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For e-ignition kits supposedly made for SIX volt systems then why isn't such a kit designed for something like 2.5 or maybe 3 volts trigger voltage instead of retaining the trigger spec for 12 volt systems????? I've never been able to get an answer to that question and i'm not exactly an EE so i havn't got a clue.
It has to do with the semiconductor properties and type. There are several families and all have different requirements and advantages or disadvantages. TTL and CMOS are the most common.
Most of these units use TTL (transistor-transistor logic). That is one of the first semiconductor families, fairly rugged & well proven. It's very reliable, capable of providing reasonable power output without additional power components and is relatively inexpensive.
It is designed to work at 5V & do not exceed limits are typically 4.7 to 5.2V. It's very unforgiving once the voltage threshold on either side is approached. Over and it burns up, under and the logic doesn't reliably trigger -- if it triggers at all. The low voltage threshold is the main downside for use on 6V. A poor connection or heavy starter load can easily drop battery below the limit. A few other issues but all in all, the good outweighs the bad for this type application.
CMOS is 12-15V. Very prone to damage by static electricity from careless handling and somewhat noise sensitive. Noise in this reference is electrical garbage on the power supply.
Newer microprocessor logic that use low voltages in the 3.3V range are becoming common but is low power and also very sensitive to handling. Would need more components and possibly even a second power supply to handle higher voltage and current output. The logic is also extremely noise sensitive and needs a lot of additional power supply filtering to ensure a very clean & stable supply voltage.
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