Automotive: Points and Ignition Booster/Amplifiers

[crutschow]

If the condenser is the slowest, then wouldn't the points booster speed up the voltage rise time?
Since the condenser is eliminated in that kit.

That's another plus for the booster kit.

The schematic for the kit shows a 0.22μF cap connected across the transistor collector to emitter.
I think that's similar in capacitance to the one used with points so there would be little change in the capacitor rise times.

 

[sign216]

The schematic for the kit shows a 0.22μF cap connected across the transistor collector to emitter.
I think that's similar in capacitance to the one used with points so there would be little change in the capacitor rise times.

Damn, okay.

How about the switching time of the transistor vs the breaker points? Is there a difference? Is the difference enough to matter? Is it even calculable?

 

[moty22]

Certainly the condenser is the slowest since it's purpose is to give the contacts time to open before the voltage builds up and the coil field starts to collapse.

The fastest would be the transistor (if the circuit is properly designed to rapidly turn off the transistor which I'm nor sure the Velleman circuit does).
For example, diode D3 prevents transistor T1 from rapidly pulling excess base charge from T2 to help speed it's turn-off.
A small capacitor across D3 would help
I don't see a good purpose for D3.

Hopefully Curmudgeon Elektroniker isn't going to get crossed with me.
T1 handles heavy current and it is possible that the on voltage is 0.5V or more, D3 makes sure it doesn't turn T2 half on. Your idea of adding a cap across it is good.

 

[KeepItSimpleStupid]

FWIW: This **broken link removed** appears to be the latest generation of the Allison XR-700 ignition system I used back in the 80's/ I would say that I should have had two just because spares would become hard to come by. I had it on two vehicles.

The Ford was the easiest. Just a slotted disc that went under the rotor and the optical pickup mounted where the points went.
The Chevy with the centrifugal weights on the top of the distributer was much more troublesome to install, but they still manged an adapter.

It allowed you to keep the same timing profile. I noticed that 6V operation is possible.

I might even have my original one somewhere.

 

[crutschow]

Hopefully Curmudgeon Elektroniker isn't going to get crossed with me.
T1 handles heavy current and it is possible that the on voltage is 0.5V or more, D3 makes sure it doesn't turn T2 half on. Your idea of adding a cap across it is good.

Nope, not crossed at all.
That's a reasonable point. The saturation voltage of T1 with about a quarter amp of collector current could be sufficient to keep T2 partially on at high under-hood temperatures.

 

[crutschow]

................
How about the switching time of the transistor vs the breaker points? Is there a difference? Is the difference enough to matter? Is it even calculable?

I think that point has been covered.
Other than the points arcing time, the switching time of the points (from closed to open) is basically instantaneous.

I remember that HP once made a pulse generator that used a mercury-wetted reed relay to generate pulses with picosecond rise times. It's speed was faster than any transistor at the time.

 

[sign216]

Hmm, fair enough. So the bottom line is perhaps the kit provides a 10% increase since there's no points-arc loss.

A good result, but not a dramatic ground-breaking break through.

 

[rumpfy]

Sign,
In one of the posts, there is reference to sparking at the points. This sparking will dissipate some of the primary energy and thus give a reduced spark energy. This wont happen to the same extent in the transistor design but still and all, some protection is applied to the transistor to restrict the peak voltage when the points open. This protection will cause some loss of primary energy as well.
The primary energy is stored in the magnetic field, and when the points open, the current is stopped starts to decay. The rate of decay of the magetic field is controlled essentially by the rate at which energy in the magnetic field can be transferred to the capacitances in the primary and secondary circuit. Mostly, the primary energy is transferred to the self capacitance associated with the coil. The rate of transfer is controlled by the resonant frequency of the primary and secondary circuits acting as a coupled pair.
In the old original Kettering coils, the back emf due to the collapsing magnetic field gave a peak voltage at the points of around 250 to 300 volt. This was transformed to say 20,000 volts at the secondary. With the coil in my 1991 Mitsubishi (a Kettering look alike) the back EMF is around 85 volt.
I would say that much of your questioning seems to be about speed of switches etc, but can I repeat that the spark rise time is all about the resonant frequency of the coil itself. The post #36 has lots of stuff in it, but I'm not sure how it is to be interpreted. The systems are hard to relate to the tabulation numbers. In post 28, the points close and there are no oscillations, Then when the points open, there is a peak of voltage. It is at this point that the plug should fire, and for the rest of the cycle, what happens doesnt matter. Any oscillations are just remnant bits of energy floating around between the parasitic capacitances and inductance. When the points close, this energy is dissipated in the switch.
As an aside, in CRT type Television receivers, the line output transformer is used to generate the flyback pulse to return the beam to the LHS side of the screen, and also to generate the 20 kV required to accelerate the beam to the screen. The Line output transformer is specially designed to have a primary resonant frequency of about 45 kHz, and the secondary is designed to be resonantly coupled to the primary with a frequency of about 135 kHz if the transformer is 'Third harmonic tuned' or 225 kHz if it 5th harmonic tuned. These transformers are a special case of a high voltage coil but it shows that a transformer, or spark coil, can be dimensioned to give some specifically well controlled characteristics.
I repeat too, that it is unlikely that an add-on kit which does not consider the coil, is unlikely to give you anything but bragging rights.
Hope this helps.

