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Spark plugs

A place to ask general spud cannon related questions.
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Unread postAuthor: kjjohn » Thu Oct 29, 2009 7:52 pm

So if I could wire the spark plugs in series it would work? Wiring it in series seems like it would have the same effect to me, but I'm not as experienced as you are either.

Would it work to wire the ignition coil up to a sparkstrip (in series, of course)? I am kind of avoiding stun guns because I want to put my large collection of electronic components to use. Of course, I won't be making the coil myself, but I will be making the 555 timer circuit which pulses it on and off.
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Unread postAuthor: Technician1002 » Thu Oct 29, 2009 11:37 pm

kjjohn wrote:So if I could wire the spark plugs in series it would work? Wiring it in series seems like it would have the same effect to me, but I'm not as experienced as you are either.

Would it work to wire the ignition coil up to a sparkstrip (in series, of course)? I am kind of avoiding stun guns because I want to put my large collection of electronic components to use. Of course, I won't be making the coil myself, but I will be making the 555 timer circuit which pulses it on and off.


In series the current in all gaps is the same. The breakdown voltage of each gap adds in series, so if one plug strikes at 6KV, a string of 5 will require 30KV to strike an arc. This is the disadvantage of a series string. Sometimes to get a series string to fire, a resistor is sometimes placed across several plugs so the voltage cross them remains low so one plug gets the full voltage, when it strikes, it is essentially a low voltage gap, so the high voltage is then developed across the other resistors and gaps so they then strike. This does require high voltage high resistance resistors. These parts are not something you will find at radio shack.

Please note the 555 won't take the inductive kick of the collapsing magnetic field in the coil or drive the current required. If you are using a circuit from online, follow it exactly. Don't substitute drive transistors. The primary voltage on the coil even when driven with 12 volts often peak at over 300 volts when the transistor switches off. Using the proper transistor is the only way to prevent instant transistor destruction.
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Unread postAuthor: D_Hall » Fri Oct 30, 2009 12:00 am

Or you can optically isoloate your 555s.
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Unread postAuthor: jimmy101 » Fri Oct 30, 2009 12:57 pm

D_Hall wrote:Or you can optically isolate your 555s.

That just means you fry the optoisolators or the power transistors. Many solid sate devices have reverse breakdown voltages of 60V or so. A large inductive load can easily generate that voltage, especially when it is being switched by a (nearly) square wave and is meant to generate several thousand volts.

You have to properly deal with the inductive kickback from the coil or the circuit is going to be "single use".

A snubber diode, and/or a largish cap, is what is usually used.
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Unread postAuthor: Technician1002 » Fri Oct 30, 2009 2:19 pm

jimmy101 wrote:
D_Hall wrote:Or you can optically isolate your 555s.

That just means you fry the optoisolators or the power transistors. Many solid sate devices have reverse breakdown voltages of 60V or so. A large inductive load can easily generate that voltage, especially when it is being switched by a (nearly) square wave and is meant to generate several thousand volts.

You have to properly deal with the inductive kickback from the coil or the circuit is going to be "single use".

A snubber diode, and/or a largish cap, is what is usually used.


FYI, normal voltage peaks in traditional kettering ignition is about 300 V on the primary. More info for the electronics types is here;
http://www.picoauto.com/automotivetopics/primary.html
This is why CDI ignition most often runs about 300 volts as a pulse into an ignition coil. This is also why a flash from a disposable camera works well with an ignition coil.

A typical voltage waveform looks like this;
Image
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Unread postAuthor: jimmy101 » Sat Oct 31, 2009 1:47 pm

Technician1002 wrote:FYI, normal voltage peaks in traditional kettering ignition is about 300 V on the primary. More info for the electronics types is here;
http://www.picoauto.com/automotivetopics/primary.html
This is why CDI ignition most often runs about 300 volts as a pulse into an ignition coil. This is also why a flash from a disposable camera works well with an ignition coil.

A typical voltage waveform looks like this;
Image


Last time I looked 300 > 60

and, the capacitance of an LED (or any other simple solid state device without any caps) is extremely small. Much smaller than the coupling cap in the AC input of an O-scope.

Just because an o-scope with a perhaps 0.01uF coupling cap doesn't show a large kick back voltage spike doesn't mean that the spike didn't occur. It just means the power in the spike was insufficient to fully charge the blocking cap.

More probably though, the cap in the ignition circuit (10 MFD?) was enough to absorb that spike. An automotive ignition circuit is a fully snubbed circuit.

Take that O-scope, switch to a DC input, remove the cap from the ignition circuit and try it. (if you don't care about cooking something in the o-scope)
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Unread postAuthor: Technician1002 » Sat Oct 31, 2009 5:20 pm

jimmy101 wrote:Last time I looked 300 > 60

and, the capacitance of an LED (or any other simple solid state device without any caps) is extremely small. Much smaller than the coupling cap in the AC input of an O-scope.

Just because an o-scope with a perhaps 0.01uF coupling cap doesn't show a large kick back voltage spike doesn't mean that the spike didn't occur. It just means the power in the spike was insufficient to fully charge the blocking cap.

More probably though, the cap in the ignition circuit (10 MFD?) was enough to absorb that spike. An automotive ignition circuit is a fully snubbed circuit.

