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Replacing master vibrator for Model T and TT Ford

12 Feb 2015  | Craig D. Merz

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Figure 2 shows the fuse, half-wave rectifier, and filter to change the magneto output to DC, which is then smoothed by a 2200µF filter capacitor. In a change from the original circuit, a 5Ω ballast resistor is added to the circuit to reduce the voltage fed to the master vibrator at higher engine RPM. This resistor is switched in and out by a normally closed contact (NC) on the "BR" relay. A 5V regulator is also included so that the BR relay coil is never supplied with more than this voltage. With the particular relay used (5V 110Ω coil) and the 10V Zener diode, the relay pulls in (opening the NC contact) at about 16V and drops out at about 14V. The Zener diode is used so that the BR, NC contact will not remain open at too low a voltage. Because the 5Ω resistor (R1) is always across the BR, NC contact, the relay contact requirement for switching DC is greatly reduced, allowing the use of a smaller relay. This particular part has a contact rating of 5A @30VDC. If you elect to use a relay with a lower coil resistance (less than 100Ω) you will need to use the larger LM7805T regulator, though it should not require a heat sink.

The DC output from this circuit is then fed to the master vibrator board via Out (+) and Ground on TB 2 (Ground must also connect to the vehicle chassis).

Referring to figure 1, a 200Hz oscillator is provided by IC1. The output is fed to a photovoltaic optocoupler (IC2). TVS1 is a Transient Voltage Suppressor to protect the MOSFET from high reverse voltage. An optional LED1 with resistor R7 provides a visual indication of the master vibrator output as the model T's timer grounds each coil in turn. It will flash at idle and more rapidly as RPM increases, appearing full on at higher RPM.

Figure 3: Both boards together at 6V.



Figure 4: One of the four Model T coils with the jumper wire in place.


About the author
Craig D. Merz contributed this article.


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