 

[crutschow]

In one of the posts, there is reference to sparking at the points. This sparking will dissipate some of the primary energy and thus give a reduced spark energy. This wont happen to the same extent in the transistor design but still and all, some protection is applied to the transistor to restrict the peak voltage when the points open. This protection will cause some loss of primary energy as well.
..............................

I think that protection circuit only works when there is an open plug wire which can give a very high secondary voltage (and thus reflected primary voltage).
For normal operation with a plug connected, the plug fires before the primary voltage reaches the zener protection break-down voltage.

 

[sign216]

Rumpfy,
I agree that coil and capacitor characteristics are important to squeezing the best performance, but we dealing with vintage machines here. So it's hard to match coil, capacitor, and other components.

In some vintage motorcycles I deal with, people use condensers from mopeds and VW bugs, because they are still available. Hardly a good fit, electronically, but it's what is available.

I'm open to ideas.

 

[tcmtech]

I agree that coil and capacitor characteristics are important to squeezing the best performance, but we dealing with vintage machines here. So it's hard to match coil, capacitor, and other components.

Actually back in the old days any decent mechanic had a condenser kit to work from that had a good range of different values of condensers to choose from. Granted it was a bit of trial and error matching process but when a person knew what they were looking at and wanting in a ignition spark honing in on the best match of condenser to whatever coil was being used on most anything was not all that difficult.

Believe me I have done some experimenting with different values of condensers on old engines ignition systems and it's surprising what some of those 'weak old coils' being fired from a set of mechanical points can do!

 

[KeepItSimpleStupid]

So get a secondary pickup, learn how to read a scope and start experimenting. Your cap in the Velleman device could be changed ad it doesnt have to be the same form factor because it doesn't have to fit inside the engine.

**broken link removed**

 

[Jnaut]

Can anyone of your experts throw light on problem causing great frustration on 1960's Mercedes. Unable to generate spark at plugs sufficient to even cause engine to fire, never mind run. Changing hardware, plugs, plug leads, coil, condensor, distributor cap & rotor arm makes no noticable difference; but creating an air gap in coil HT lead by removing it and holding it +/- half inch from its contact in the distributor cap produces instant start and smooth running at all speeds? HELP!

 

[Tony Stewart]

After Calibration of points gap, and distributor timing with TDC offset on CYl #1 and then verifying spark to plug by disconnecting has been done, these are the basic steps.
If the any of these steps fail, the engine wont start. Adding a gap to the HT lead suggests it could be some invisible short inside HT cap socket to ground only when the wire is inserted. The gap would not add much delay. To check the function of the distributor, unplugging from any spark plug should produce a spark to chassis ground when engine starter is turned.. Pls verify.

 

[Jnaut]

Thanks for your wisdom Tony, & apologies for delayed response, I wanted to go through your instructions first. With coil lead fitted correctly there is spark on grounding spark plug lead but no where near the intensity of spark on grounding coil lead. With spark lead held at distance from coil tower spark at grounded plug is of similar intensity as coil lead (and hence the car runs)?

 

[alec_t]

Adding a gap to the HT lead suggests it could be some invisible short inside HT cap socket to ground only when the wire is inserted.

I agree. You changed the plug leads, but did you change the coil lead? Does the distributor cap clamp the coil lead in place? Compression of the coil lead end might be creating a partially conductive path to ground (I'm clutching at straws here).

 

[dr pepper]

Creating an air gap will force the coil to produce a higher voltage in order to flash over to ground, this tells me there is a leak somewhere to ground.
Check the dizzy cap for cracks, a powerfull led torch is good for this, sound slike this is affecting all cylinders so it doesn tsound like a individual plug has grounded, maybe the coil itself is at fault, hav eyou tried connecting 1 plug direct to the coil?, do you get a good spark, if so summats up in the dizzy, or leads, if not possibly the coil.
You can tell a fair bit by looking at the current through the coil on a 'scope, if you have a scope you can rig up a simple current transformer using a junkbox torroid choke and resistor, with a little experience you can tell whats up just by the trace, from a short to ground on one plug to low compression on one pot.

 

[Tony Stewart]

I wonder if you changed to non-resistance wiring. Resistance wire is normally to reduce EMI with 10K+ ohm carbon wiring, but wondering if there is something else going on?

 

[Jnaut]

Thanks for all contributions guys - all combinations of new spark plug/coil leads, carbon & copper have been tried.
The strong coil lead spark and weak plug lead spark pointed to energy being somehow 'lost' in the distributor; but would not the more enhanced spark created by having an air gap be even more likely to exploit any weakness at the distributor rather than enhance the plug spark?
To provide for starting and moving the vehicle around, the rubber boot on the coil lead is currently securing the lead to the coil, but instead of the lead being pushed into the receptor in the coil tower, it is withdrawn within the rubber boot to provide the air gap within the boot. Unfortunately no scope available.

 

[dr pepper]

Not necessarily an air gap forces the coil to make more voltage, and more apt a higher rise in volts over time, this seems to combat leakage and makes a good spark on a manky system, when was the last time you had to use wd40 to get a newer car going.
Are you using the right coil?, coils for programmed ignition systems on later vehicles are usualy constant energy (or more appropriately constant dwell time, consistant spark over the rev range instead of a constantly weakening spark with increasing revs), this would certainly cause issues with a points system, you can usually tell by the dc resistance of the primary, 3 to 6 ohms for an unballasted points type coil, and subtantially less than an ohm for a constant energy one, using the latter on a points system can end up with a melted points or coil.