Download 05 May 2012 MGB Tachometer Circuit (please report errors to Terry

PNP Germanium
X1-1, X1-2 and X1-3
are windings on
a Toroidal Core
MT59/A
7027
Q1
E
B
05 May 2012
MGB Tachometer Circuit
(please report errors to
[email protected])
B
E
C
C
Bottom View
Typical pulse duration is 1.2 mSec.
At 6000 RPM the duty cycle is
about 50%.
Pulses to meter are near battery
voltage. If V+ < 12.8v then tach
reads increasingly low.
39 
Ground
Ign
R2
X1-3
At least two different meters were
apparently used. One had a resistance
of 10 Ω and the other 70 Ω
black
A
50 
X1-2
(3.2 Ω nominal
resistance)
Ign
+12v
yellow
~3.5 Ω cold
R1
Some specimens show values of 8 - 10 Ω cold.
Differences may be due to age-related drift or
changes to match different meter resistances.
The NTC thermistor may compensate for PTC
characteristics of the copper meter windings.
X1-1
NTC Thermistor
2.2  or 3.3 
red
Q1
+
2.5 F
C1
Circuit explanation - Terry Ingoldsby:
This explanation is based on information from “Transistors - Theory and Use”, the book’s author is listed
only as “Prepared by the U.S. Department of the Army”. Copyright 1977. Strangely, the copy I have was
published in Canada. There have been significant contributions to my understanding of this circuit by
Tom Hayden and Herb Adler.
Most tachometer circuits are monostable multivibrators. Each ignition pulse triggers the monostable to
generate a fixed width pulse. These pulses are integrated by a moving coil meter which displays the RPM.
Several tachometer circuits were used in the MGB. This diagram is for the single transistor, current sensing
tach used up to about 1973.
As drawn, the circuit is a simple, common emitter transistor configuration. Note that in the quiescent state,
both the base and emitter of transistor Q1 will tend to the same voltage (12v). This biases the transistor
OFF and no current flows.
When the ignition points close, the full ignition coil current flows through X1-1, a two loop winding on the
toroidal transformer. The resulting current pulse creates a magnetic field that induces a negative pulse in
X1-2. This pulse drives the base negative w.r.t. the emitter (forward bias) and causes a collector current
(through the ammeter).
So, how does this produce a fixed width pulse? It turns out that the tach circuit is very similar to the
monostable blocked oscillator found on page 201 of “Transistors - Theory and Use”. I have adapted the
text to refer to the tach diagram - there are differences between the two circuits and I apologize for any
errors introduced into the original explanation.” We read, “In the monostable mode of operation, the
base-emitter junction is reverse-biased and the transistor is held at cutoff. The output obtained is a single
cycle for each pulse applied at the input.” {Which in our case, is induced via the X1-1 and X1-2 transformer
windings} “Transformer windings X1-3 / X1-2 provide regenerative feedback and couple the output signal
to the load.” The rest of the explanation is mostly mine (and may have errors!)
Initially, it was believed that the capacitor C1 contributed to the timing of the circuit. However, calculating
the RC time constant provided by C1 results in a value far too short to have any timing impact. Herb Adler
has conclusively proven this to be true – the circuit functions even if C1 is removed! An alternate
explanation of C1 is provided below.
A negative trigger pulse is applied at the input {by an ignition coil pulse through X1-1}. This causes the
base of the transistor to become forward biased to saturation and the transistor begins to conduct.
Collector current flows through transformer winding X1-3 and produces a varying magnetic flux that
induces a regenerative voltage across transformer winding X1-2, thus maintaining the forward (negative)
bias on the base of the transistor.
The current does not reach its maximum value instantaneously. Rather, current increases gradually due to
the inductance of X1-3 and meter coil itself, and reaches a limit defined by the series resistance of X1-3,
the meter’s coil and R2. Once the current steadies this ends the regenerative transformer action of X1-3
and X1-2, and therefore the forward bias to the transistor.
If the full output of X1-2 were applied directly to the base of the transistor, the time period for which the
transistor remained biased on would be determined by the LR time constant, where L is the series
inductance of X1-3 and the meter coil, and R is the series resistance of all of the components in the
collector circuit. This time period represents the upper bound of the duration for which the transistor
could be biased on.
However, the designers of the tachometer needed a way to tune the pulse width in order to calibrate the
instrument. This is done by limiting the feedback signal from X1-2 to the transistor’s base junction using a
resistor network. The main element of the resistor network is a 50 ohm adjustable rheostat. Other
elements of the resistor network include a NTC thermistor. The thermistor is believed to compensate for
the PTC characteristics of the copper meter windings. Tom Hayden comments that he believes that the
compensation was not perfect, perhaps because British engineers did not anticipate very high desert
temperatures. If operating in this environment please contact Tom Hayden for suggestions on improving
the accuracy over a wider temperature range.
Note that when a trigger from the ignition system begins the pulse cycle, the current through X1-3 (and
the meter) initially increases at an almost linear rate. However, as the current approaches the maximum
value, the rate of change decreases and rolls off asymptotically. Hence, the induced signal in X1-2
decreases in a similar fashion. The rheostat is used to adjust the exact point at which the regenerative
signal will no longer be enough to maintain the transistor fully biased on. As soon as the transistor’s
conductance begins to drop, so does the current through X1-3. This reverses the regenerative feedback
and induces a positive voltage on X1-2 which quickly clamps the transistor fully off - ending the pulse.
There is some speculation as to the function of C1. As mentioned, it definitely does not affect the timing of
the circuit since its absence does not prevent the circuit from working. One theory is that it is simply a
filtering capacitor. It is of note that the capacitor is reverse polarized from the normal configuration. This
suggests that the purpose of the capacitor is to absorb the reverse regenerative feedback pulse (created
when the transistor pinches off the pulse) and protects the transistor from excessive reverse bias on the
base junction.
Of final note is Tom Hayden’s research that the house numbered PNP Germanium transistor originally
supplied by the manufacture can be replaced by NTE158.