Production of Motorcycle Electronic Ignition

The author has an idle power-assisted bicycle, which originally had platinum ignition. As an electronics enthusiast, in order to overcome the many shortcomings of platinum ignition, he decided to change it to electronic ignition.

After thinking about it, the author's idea is not to remove the platinum contacts, and use the disconnection of the original contacts to trigger the thyristor ignition. The plan is as follows: The ignition part of the original magneto is shown in Figure 1. When the contacts are disconnected, the engine ignites. In order to use the original contacts, disconnect them at the × point and connect a lead to point A. The entire modified igniter circuit is shown in Figure 2.

working principle

The AC voltage output by the magneto ignition power supply coil is divided into two paths: one is input by line 1, one is connected to the low-voltage end of the transformer, and the other is rectified by VD1 and filtered by C1 to supply the trigger circuit composed of V1 and V2. When the engine starts, the magneto outputs an AC voltage of 4V-5V, which is rectified and filtered to obtain a DC voltage of 5V~6V. In the trigger circuit composed of V1 and V2, when the platinum contacts are combined, V1 is saturated and turned on, and V2 is turned off. The collector of V2 is at a high potential, and this voltage is used to trigger the bidirectional thyristor VD3, and VD3 is turned on.

In addition, the low-voltage AC is boosted by the transformer, and VD2 is rectified to charge C2. When the engine cam disconnects the contact, V1 cuts E, the collector of V1 is at a high potential, and the thyristor VD5 is triggered by R4, VD5 is turned on, C2 is discharged, and the ignition coil ignites. On the other hand, due to the different designs of the engine cam, the protrusion of some cams reaches more than 100 degrees, that is, the contact is disconnected by more than 100 degrees. Since the magneto has four poles, the alternating voltage induced by each set of coils changes twice when the positive wheel rotates one circle (waveform as shown in Figure 3).

In order to prevent the high voltage side of the transformer from being at a positive potential during the contact disconnection period, that is, during the conduction period of VD5, so that the high voltage of the positive half cycle is short-circuited through VD2 and VD4, a switch circuit composed of a bidirectional thyristor VD3 is specially installed for this purpose. As mentioned above, when the contact is disconnected, V1 is cut off, the SCR is turned on, and at the same time, the collector of V1 is at a high potential, and this voltage saturates V2. The collector of V2 is at a low potential, and VD3 is turned off, so that the high voltage can be prevented from being short-circuited during the contact disconnection period.

Since the original platinum contacts are used, the ignition is at positive potential, which solves the biggest problem of modification.

Component selection

As shown in Figure 2. R2 and R5 should not be less than 1W, and the B of V1 and V2 should be larger. The transformer should not be lower than 12V (because the transformer used in general modification is 6V-9V), because the voltage of 4V~5V can be started with a 12V transformer, and when the engine is at medium and high speed, the output voltage of the magneto can reach more than 20V. Therefore, under the premise of ensuring triggering, try to use a transformer with a higher voltage. VD3 uses a thyristor with high trigger sensitivity.

Installation and Debugging

After the igniter is assembled, connect the ③ and ④ lines in Figure 2 to the ground and ignition coil respectively. At the same time, take out the spark plug and place it on the engine case. Find a transformer with an output voltage of 3.5V, or use a voltage regulator to output 3.5V. Do not use a thyristor voltage regulator. Connect 220V to the mains, and 3.5V to lines ① and ③ in Figure 2 to replace the magneto power supply. Do not connect the platinum contacts first, but manually connect the emitter of line ② V1 to line ③ for a connection and disconnection test. If the c pole of V1 is not around 0V when lines ② and ③ are connected, it is because the B of V1 is too low. R1 can be appropriately reduced.

If the e pole of V2 is not close to 0V when the ② and ③ lines are disconnected, then reduce R3. When the ② and ③ lines are connected, if there is a voltage of about 100V across VD3, it proves that VD5 has been triggered and turned on. Otherwise, reduce R5 until VD5 is turned on. When there is voltage across VD3, disconnect the ② and ③ lines, and the spark plug should ignite. If it does not ignite, reduce R2 to trigger VD3 to turn on. Each time the ② and ③ lines are connected and disconnected, a spark is generated, which means success.

Point B of the ignition switch can be connected to line ②, or points B and C can be connected without disconnection, retaining the original wiring. The wiring installation on the car will not be described again, and it can generally be successful in one go.