INPULSE TACHOMETER
GENERAL DESCRIPTION
The equipment consists of an indicator head and a pulse lead. The pulse lead, when
connected in series between the vehicle ignition switch and the
low tension terminal of the engine ignition coil will transmit voltage pulses to
the indicator head.
The tachometer system has been designed to cover a wide range of internal
combustion petrol engines using either 6V or 12V coil ignition electrical systems;
positive or negative ground. This range covers 2, 3 and 4 cylinder 2 stroke
engines and 4, 6 & 8 cylinder 4 stroke engines. Any one of these applications can
be obtained by recalibrating the tachometer. This operation is explained in the
section headed CALIBRATION INSTRUCTIONS on page S.7.
TECHNICAL DESCRIPTION .
(For Component Reference see Circuit Diagram Fig. 2 Page S.10)
The object of the circuit as shown, on Page S.10 is to provide a pulse of constant
height and width to the coil of a meter every time the engine fires. The cycle of
operations consists of a rest period followed by a pulse followed by a further
rest period.
At rest the collector-emitter voltage of transistor TR1 is very low due to the
base current flowing via resistors R8, R5 and R6. Under this condition transistor
TR2 will not be conducting, since its base is effectively shorted to earth by R1
and the conducting TR1. Capacitor Cl is charged to the zener voltage with its
right hand plate negative with respect to earth. No current flows through the
meter M.
The primary of the triggering transformer (T) is connected in series with the
primary of the engine ignition coil, so that when the contact breaker in the
engine distributor closes, the current flowing to feed the ignition coil passes
through the primary of the transformer energising the core. When the contact
breaker opens to provide a spark to the engine, the flux in the transformer core
collapses and appears as a short duration voltage pulse across the secondary of
the triggering transformer.
This pulse causes TR2 to conduct, which effectively brings the right hand plate of
Cl to earth potential so that the left hand plate is positive with respect to
earth. Cl starts discharging through R2, R8, R5 and R6 (driving the base of TR1)
positive making it non-conducting. Simultaneously, the collector voltage of TR1
switches towards the zener voltage and TR2 conducts due to the base current
flowing via R4 and Rl. During this state current flows through the meter via the
conducting TR2 and the temperature compensating circuit
thermistor Thl and R3. This state continues until the charge in Cl is unable to
maintain the positive potential on TR1 and the circuit reverts to the stable
state.
The time taken for Cl to discharge is a function of Cl, R6, R5, R8 and R2. Hence
current pulses of constant charge are applied to the meter at a frequency
depending on engine speed. The voltage applied to the circuit is stabilised by the
zener diode Dz, C3 and R7. The capacitor C2 prevents the circuit being triggered
by spurious voltages generated by auxilliary equipment or faulty contact breaker
points.
