I see people using coils, spark plugs and condensers to ignite their flame throwers, but why would they need a condenser when there are no points?
Just thinking, having never seen one at work, but is the switch toggled and acting like a set of points?
You'd have to be more specific, because you are correct, there are no ignition condensers required for electronic ignition. Sometimes people use what look a lot like ignition condensers as RFI filters.
A coil can't make sparks on its own, it needs either electronic control that can interrupt the current to it suddenly, or some sort of mechanical switch (such as points) together with a condenser to delay the coils primary winding voltage spike so it can't create a spark in the switch during the initial opening when the gap is so small the voltage required to make a spark is very low.
Why is it ignition condensers are not necessary for an electronic switch? Generally speaking solid state is a lot less forgiving than good ole' stone knives and bearskin tech.
It's a matter of choosing the right electronic components, that can switch between on and off fast enough, and can handle the voltage spike that the coil produces when the current is cut off suddenly. If you get a 500V spike and have used a transistor that can handle 1000V you are fine. (Then again, pull the wire of a spark plug and the voltage may be pushed up when the spark "can't go anywhere" and even the 1000V transistor may be overloaded, you never ever want to do that.) On the other hand, if you use some other kind of electronics where spikes weren't expected, say some modern LED light controller just to take a possible example, there may be transistors that just can handle 50 or 100V for example. Fine for the job they are meant to do, but hook up a regular relay to the output to help control some other lights from the same output and the spike the relay coil creates when turned off is likely to kill those transistors and/or other connected LEDs. Solid state electronics are indeed far less forgiving than old electromechanical devices in general, when you go outside the specs they are designed to handle - voltage, current or temperature usually. On the other hand, make sure the working conditions are right and the electronics can be extremely reliable. (Can be, unless for example someone cut corners to save money in manufacturing, or didn't correctly account for what reality had in store for the future such as dirt reducing the cooling capacity of a cooling fin.)
OK, but in a normal ignition system, the cam lobe is opening and closing the points which is the switch. If you eliminate the distributor and use a momentary switch.....?
It's still a mechanical switch, even if a switch may open faster than ignition points usually do it's still painfully slow when you compare to the speed the electricity react at. Here's the spike from the coil in a ordinary 12V relay. It dissipates in about 2 milliseconds total (one full division is 2ms), the rise time from zero to the -200V peak (well, the bottom as it's a negative voltage, but you get the idea) is a fraction of that time, and that's the voltage rise the switch movement would have to keep up with. They just can't move fast enough, as they start with zero opening the voltage needed to start a spark there is also very low, you can't outrun the train if it already ran you over before you started moving. The hump you can see halfway through the spike is avtually caused by the mechanical movement inside the relay, as the coil has released the contacts. Says a little about how slow the contacts in a relay moves, at least slow as electricity sees it.
Without a photo, I'm assuming the condenser is external. It could be that it's non-functional; it's purpose is to maintain the look of an original, unaltered ignition system. People have done the same thing with voltage regulators when replacing generators with alternators that look like generators.
Good discussion, but I have seen explanations of how a points style ignition system works that claim the condenser is doing more than just absorbing a voltage spike to prevent arcing at the points, the claim is that the condenser results in a high flyback voltage that is what results in the high voltage in the secondary side of the ignition coil (some have even called this an AC system due to this flyback voltage); that if it were not for this very high flyback voltage, the high voltage out of the coil would be too low to jump the sparkplug gap. Care to comment on that?
Well, the flyback voltage is the voltage spike we want and need to make an ignition coil work. We just want to have it under control so the energy stored in the coil goes into giving a high voltage, instead of keeping current flowing past the slightly open points (spark at the points). The condenser isn't so much absorbing the (flyback) voltage spike from the coil, as it is slowing down the voltage rise. It takes a bit of time to charge the condenser, in that time the points have opened enough to prevent an arc to form there, and the result is that the voltage pulse in the primary winding can keep rising to a good, high peak, inducing a proportional, but much higher voltage in the secondary winding. The condenser isn't helping the voltage rise in itself, it just prevents a too high voltage at the points at a time when the point's can't handle a high voltage. But there's a lot going on in an ignition system, most of it we never need to have any idea about to get a working system. Here's the oscilloscope result from some playing around with an improvised points ignition (an ignition coil, condenser and regular switch hooked to a 12V power source on a work bench). The red trace is primary voltage, blue is secondary. The strong oscillations lasting a couple of milliseconds from the start of the spark is a resonance between the condensers capacitiance, and the coils inductance, the frequency is a result of their size in relation to each other. In my electronic ignition also used for bench tests doesn't get those oscillations from the start, as there's basically no capacitance in it. The weaker, lower frequency after the spark has died it more or less the same in both systems, that's the last remaining energy in the coil that wasn't enough to keep the spark alive. Just to mention something that goes on in there, that we rarely need to know anything about.
You're right, we don't need to know some of this, but some of us appreciate a deeper understanding of what's going on. So, this this graph, at what point is the opening of the contacts?
The switch open where the red trace starts going steep down. Looks like the switch closed just ~1.2 ms before that, not much time to charge the coil that test, but that's the joy of improvised test setups on the bench and a storage oscilloscope hooked to them, you get proof of what you messed up, easy to view and save so everyone can se what you did wrong.
That is the best description of how a condenser works with an ignition system that I have seen text books included. Thanks G-son, I'd expect that this thread will get referred to in the future a number of times when someone asks about condensers in ignition systems.
Well, I was thinking that was the point when the switch closed, but that should go up then... Been a while since I did this test so i don't remember the details. Now I'm thinking the switch I used for the test didn't break the current abruptly quite as it should, the voltage drop could be from a change in resistance through the switch before the final break perhaps. There were other times I did not get that voltage drop there, like this.
Back in the 50's I built flame throwers using a doorbell buzzer with a condenser across the buzzer points, then connected this to a common ignition coil. If you can find a new buzzer to buy you will find the points are so shoddy that this method will not work ( Oh, for the good old days) Greg
I think you could get continuous sparks using a 2-pole switching relay. Connect the coil to the breaking side on one pole, so it automatically turns the coil off when the relay has activated. The other pole feeds the ignition coil, with a condenser ofcourse.
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