and explains the operation in detail - just be aware the text refers to a 6V system and the TWN is a 12V system:


How The Regulator Works

When the voltage produced by the generator is low, the current flows through the field windings to ground. This works to increase the voltage produced.

When the voltage increases above 7 to 7.5 Volts, the current passes through a resistor which prevents the voltage from rising so fast.

If the voltage continues to rise, the voltage coil shunts the field winding (both ends connected together), which prevents it from flowing current. The voltage produced by the generator then drops to zero.

The current coil of the regulator measures how much current is flowing from the generator to the battery and all other electrical components. It works to drop the voltage if the current rises too much.

The regulator also has a circuit breaker - this is normally open until the generator voltage rises above 6.5 Volts, when it closes and opens a path from the generator to all the consumers. When the voltage drops, the current coil is wound so that the battery drains into the generator, and the current coil presses the circuit breaker open.

Replacing the Regulator

The regulator is a big compromise in order to get good results in normal conditions.

The biggest problem is the function of the current coil. It protects the generator from currents that are too high, but as soon as there is a current flowing it is acting to regulate the voltage down. So with a rising current, the voltage drops gradually. This produces a compromise - the voltage is too high when the lights etc are turned off, boiling the battery, and it is too low when everything is switched on, causing the battery to be drained.

The regulator is also sensitive to temperatures. The higher the temperature, the higher the generator voltage. The regulator is mounted in quite a hot place, so at least the battery shouldn't starve - but it might
cook in summer.

The best compromise is to use a 14Ah battery, and have the regulator adjusted at the low end of the voltages. Keep an eye on the battery, and if it is draining, give it extra load, such as by using the headlights all the time, for example.

The circuit breaker isn't great either. It is switching currents of between 2.5 to 9 Amps, which isn't healthy for a mechanical switch. This is a major issue when riding for long times at slow engine speeds.

The solution is to use an electronic regulator - this does away with the problems of mechanical switch contacts, and most electronic regulators have a constant voltage over the whole current range.

An electronic regulator also has a diode to replace the circuit breaker. This acts as a one-way valve and only lets current flow in a single direction.

Not all electronic regulators are suitable - they must have over-current protection as well, so that when the current is too high for the generator, they switch off the field current.

But they are still temperature sensitive - they need to be kept below 80°C, so should be mounted outside the engine, under the tank for example. Don't put it in the headlight - they don't like vibration and headlights vibrate a lot.



Regulator / Ignition Coil - original condition


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