I'm thinking about designing a circuit for a tach. I've been reading about the signal from the Tach on the HEI.
The Tach signal for an HEI is reasonably similar to that found in a normal points ignition, with the exception of the dwell period duty cycle. A points system has a constant 67% duty-cycle angle (30*/45*), while an HEI strives for a constant dwell time (of perhaps 4-5 mSec). These are primary current duty-cycles, and a voltage reading at the Tach/C- lead (via an oscilloscope) would show a reciprocal duty cycle. An oscilloscope signal trace on a points system would show a constant appearance over the RPM range (only a change of frequency), while the HEI signal would show a continually decreasing voltage duty-cycle because while the period is decreasing, the dwell time (C- at 1.5v) is attempting to stay the same 4-5 milliseconds. At high RPM the voltage duty cycle reaches a minimum, as the module preserves a short amount of time for the coil to dump its energy to the plugs. As you can see, the signal is reasonably simple, with the exception of when the dwell stops and the plugs fire. We can divide it up into two parts.
The Tach/C- lead sits at 12-14v during non-dwell periods (as the HEI Darlington power transistor is off and there's no current flowing through the coil inductance/resistance). During dwell the power transistor is on, and most of the supply voltage is dropped across the coil, with the exception of the transistor's saturation voltage (about 1.5 volts). So essentially the Tach is a PWM signal bouncing around between 1.5v (during dwell) and 12-14v between dwell. [ For additional technical minutia, there is a short time just prior to the end of dwell when, if the primary current has hit its calibrated amperage target, the Darlington power transistor comes out of the saturation mode and into active mode, and the collector voltage (same as C- voltage) will reach about 6-8 volts. This mode keeps the current limited to about 5.6 -5.8 amps, negating the need for a ballast resistor to limit the peak primary current. This short event is only visible on an oscilloscope.]
The main concern when connecting the Tach to any instrumentation is the inductive ringup/flyback voltage when the transistor shuts off (and the plugs fire). Depending on the plug gap breakdown voltage, the Tach/C- signal will spike up to 100-300 volts, and then decay with a bunch of ringing on down to 12v again. This is the point where some RC filtering (and perhaps some voltage clamping) is required to keep the Tach/instrumentation circuitry from being damaged from the high voltage spikes each time the coil/plug fires.
So, it's basically a a DC PWM signal with a high-voltage ringing/AC component added onto it.
Forum
http://www.digitalcorvettes.com/forums/showthread.php?t=149655
http://www.youtube.com/watch?v=mJY1xftst94&feature=related
This is the "tach" signal (negative terminal of the ignition coil) of a GM HEI distributor after being run through an RC filter.
http://www.youtube.com/watch?v=6OlRJN57Q2Q
This is the unfiltered signal coming off the negative terminal of the ignition coil (the terminal labeled "tach") of a GM HEI distributor.
The Tach signal for an HEI is reasonably similar to that found in a normal points ignition, with the exception of the dwell period duty cycle. A points system has a constant 67% duty-cycle angle (30*/45*), while an HEI strives for a constant dwell time (of perhaps 4-5 mSec). These are primary current duty-cycles, and a voltage reading at the Tach/C- lead (via an oscilloscope) would show a reciprocal duty cycle. An oscilloscope signal trace on a points system would show a constant appearance over the RPM range (only a change of frequency), while the HEI signal would show a continually decreasing voltage duty-cycle because while the period is decreasing, the dwell time (C- at 1.5v) is attempting to stay the same 4-5 milliseconds. At high RPM the voltage duty cycle reaches a minimum, as the module preserves a short amount of time for the coil to dump its energy to the plugs. As you can see, the signal is reasonably simple, with the exception of when the dwell stops and the plugs fire. We can divide it up into two parts.
The Tach/C- lead sits at 12-14v during non-dwell periods (as the HEI Darlington power transistor is off and there's no current flowing through the coil inductance/resistance). During dwell the power transistor is on, and most of the supply voltage is dropped across the coil, with the exception of the transistor's saturation voltage (about 1.5 volts). So essentially the Tach is a PWM signal bouncing around between 1.5v (during dwell) and 12-14v between dwell. [ For additional technical minutia, there is a short time just prior to the end of dwell when, if the primary current has hit its calibrated amperage target, the Darlington power transistor comes out of the saturation mode and into active mode, and the collector voltage (same as C- voltage) will reach about 6-8 volts. This mode keeps the current limited to about 5.6 -5.8 amps, negating the need for a ballast resistor to limit the peak primary current. This short event is only visible on an oscilloscope.]
The main concern when connecting the Tach to any instrumentation is the inductive ringup/flyback voltage when the transistor shuts off (and the plugs fire). Depending on the plug gap breakdown voltage, the Tach/C- signal will spike up to 100-300 volts, and then decay with a bunch of ringing on down to 12v again. This is the point where some RC filtering (and perhaps some voltage clamping) is required to keep the Tach/instrumentation circuitry from being damaged from the high voltage spikes each time the coil/plug fires.
So, it's basically a a DC PWM signal with a high-voltage ringing/AC component added onto it.
Forum
http://www.digitalcorvettes.com/forums/showthread.php?t=149655
http://www.youtube.com/watch?v=mJY1xftst94&feature=related
This is the "tach" signal (negative terminal of the ignition coil) of a GM HEI distributor after being run through an RC filter.
http://www.youtube.com/watch?v=6OlRJN57Q2Q
This is the unfiltered signal coming off the negative terminal of the ignition coil (the terminal labeled "tach") of a GM HEI distributor.
