Technical Points ignition/condenser question

What determines the ‘rating’ (ohms?) of the condenser in a points ignition system? The only variable that I can think is the amount of dwell.

I am electrically challenged.

@squirrel

 

Wonder why we call them condensers instead of capacitors?

would the ohm rating depend on 6 or 12 volt applications?

 

Condensers (capacitor is another name for the same thing) are rated in Farads, most for automotive ignition use are rated 0.22 Micro Farad (abbreviated uF). They also have a voltage rating, 600v is typical for these.

The time constant of the ignition coil and condenser LC circuit usually determines the needed capacitance. I expect you won't understand that last sentence. that's ok.

 

This is why I buy them by part number.
I’ll have my electrician friend translate your comment for me.

 

" I expect you won't understand that last sentence. that's ok." I understood it.

 

I think he means that one of the factors the determine the desired capacitance is engine speed; the faster you run the engine, the more capacitance is appropriate; this is why the old Mallory "trash cans" and the currently available Vertex magneto condensers are rated at .28 to .36 micro-farads. The inductance of the coil comes into play as well.

This is true, but in the real world, it doesn't make a lot of difference on how an engine runs. When I was developing my "trash can" replacements. we initially used anything we had handy; the engines ran fine on anything for .047 microfarads up to .5 microfarads. The only difference is the effect it has on point wear, and it takes a long time to figure out what that is. As to voltage, condensers don't care; they are however, sensitive to voltage spikes and heat. A generator can raise hell with a condenser that doesn't have a high enough voltage rating.

 

Thanks…..I think!!!

Actually makes sense.

 

When the current through the coil primary is interrupted (by points opening, for example) the collapsing magnetic field in the coil will attempt to keep the current flowing. If current can't flow, the voltage instead goes up - ignition coils can easily reach several hundred volts on the primary side.

As the points opened a fraction of a second ago the opening is very small, and it only takes a relatively low voltage to jump a small gap. So, a spark jumps the points opening, some current flows through the spark, and the energy in the coil bleeds of pushing current through the spark on the points. We don't want that, as we won't get a good high voltage spark from the secondary winding.

If we add a small capacitor parallell to the points the situation changes. The points open, and for a fraction of a second a small current from the coil charges the capacitor. This delays the voltage rise, not much but just enough to let the points open further, so when the voltage has risen enough to make a spark at the points the points have opened too far - the voltage isn't high enough now. This means we don't waste the energy in the coil pushing current past the slightly open points, all the energy goes into creating a higher voltage pulse in the primary winding, which in turn makes a nice high voltage pulse in the secondary winding.

The size of the capacitor does matter, if it's too small the voltage may rise too fast, if it's too large energy will be wasted charging the capacitor instead of producing sparks where we want them, basically the size needs to be tuned in combination with the coils properties. And that's where I stop trying to explain how it works in a way anyone can understand, because I don't have a good grip on that kind of black magic.

I'll throw in an oscilloscope waveform of the voltage pulse when the current to the coil in a simple 12V relay was turned off. It peaks at around -270V, and that's probably limited by sparks in the switch used to switch it off - a small capacitor could probably have bumped that up even higher. Anything with a coil can be quite mean to whatever interrupts the current through it...

 

I think the required capacitance has more to do with damping the LC oscillations in a way that the arc at the points is minimized...it doesn't have much to do with the RPM you are running. Keep in mind that coils have both resistance and inductance, and the inductance is what applies to AC situations like this. Inductance is measured in Henrys...not Ohms....

 

Would it not be possible to measure current past the points with a storage oscilloscope, to determine what size capacitor gives the cleanest interruption, i.e. the least amount of spark on the points? I'm thinking that would get you pretty far along the way of getting the least points wear.

 

Agreed. The capacitor is there to "manage" what happens every time the points open, and that's basically the same every time, no matter if it's at idle or howling at 7000rpm. Well, the coil may not have had enough time to reach saturation current at high rpm so there's a little less current and energy to deal with, but the capacitor/coil has the same resonance frequency no matter the current or voltage.

 

been a while since I was playing with this....

 

Sounds like a great idea if you have the skill, the proper equipment, and a lot of time on your hands.

When all is said and done, you might get 500 miles more use out of a set of $15 points (I'm just guessing at this, because it's just not important to chase down for the minimal return you might get.)

 

With the scarcity of quality points these days, it might even be worth the effort, who knows.

I've been running a 0.22 at 1000v capacitor in the little Chevy for several years, I had to clean up the points not too long ago. But I'm happy with point life. This is with a stockish coil.

