21-02-2018, 10:24 AM
Five years ago I looked around for an electronic ignition for my 6 volt Pearl, but couldn't find anything suitable. I ended up making my own, which is of the simplest 1960's type, categorised as "Transistor assisted contacts" and needs only two transistors and a handful of passive components.
The standard (good condition) DK4A with its points and condenser are retained, but the points current is reduced to 50 mA, so contact wear is negligible and the stress on the condenser is eliminated. It took some time to select a switching transistor with a low enough voltage drop when on, as a 6 volt system has little margin. I settled for an IGBT which drops less than a volt. Whilst this reduces coil current a little compared to a new set of points, the very rapid switching more than makes up for this. One gets a good spark even with slow cranking speeds, and starting (even with "old" petrol) and idling is noticeably better. One of my aims was to avoid occasional misfires which can lead to fouled plugs, and this has been successful.
The unit is mounted low down in the engine bay in a small black metal box with period looking terminals. I took care with the construction to use generously rated components, mounted on tagstrips and with the larger items araldited to the inside of the box to avoid vibration. The other precaution was to use suppressed spark plug connectors, as there was a small risk that the high level of electrical noise in a unsuppressed Seven engine could upset nearby electronics.
In principle the original ignition setup can be restored at the roadside by swapping over a few wires, however in 5 years usage it has proved completely reliable.
As a general observation, my experience of consumer grade electronics from the far east is that there is often no testing of any description, as this would push up the cost too much. Instead, the manufacturer's philosophy is to offer a replacement for anything which fails. For a clock radio or a kitchen timer this is merely annnoying, but for something you are going to rely on it's unacceptable. The only thing I can suggest is to try and give items of this nature several hours of soak test before fitting, because failures tend to be in the early stages.
The standard (good condition) DK4A with its points and condenser are retained, but the points current is reduced to 50 mA, so contact wear is negligible and the stress on the condenser is eliminated. It took some time to select a switching transistor with a low enough voltage drop when on, as a 6 volt system has little margin. I settled for an IGBT which drops less than a volt. Whilst this reduces coil current a little compared to a new set of points, the very rapid switching more than makes up for this. One gets a good spark even with slow cranking speeds, and starting (even with "old" petrol) and idling is noticeably better. One of my aims was to avoid occasional misfires which can lead to fouled plugs, and this has been successful.
The unit is mounted low down in the engine bay in a small black metal box with period looking terminals. I took care with the construction to use generously rated components, mounted on tagstrips and with the larger items araldited to the inside of the box to avoid vibration. The other precaution was to use suppressed spark plug connectors, as there was a small risk that the high level of electrical noise in a unsuppressed Seven engine could upset nearby electronics.
In principle the original ignition setup can be restored at the roadside by swapping over a few wires, however in 5 years usage it has proved completely reliable.
As a general observation, my experience of consumer grade electronics from the far east is that there is often no testing of any description, as this would push up the cost too much. Instead, the manufacturer's philosophy is to offer a replacement for anything which fails. For a clock radio or a kitchen timer this is merely annnoying, but for something you are going to rely on it's unacceptable. The only thing I can suggest is to try and give items of this nature several hours of soak test before fitting, because failures tend to be in the early stages.