Diode across coil?

MrTube

MrTube

Well-known member
Hey,

Sorry, long time no post!

Is there any reason we can't or shouldn't use a large diode across the coil instead of a capacitor across the points?


If the capacitor is there merely to keep the points from burning I would think a diode could do a far better job and take up less room and last a whole lot longer as well as being easier to hide.

Sourcing quality parts would also not be a problem as it is with the capacitors.
 
MrTube said:
Hey,

Sorry, long time no post!

Is there any reason we can't or shouldn't use a large diode across the coil instead of a capacitor across the points?


If the capacitor is there merely to keep the points from burning I would think a diode could do a far better job and take up less room and last a whole lot longer as well as being easier to hide.

Sourcing quality parts would also not be a problem as it is with the capacitors.
Click to expand...

There is the problem, the cap also discharges into the coil adding to the efficiency of the coil.
 
Mike V. Florida said:
There is the problem, the cap also discharges into the coil adding to the efficiency of the coil.
Click to expand...

How does it do that?
When the points are closed the capacitor has both ends connected to ground. When the points are open the coil discharges and while the cap charges at this point it gets discharged as soon as the points close?

I cannot see the capacitor effecting the output of the coil at all but I may be missing something. :(
 
A helpful description from Les Andrews, Model A Ford Troubleshooting & Diagnostics (blue book), page 4-3

Points and condenser: The distributor points are made of tungsten to reduce the transfer of metal from one contact to the other caused by arcing. When the points open, the ignition primary circuit is interrupted. But the flow of current cannot be stopped instantaneously. The current continues to flow and arc across the points. To stop this flow, a condenser is connected across the points so that when the points start to open, the current surges into the condenser. By the time the condenser is fully charged, the points are open wide enough to prevent an arc and the current flow stops and the magnetic field in the coil collapses quickly. The speed of collapse is aided by the discharge of the condenser back through the coil primary windings in reverse direction.Therefore, the purpose of the condenser is to reduce arching at the points, thereby increasing point life, and to speed up the collapse of the magnetic field of the coil.
Click to expand...
 
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MrTube said:
Hey,

Sorry, long time no post!

Is there any reason we can't or shouldn't use a large diode across the coil instead of a capacitor across the points?


If the capacitor is there merely to keep the points from burning I would think a diode could do a far better job and take up less room and last a whole lot longer as well as being easier to hide.

Sourcing quality parts would also not be a problem as it is with the capacitors.[/QUOTE

You may be on to something good !!! I've got an open mind. I admit that I know next to nothing about diodes. My only experience with diodes is installing the ones that Brattons and Snyders sell to be installed in old cutout shells. My experience with diods in old cutouts have been excellent and never a problem. Still some say that they can't work because of heat, this hasn't been a problem for me. You seem to be pretty sharp on electrical stuff. I have no idea of how to try a diode in place of the condenser. You should try it and tell us how it works and how to do it . I have people telling me that hooking the condenser to the coil won't work, even after I have done it for over eight years and have shown pictures of the set up.
Click to expand...
 
Purdy Swoft said:
MrTube said:
Hey,

Sorry, long time no post!

Is there any reason we can't or shouldn't use a large diode across the coil instead of a capacitor across the points?


If the capacitor is there merely to keep the points from burning I would think a diode could do a far better job and take up less room and last a whole lot longer as well as being easier to hide.

Sourcing quality parts would also not be a problem as it is with the capacitors.[/QUOTE

You may be on to something good !!! I've got an open mind. I admit that I know next to nothing about diodes. My only experience with diodes is installing the ones that Brattons and Snyders sell to be installed in old cutout shells. My experience with diods in old cutouts have been excellent and never a problem. Still some say that they can't work because of heat, this hasn't been a problem for me. You seem to be pretty sharp on electrical stuff. I have no idea of how to try a diode in place of the condenser. You should try it and tell us how it works and how to do it . I have people telling me that hooking the condenser to the coil won't work, even after I have done it for over eight years and have shown pictures of the set up.
Click to expand...


This is my thought.
The diode would not conduct while the points are closed, but in theory would conduct when the points open to protect the points.

It seems, according to others this is not the only purpose of the capacitor though. :(
Click to expand...
 

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MrTube said:
The diode would not conduct while the points are closed, but in theory would conduct when the points open to protect the points.
Click to expand...
My guess... as soon as the points open, the coil's primary winding is shunted by the diode. The primary field never collapses. No voltage is developed in the secondary.
 
If you use theA&L condenser you won't have any condenser problems, I have cooked mine hot enough to melt solder---- and it is still good , so far 25 years of abuse
 
newshirt said:
My guess... as soon as the points open, the coil's primary winding is shunted by the diode. The primary field never collapses. No voltage is developed in the secondary.
Click to expand...