Take that O-scope, switch to a DC input, remove the cap from the ignition circuit and try it. (if you don't care about cooking something in the o-scope)


The voltage rise is directly related to the rate of flux change in the magnetic core. It's much like landing a spud on plywood has lower peak pressure on impact than a steel ball bearing impacting an anvil. Snubbers are an energy absorbing device. A cap alone only stores some of the energy from the collapsing magnetic field and then returns it once the arc is established. Because current continues to flow with a cap when the circuit is interrupted, the peak voltage is reduced. In a mechanical switch, this allows an arc on the switch to extinguish. Without a cap, the points burn and spark energy is lost to an arc on the points instead of the plug. When the decay is complete the inductor and capacitor ring in a traditional resonant circuit.

With the cap missing the peak amplitude is much higher, but the duration is much shorter. This provides a weak thready spark instead of a nice fat spark.

On the misinformation on the scope input capacitance.. LOL.. 0.01 uF. Mine is 15 pF, several orders of magnitude less. Have you used a scope lately? They are designed to meter a circuit, not add a heavy load. Many LED's with a large die size have more capacitance than my scope probe.
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Unread postAuthor: jimmy101 » Sun Nov 01, 2009 7:32 pm

Technician1002 wrote:The voltage rise is directly related to the rate of flux change in the magnetic core. It's much like landing a spud on plywood has lower peak pressure on impact than a steel ball bearing impacting an anvil. Snubbers are an energy absorbing device. A cap alone only stores some of the energy from the collapsing magnetic field and then returns it once the arc is established.

That is what the fairly large cap in an auto ignition does. It stores energy on the "up" side but also reduces point sparking on the "down" side. That cap is there to do two things. For other circuits the cap is not an energy storage device for the "up" side. They are dampers on the "down side". The peak voltage depends on how fast the energy is released. Not only in how fast the field changes but also in how well something can absorb the voltage.

For most solid state devices over voltage will fry'm pretty much instantaneously. Hence all you need to protect them is a big enough cap to hold the peak voltage down. Doesn't matter that the energy is eventually still transfered through the circuit, it just matters that is does it slowly to keep the voltage down.

With the cap missing the peak amplitude is much higher, but the duration is much shorter. This provides a weak thready spark instead of a nice fat spark.

which is what I said. Most solid state devices will be instantly cooked by 300V, let alone a couple KV. Voltage in a solid state device doesn't behave the same way current and power dissipation does. With current and power a short pulse way over spec can be tolerated. For voltage it cooks the device very quickly.

On the misinformation on the scope input capacitance.. LOL.. 0.01 uF. Mine is 15 pF, several orders of magnitude less. Have you used a scope lately? They are designed to meter a circuit, not add a heavy load. Many LED's with a large die size have more capacitance than my scope probe.

Even 15pF is of course insignificant to a automotive ignition circuit but it isn't to say a piezo sparker. (667x is "several orders of magnitude"? I would call it a "couple of orders".)

Besides, don't most scopes have a fairly large resistor on the input line, if not in the probes themselves, as well as the cap?

Go ahead and bypass the input cap and resistor on your scope, remove the ignition cap from the car and try to get a recording of the car ignition's V vs T.

The scope isn't going to be happy.
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Unread postAuthor: Technician1002 » Sun Nov 01, 2009 10:26 pm

Even 15pF is of course insignificant to a automotive ignition circuit but it isn't to say a piezo sparker. (667x is "several orders of magnitude"? I would call it a "couple of orders".)


100 is 2 orders. 1,000 is 3 orders. With normal rounding, you validated my point.

That cap is there to do two things. For other circuits the cap is not an energy storage device for the "up" side. They are dampers on the "down side".


We appear to be using two differing dictionaries. A snubber and damper in mine are energy absorbing devices designed to dissipate energy. A cap is an energy storage device. It is often used in resonant circuits. I will admit that they store energy on the up side and return it on the down side. This is why when a cap is the wrong value the points get metal transfer. Too large a value transfers more on close and too small and the metal transfer is larger on the opening. A proper size cap will extend the point life.

Using the proper attenuator on measurement devices is standard safe industry practice. Why would you recommend removing the attenuating probes from my scope? Over voltage does damage semiconductors. There are protection measures besides simply large capacatance. Other measures include spark gaps, discharge tubes, zener diodes, metal oxide varistors, steering diodes, series inductance, and others.

Many MOSFET transistors are protected by an internal steering diode as well as a zener diode. This was part of my reason for mentioning it is important to not substitute parts in making a solid state ignition driver.
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Unread postAuthor: jimmy101 » Mon Nov 02, 2009 5:59 pm

Technician1002 wrote:Using the proper attenuator on measurement devices is standard safe industry practice. Why would you recommend removing the attenuating probes from my scope? Over voltage does damage semiconductors. There are protection measures besides simply large capacatance. Other measures include spark gaps, discharge tubes, zener diodes, metal oxide varistors, steering diodes, series inductance, and others.

Because my reading of your earlier post and traces implied that a coil only circuit did not generating a HV kickback pulse. Only a fairly small kickback is shown in the traces.

My points is that the circuit being measured and the circuit doing the measurement are both damping the kick back pulse. The scope trace is not representative of a home brew HV coil setup.

Without suitable damping (snubbing, whatever you want to call it) a typical HV coil will cook solid state devices, like an optocoupler, hooked up to it.
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