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P-lead as a source for a tachometer

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ChangDriver

Joined: 15 Sep 2007
Posts: 266

Posted: Tue Sep 15, 2009 3:49 am Post subject: P-lead as a source for a tachometer

I am trying to figure out how a p-lead is used as a signal source for a tach.  Not how it is wired, but more of the theory behind it.  Questions like:



1) what does the signal look like (square wave, sawtooth, sine, etc)



2) What is the typical amplitude



3) why is the p-lead a typical source of noise in the RF range



4) how is the signal produced when the original intent of the p-lead was as a way to ground the magneto and shut down the engine.



5) how do p-lead tachometers typically prevent from a failure that would ground the p-lead.



Thanks,



Craig

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jindoguy(at)gmail.com
Guest

Posted: Tue Sep 15, 2009 4:42 am Post subject: P-lead as a source for a tachometer

Just curious, how will you check for rpm drop between mags if the tach goes dead when one p lead is grounded?



Rick Girard



On Tue, Sep 15, 2009 at 6:49 AM, Craig Winkelmann <capav8r(at)gmail.com (capav8r(at)gmail.com)> wrote:

 [quote]--> AeroElectric-List message posted by: "Craig Winkelmann" <capav8r(at)gmail.com (capav8r(at)gmail.com)>

  

 I am trying to figure out how a p-lead is used as a signal source for a tach.  Not how it is wired, but more of the theory behind it.  Questions like:

 

 1) what does the signal look like (square wave, sawtooth, sine, etc)

 

 2) What is the typical amplitude

 

 3) why is the p-lead a typical source of noise in the RF range

 

 4) how is the signal produced when the original intent of the p-lead was as a way to ground the magneto and shut down the engine.

 

 5) how do p-lead tachometers typically prevent from a failure that would ground the p-lead.

 

 Thanks,

 

 Craig

 

 

 

 

 Read this topic online here:

 

 http://forums.matronics.com/viewtopic.php?p=263104#263104

 

 

 

 

 

 

 

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 [b]

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nuckolls.bob(at)aeroelect
Guest

Posted: Tue Sep 15, 2009 4:54 am Post subject: P-lead as a source for a tachometer

At 06:49 AM 9/15/2009, you wrote:

 	  Quote: 	
		  

<capav8r(at)gmail.com>



I am trying to figure out how a p-lead is used as a signal source 

for a tach.  Not how it is wired, but more of the theory behind 

it.  Questions like:



1) what does the signal look like (square wave, sawtooth, sine, etc)


	


   Very ragged sorta square wave. At least when the

   points are closed, the waveform is steady at zero

   volts.  When the points open, the magnetic field

   stored in the inductive iron is allowed to collapse

   rapidly. Like the battery powered points/condenser/

   coil Kettering system, this mechanically powered

   energy storage system delivers a fast rising, high

   voltage transient in the fine turns of the secondary

   winding (spark energy) while a ratiometric image of

   the same waveform appears across the now open windings

   of the primary . . . or P-lead.

 	  Quote: 	
		  2) What is the typical amplitude


	


   The peak voltages can be on the order of 200-300

   volts. It's an damped oscillatory waveform who's

   period is set by the inductance/capacitance

   combination of the system and damped by the loss

   of energy that is running out to the spark plug.

 	  Quote: 	
		  3) why is the p-lead a typical source of noise in the RF range


	


   Any and all perturbations of current/voltage

   magnitude can be called "noise". The RATE at which

   they vary combined with amplitude of the change

   sets the parameters for the frequency spectrum

   over which the energy is distributed and propagates.

   When I mentioned FAST rise time earlier, this suggests

   an unusual opportunity for effects to extend well

   up into the RF spectrum. When we combine that with

   HIGH voltage, the energy signature becomes potentially

   antagonistic.



   FAST rise times is the key phrase that suggests why

   shielding p-leads is useful. It's waveforms with

   high dv/dt (differential) numbers that propagate

   electro-statically from one conductor to another.



 	  Quote: 	
		  4) how is the signal produced when the original intent of the p-lead 

was as a way to ground the magneto and shut down the engine.


	


   The P-lead is connected in parallel with the points

   which are themselves connected in parallel with the

   primary winding of the magneto's pulse transformer

   and the damping condenser. Closing the switch at

   the end of a P-lead essentially keeps the "points

   closed" all the time thus terminating the train of

   sparks.

