I've been poking around with an old Piranha electronic ignition module I
have lying around, trying to get a better feel for why these things
interfere with the operation of the Stag MK I tachometer.
Now the Piranha is an older technology, so I'm not sure how well what
I'm about to say relates to units that were designed afterwards, but for
those of you who are using an older design like the Piranha or the Crane
unit, this could be relevant.
The actual device that switches on current through the coil on the
Piranha system is a darlington NPN transistor that was specifically
designed to drive the coil on electronic ignition systems. However
there's a gotcha here: It was designed to drive the coil on ELECTRONIC
IGNITION SYSTEMS. Not to drive an ordinary coil. The transistor has a
voltage drop of more than 2 volts when turned on. This can result in a
loss of up to 20% or more of the output voltage of the coil in an
ordinary system. In a system with a resistor to drop the coil to 6
volts, the loss can be even greater.
In a system with an electronic tachometer like the Stag, that voltage
drop and the accompanying current drop can affect the functioning of the
tachometer.
I guess the rule should be to make sure you add a higher power coil when
you have electronic ignition. On the flip side, when with a higher power
coil, the voltage drop on the transistor will go up a bit.
For myself, I'm looking into replacing the transistor (BUX37) with a
power MOSFET with a low turn on resistance. One nice thing about the
Piranha (now Newtronics) system is that you can open it up and get at
the components.
--
George Richardson
Wyvern - '57 Triumph TR3, TS15559LT -
http://www.merlingroupinc.com/tr3.htm
Griffin - '71 Triumph Stag - undergoing restoration
Pikachu - '75 Triumph TR6 - undergoing repair
Kitty - '83 Jaguar XJ6 Vanden Plas - Daily Driver
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