<VV> Monza clock diode install

[Frank DuVal]

Let's see if I can explain better.

You know there is voltage produced when a wire is passed through a 
magnetic field. Voltage is also produced if the magnetic field passes 
over a wire standing still. When you shut off the current to a coil of 
wire, like a solenoid, ignition coil, etc, the magnetic field collapses. 
This is the moving magnetic field over the still wire. So there is 
voltage produced by the coil when the coil is shut off. Back emf is 
another term for this voltage.

But, I digress, as you already know how the ignition coil works.

And you already know the condenser (capacitor) will help minimize the 
arc of the points. This capacitor helps dissipate the back emf., as it 
passes AC current and the changing current (back emf) mimics AC in its 
varying waveform. It also forms a tuned circuit with the coil.

A diode will pass voltage in one direction and not the other. Oops, it 
will pass current in one direction and not the other. It passes current 
in the direction of the arrow. OK purists, electron flow is the other 
way around, don't confuse my explanation. Conventional current flow is 
in the direction of the arrow (towards the banded end of the diode).

The back emf produced by the collapsing magnetic field is the opposite 
polarity of the normal operating voltage. A diode placed across the 
opening switch (points) with the arrow pointing towards the positive 
voltage source will not conduct under normal operating conditions. But, 
it will conduct jst fine the opposite polarity voltage (back emf) 
produced when the circuit is shut off.

This diode is standard design for all my DC relay circuits. It keeps the 
back emf from damaging solid state drivers.  It also makes the switch 
(contacts of relays, etc) last longer since there is no arc.

Frank DuVal

EE in one of my lives

On 2/13/2012 8:39 PM, Daniel Monasterio wrote:
> I'm really concerned about this topic. What for is the addition of a diode in this case ?Not sure about my memory but, remember on using a capacitor (or condenser ?) to make the clock points last for a longer time, as it minimizes the arc when the points open.What for is the diode ?
> Daniel Monasterio
>
>
>>   Actually, Matt, putting the diode across the clock terminals won't do anything. The diode needs to dump the back EMF when the points open. So, the diode needs to either be a shunt across the coil or across the points. And the points end of the coil isn't accessible from outside the clock housing.
>>
>>
>>
>> John Roberts
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