Dynamo Current and Voltage Regulator

Posted on Jul 24, 2012

In the typical dynamo charging circuit, B+ and B- are the battery connections. D+ and D- go to the dynamo brushes, while DF is the field connection, with its other end returned to D+ inside the dynamo. Please note that not all dynamos return the field winding to the positive output! My regulator circuit, as shown here, works only with dynamos connected in this way. For further clarity, here`s the internal wiring of a standard dynamo. A dynastarter is just like this but with an additional field winding added, connected from D+ to an additional terminal, which is used to apply battery current for starting.

Dynamo Current and Voltage Regulator
Click here to download the full size of the above Circuit.

This regulator operates with a rather slow time constant when in voltage regulating mode. The time constant is given by C4 and the parallel resistance of R11 and R12. When operating in current limiting mode, the high gain of U1D makes the time constant a lot faster, and asymmetrical: The regulator will adjust the output down faster than back up. In the presence of an abnormal pulsed load, this will make the circuit adjust for an average current somewhat lower than 20A, protecting D1 and the dynamo. Also, if the fuse is open, dynamo voltage limiting will have a fast attack time. The fuse was included mainly as a safety measure against failure of D1. This diode gets pretty warm, and could conceivably fail if it comes loose from the heatsink. If it fails shorted, F1 would avoid the destruction of the dynamo through reverse current. Anyway the risk would be low, because a failure like this would most likely end up with D1 opening rather than shorting, but fuses are cheap, so it's not bad to include one... If F1 is open, the dynamo voltage can no longer be regulated by sensing battery voltage, and also there would be no current to sense. In order to keep the dynamo voltage from soaring, D5 will inject some current to pin 2 of U1A, and so the circuit will now regulate the dynamo voltage, not battery voltage, to about 16.5V, keeping matters safe until someone replaces the fuse - hopefully before running the battery down! D18...

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