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Old 04-10-2012, 07:49 PM   #1
cap01
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Default Key Ignition vs. Toggle Switches

I agree with dorsal , the addition of left and right mag toggle switches in parallel with the key switch is well worth the time .
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Old 04-10-2012, 08:19 PM   #2
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Default Re: Show us your panel

The guys from both LEAF and Lockwood say repeatedly that the ACS ignition switch (Off-Left-Right-Both-Start) should not be used with the 912 engines. These switches are not designed to handle the ac voltage that the ignition system sends through them, they are designed to ground magnetos.

According to LEAF these switches are responsible for more ignition box failures than anything.

(I'm still running one, too, so this should be warning to me as well... )
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Old 04-11-2012, 03:50 AM   #3
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Default Re: Show us your panel

Quote:
Originally Posted by cap01 View Post
I agree with dorsal , the addition of left and right mag toggle switches in parallel with the key switch is well worth the time .
Actually I only added a start button, still use the key for L-R-Both, not sure if that helps with the issues identified by LEAF and Lockwood.
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Old 04-11-2012, 08:16 AM   #4
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Default Re: Show us your panel

Re: Magneto switch and Rotax

Larry,

I wouldn't worry about your mag switch.

There are some sites that say a “standard” mag switch cannot handle AC current from the ignition shorting wire on the Rotax 912. This is probably not correct.


In fact, any switch designed for DC can handle a much higher voltage and current when used in AC. The limiting factor on switches is arcing of the contacts. When a DC switch opens the current continues to flow across the gap until the air resistance is high enough to break the connection. It is the continued arc that causes switch contacts to burn or pit.



In AC circuits the current is constantly reversing. Each time the current reverses the voltage drops to zero. When the voltage drops the arc is temporarily stopped. Restarting the arc takes a much higher voltage for a given air gap, thus the arc is broken sooner than it would be with a DC circuit. Less burning and less pitting with AC.


A switch can often handle 10 times the voltage and 50 percent higher amperage on AC that it could on DC. Many switches have voltage rating printed on them. A typical rating might be AC 250 volts at 6 amps and DC 28 volts at 4 amps.


The Rotax “kill” wire shorts the low voltage input side of the ignition module, not the high voltage output. That voltage is about 40 volts at maximum which is comparable to a magneto P lead voltage.


The Magneto ignition P lead is not pure DC either. It is pulsed, starting at zero and rising to peak then falling back to zero on each opening of the points.



The contacts in a “standard” magneto key switch are very robust and can handle AC voltages far in excess of anything put out by the Rotax dynamo. If you install toggle switches, you may actually end up with a switch that is less reliable than what you have now.



I would continue to use your magneto key switch with confidence.


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Old 04-11-2012, 10:00 AM   #5
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Default Re: Show us your panel

What is the key switch that Kitfox sells in their parts catalog? I assume it is fully compatible with the Rotax 912.

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Old 04-11-2012, 10:48 AM   #6
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Default Re: Show us your panel

John - with all due respect, the information I passed on was from several people I have heard speak and have spoken to directly, not random "internet experts". People like Phil Lockwood and Brian Moyerhauser from LEAF, who I have heard address this directly, are far more expert on this subject than you or I.

Both of these gentlemen have stated that the ac voltage is sufficient to erode the contacts in these switches which causes a feedback through the Ducati ignition boxes which can cause damage. Both of them recommend individual, heavy duty, 250v ac switches for the ignitions. A security, keyed switch can be used elsewhere for the start and/or master functions.

Your Corvair automotive ignition is most likely different and this may not apply. As to the 912 ignition system I plan to head this advise.
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Old 04-12-2012, 11:12 PM   #7
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Default Re: Show us your panel

My local airport is KGVT, Greenville, Texas. It is home to the defense contractor L3 – Mission Integration Systems. L3 employs thousands of engineers. I am fortunate to rub shoulders with many of these engineers both active and retired.