 

In the final analysis, trying to find the optimum capacitance for the condenser in an automotive ignition system is like trying to nail Jell-O to a wall.

 

Not quite that difficult, because car makers answered the question decades ago. Start with what they used.

 

Ah, that's an excellent point. Also, capacitors can vary a bit in capacitance with temperature (and I think they're some of the worst components when it comes to acceptable manufacturer tolerances, 20% off can be perfectly normal), so the same capacitor size may not be ideal in the summer and in the winter, and the new .22 microfarad capacitor may be quite a bit different from the "identical" old .22 cap you replace.

Apparantly the ignition works well as long as the capacitor size isn't completely crazy. Good enough probably IS good enough.

 

What’s old is new again.
The last couple update training classes I was sent, I sit in with the mechanic guys (I’m a collision guy) oscilloscopes were being discussed. New high end scanners are using this info to diagnose ignition performance.
Me and our automotive instructor rescued a couple oscilloscopes from the dumpster at school.
I’ll think I’ll learn how to use the dwell meter I bought at a yard sale first.

 

About condensers and rpm....Bikes with points that turn 10,000 rpm generally have .25 mf condensers..

 

sqμirrel>>>0.22 Micro Farad (abbreviated uF)>>>

I doμbt it. It's a μ not a u.

Trμckedμp>>>Bikes with points that tμrn 10,000 rpm generally have .25 mf condensers >>>

I doμbt it. It's a μ not a m.

 

I'm glad someone can figure out greek letters on this forum.

 

Back more years than I want to count I had an instructor explain that you could tell if the rating of a condenser was high or low by which side of he points had the deposits and or burned area on it.
I don't know if it was one of my high school or trade school instructors or the Echlin factory tune up expert that the regional rep brought around to the school shop when I was teaching. My local Napa dealer who was also on my advisory committee would arrange for the factory training dudes to spend some time with my classes when they were in town, The Echlin rep and the Champion spark plug rep were there quite often.

Still G Son gave the best description of what a condenser actually does that I have seen in the past 30+ years.
You are not supposed to get any spark across the points when they open and that is why the condenser is there. One instructor described it as a shock absorber for the points in one class I had.

 

The shock absorber analogy is interesting, but not quite accurate. When you analyze AC circuits, you use pretty much the same equations as when you analyze mechanical vibrations. The coil is the mass, the capacitor is the compliance, and the resistance is the damper (shock absorber). So he had sort of the right idea, but not quite.

Anyways, a quote from my oldest Motors manual, 1935-53:

"If the mound is on the positive point, install a condenser of greater capacity; if the mound is on the negative point, install a condenser of lesser capacity"

 

One of the very first things my uncle Norm taught me!

 

We use oscilloscopes all the time on modern stuff.

 

This has been posted before but I guess another time won't hurt.

The PRIMARY purpose of the capacitor in a coil /points/capacitor circuit is to tune the circuit to RESONANCE. When this is achieved, the impedance of the circuit is at maximum and current flow is minimum. Current flow being minimum is where the points will have the longest life.
Resonance peak is also dependent on frequency. RPM in this case.

The efficiency or “useful quality” of resonant circuits is measured in “Q’s”. An iron core coil has a very low Q or broad resonance peak, therefore can tolerate a fairly wide variation in capacitor value before circuit efficiency falls off enough to shorten points life so you can notice it. Since the resonance peak will vary with RPM, the capacitor is usually chosen to tune the circuit in the middle of the operating range.

 

What it all boils down to, is that in the end, it is a compromise. Sixty years ago, Echlin made a condenser for a 283 Chevrolet that would work well for grandpa, who putted around in his Bel Aire at 30 MPH all day and his grandson, who ran it every weekend at the drags. It just turns out that the value is somewhere in the mid-20's micro-farads for most everything out there.

 

The problem today is the contact point ignition system is obsolete, the demand isn't there anymore. Consequently nobody can manufacture them at a price point where they meet the minimum electrical specifications. Ignition service is rough duty.

Most people don't want to get off into the weeds on this stuff. They just want to drive their hot rod. They go to the auto parts store and see brand new parts, it physically fits inside the distributor, and think they can depend on them to operate correctly and give good service. Years ago that might have been a safe bet, though not anymore.

 

Electricity is not real! You electrical engineers believe in the square root of negative 1, i, an imaginary number. Without it you guys can't make your equations work...

Not like us me's. We don't use imaginary numbers...

 

Ah, but we have the Turboencabulator to verify it.