I'm curious how modern ignition systems work? But I can't seem to find a schematic anywhere.

It's very possible you are 100% right.
 
It won't work because about 5 years ago I tried it just to demonstrate that the condenser has a dual purpose. I also tried a light bulb in place of the condenser. Both the light bulb and diode kept the points from arcing, but they also kept the secondary windings from developing the high voltage needed to fire the plug.
 
Modern ignition circuits likely use some low resistance mosfets to turn on the coils.

The reason why a condensor at the coil is bad is because there is resistance in the wires and it will cause some arcing at the points. Is you have any marginal connection you increase the chance of the engine not quite working right or damage to the points. Keep in mind you may not see any problems, but that does not mean they are not there. Since you do not drive a lot then you may not see a real problem at the points.

The fact is you are best to use the A&L condensor in the dist as they use a very high quality capacitor that can handle the heat and last for decades.

Quite frankly. Given the wide body of positve results I think one would unwise to not use the A&L units since they are so stinking cheap and they just work.
 
Tom Wesenberg said:
It won't work because about 5 years ago I tried it just to demonstrate that the condenser has a dual purpose. I also tried a light bulb in place of the condenser. Both the light bulb and diode kept the points from arcing, but they also kept the secondary windings from developing the high voltage needed to fire the plug.
Click to expand...


Interesting!

Kevin in NJ said:
Modern ignition circuits likely use some low resistance mosfets to turn on the coils.
Click to expand...


Wouldn't you still need the capacitor if it does indeed have a dual purpose? I would also expect the coil to fry any mosfets without some sort of protection same as the points no?
 
U gotta have a capacitator. Just ask Charlie Kettering who invented the L-C circuit we know as the ignition circuit. That it keeps the points from burning is true but irrelevant and overrated. A "myth" to hang onto since none of us other than Charlie understands what the hell an L-C circuit does

Once Charlie did that he put the magneto boys out of business.

U don't have to mount the capacitator inside the dizzy if U don't want to. It can be anywhere. I have slapped a spare one on the firewall when I was caught short and in a hurry. Just as long as it is on the points side of the coil it will work fine, and could be in Arkansas and would still work

I know ppl who have put it on the firewall deliberately to keep heat away from it
 
That extra couple feet of wire means nothing to a condenser/capacitor installed on the firewall. Electrons are pretty quick little critters. They get where want to go in less than a nano-second.
All I can say is, if you think a diode will work in place of the condenser then try it and post the results. I'd recommend having an extra condenser and set of points on hand for when you decide to drive the car somewhere though.
Plus I think about the only folks that can understand this are Charles Kettering or Nickola Tesla.
 
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1930 coupe said:
NO IT WILL NOT WORK!!
A diode and a capacitor are completely different animals and not interchangeable.
The capacitor does more than prevent arcing at the points, it also forms a L-C resonant circuit to boost the voltage. If you remove the capacitor your engine will die or run very bad.
The L-C osculations also make the spark last longer.

http://en.wikipedia.org/wiki/LC_circuit
Click to expand...

This is correct, if you do not understand electronic theory, trust us that have had to learn it in depth and can do this type of circuit design in our sleep, the capacator is needed for the LC circuit, that is its primary purpose, the fact that it helps reduce point arc is a secondary benefit. A diode in this particular LC circuit will not work. There are LC circuits that use for example, a PIN Diode, but those are in high frequency RF systems, not automotive ignition systems.
 
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As others have noted, the cap is used to both reduce the arc and to boost the coil voltage. It's a L-C circuit and it's invention was a great step forward for good ignition systems.

In order to understand the operation, you need to study L-C circuits.

Diodes are used often as "snubbers" with relays in order to stop the sparking, but it's not the same as an ignition coil.

Marc
 
"A "myth" to hang onto since none of us other than Charlie understands what the hell an L-C circuit does."

Speak for yourself.

"Once Charlie did that he put the magneto boys out of business."


A magneto needs a capacitor just like a battery ignition.
 
1930 coupe said:
NO IT WILL NOT WORK!!
A diode and a capacitor are completely different animals and not interchangeable.
The capacitor does more than prevent arcing at the points, it also forms a L-C resonant circuit to boost the voltage. If you remove the capacitor your engine will die or run very bad.
The L-C osculations also make the spark last longer.

http://en.wikipedia.org/wiki/LC_circuit
Click to expand...

Interestingly enough, there is an animal called a quench arc, that is a combination of a diode and a capacitor. Not for cars tho, but to stop a collapsing field from tripping a gfi.
 
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