I spent much of the day with a pilot/electrical engineer who has over 35 years designing and specifying aircraft electrical systems. We discussed the magneto switch when used with the Rotax Ducati ignition system. He agreed the notion of a magneto switch causing a failure of the solid state module with some type of "feedback" was poppycock. He also agreed the AC voltage on the Ducati kill circuit was less damaging to a switch than the P lead circuit on a magneto ignition.

Let’s look at the facts, how the system operates, and possible failure modes.

The Rotax Ducati ignition uses a shorting wire to “kill” the ignition. It is only grounded on engine shutdown and during ignition checks. The shorting wire is grounded through the “mag” switch. If the shorting wire is disconnected or breaks, the engine continues to run. The only electrical things that could damage the ignition module are overvoltage and improper or shorted wiring.

The Rotax engine dynamo supplies power to the modules. Voltage is limited by engine rpm. Current is physically limited by the size of the dynamo coils. It is impossible for the dynamo to supply excessive voltage or current to the modules. The vast majority of Ducati ignition module failures are wires breaking inside the insulation close to the module due to vibration.

The premise that AC voltage present on the grounding circuit is too high for a magneto switch is unfounded. The voltage involved when grounding a Ducati module is similar to the voltage present on the P lead of most magnetos. The internal arcing with AC voltage is less than what would be present with a pulsed DC signal from a magneto. There is no reason to assume one or more toggle switches will last any longer than a standard magneto switch.

While we have no doubt some standard type magneto switches have failed while in service on the Rotax engines, we do not believe they are the cause of ignition module failures. If the contacts on the magneto switch were to corrode or burn to the point where they no longer make continuity, the respective module would simply not shut down with the switch in the off position. I don’t think LEAF, Lockwood or anyone else can explain how an open kill circuit due to a bad switch is the cause of ignition module failure.

Advising aircraft owners to change the magneto switch to toggle switches is questionable. The magneto switches are aircraft quality devices with large contacts capable of handling high voltage and current loads either AC or DC. Magneto switches are used successfully on many Rotax 912 equipped aircraft without incident. The replacement toggle switches are largely unspecified. A 250 volt toggle switch can be a totally inadequate miniature device capable of only 0.5 amps. with quality or origin unknown.

In summary, the Rotax Ducati ignition module kill circuit is not likely to damage a magneto switch or a replacement toggle switch. Nor is any switch, magneto or toggle likely to cause a failure of the ignition module.

John Pitkin








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Old 04-13-2012, 07:45 AM   #8
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Default Re: Key Ignition vs. Toggle Switches

Thanks John for clearing that up , I was having a hard time understanding how the open contacts of a mag switch could cause problems with the module . The rotax installation manual does give a 250 volt .5 amp spec for the mag switches . Granted there are quality issues out there with so much crap from china . Thankfully there is some mil spec stuff still available.
There are advantages to having both the toggle switches and the keyed switch . For the times that require motering the engine without starting it ,such a purging the oil system also no chance of bumping the starter while doing a mag check using the toggle switches
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Old 04-13-2012, 04:38 PM   #9
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Default Re: Key Ignition vs. Toggle Switches

I used the ACS switch for a number of years with no problem until about a year ago. The ACS switch failed when I was trying to start my 912ul by erratically grounding the ignition modules while cranking causing the propeller to stop dead in it's tracks. The first time this occurred I suspected that the battery was getting weak, so I replaced it. The next time this problem occurred I was unable to obtain an engine start. Determined cause to be ACS switch by leaving it in the "Both" position and jumping the start circuit beyond the ACS switch, instant start with smooth operation. I have since replaced with 250V toggle switches and a momentary push button for the starter. Now all starts are quick and smooth.
I originally thought that there is no reason the ACS switch shouldn't work just fine as it appears to be constructed with heavy duty contacts. I disassembled the switch after failure and it looks like new inside, but there is definitely a problem when used with the Rotax ignition modules.
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Old 04-13-2012, 07:25 PM   #10
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Default Re: Key Ignition vs. Toggle Switches

I consider LEAF and Lockwood experts on Rotax engines. They have years (decades) of experience with these engines in particular. I have added the switches on my plane and replaced the ACS switch per their advise. This thread has brought it to the front burner for me, so I did the conversion.

Everyone has the free choice to do this or not.
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