 	  Quote: 	
		  5) how do p-lead tachometers typically prevent from a failure that 

would ground the p-lead.


	


   If poorly designed, yes. I've never seen one that

   offer this risk. The design goal for a tachometer that

   watches magneto p-leads is to first offer a load

   impedance that is sufficiently high as to NOT load

   normal operation of the magneto. The next task is

   to figure out a way to ignore the trash on the p-lead

   waveform that might be mis-interpreted as a signal

   proportional to engine RPM. My favorite way to do

   this is with an active band-pass filter that only

   allows the frequency of interest (30 to 150 Hz)

   to pass. My last crack at a smoothing circuit followed

   the band-pass filter with a phase-locked loop that

   produced a pristine square wave that would not

   upset the poorest of tachometers. The input impedance

   to the system was over 1000 ohms. Incapable of

   causing mischief in the ignition system



   Bob . . .

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nuckolls.bob(at)aeroelect
Guest

Posted: Tue Sep 15, 2009 5:14 am Post subject: P-lead as a source for a tachometer

[url=http://www.matronics.com/forums/viewtopic.php?p 62838#262838]  Posted: Sun Sep 13, 2009 3:46 pm    Post subject: Re: Tachometer Generator Wiring   [/url] 

  Bob: 



 The problem I have is that at RPM below 1000, I get noise in the signal and the tach jumps all over the place. Over 1000 RPM and all is fine. The radial engine idles at 500 rpm so this is a problem. 



 The tach gen wires go to a circuit that is a basically a voltage divider and a comparator to provide a clean pulse for the engine monitor system. 



 It has been suggested to 1) shield the tach gen wires to reduce noise, 2) add a 0.1 microfarad ceramic capacitor as a filter, and/or 3) change the voltage at which the comparator triggers (raise the limit for it to trigger). 



     Craig,



     Sorry, I got side tracked by the use of the term

     "tach generator" in your original posting. The

     2-wire device you're describing is not the traditional

     tach generator but SOME form of transducer. If it's

     just a 2-wire device, it MIGHT be a permanent magnet

     generator of some kind, most hall effect sensors

     require a third wire to carry 5-12 volts out to the

     tach TRANSDUCER's electronics. But a clever designer

     COULD make halls work on the end of a two-wire loop

     too. I suspect that the 500 rpm thing is the problem.

     Your tach/instrument combination may well have been

     crafted for the more common aircraft engines that don't

     even IDLE at 500 RPM. If the transducer is a 

     magnet/coil signal source, the low speed operation may

     well be producing a signal below the design goals by

     which the system was crafted. The fact that your

     difficulty clears up at higher speeds reinforces this

     suggestion.



     So you may be correct in your supposition that the

     circuitry needs to be tweaked to new design goals

     (stable ops at low speed). Without a schematic,

     parts values and waveform data, it's not possible to

     offer more specific assistance. Do you have access

     to such information?



     In any case, this isn't a noise problem in the classic

     sense that the tachometer system is talking to or being

     upset by another system.



     Bob . . .    [quote][b]

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ianxbrown

Joined: 16 May 2009
Posts: 80

Posted: Tue Sep 15, 2009 5:28 am Post subject: P-lead as a source for a tachometer

When I installed p-lead signal-sourced RPM monitoring, I learned that the signal is basically a pulse,  not a nice sig. gen. produced signal.   It's going to be around 100V and in my installation (into a Rocky Mountain engine monitor) there is no noticeable noise that would be attributed to P-lead.   In speaking with Unisom I learned that my Slick magnetos do not need shielding on the P-leads, and that seems to be true.

 

 As to how it works, I like to think of it a bit like the ear.  There is air on one side, transmitting sound, and air on the other side of the ear drum (capacitor) receiving the vibrations and passing them to the middle ear.  Unless the ear drum breaks (capacitor fuses into a dead short) there is no communication between the air on the outside and the air on the inside.  

 

 Ian Brown

 On Tue, 2009-09-15 at 04:49 -0700, Craig Winkelmann wrote:  	  Quote: 	
		    	  Quote: 	
		  

--> AeroElectric-List message posted by: "Craig Winkelmann" <capav8r(at)gmail.com (capav8r(at)gmail.com)>



I am trying to figure out how a p-lead is used as a signal source for a tach.  Not how it is wired, but more of the theory behind it.  Questions like:



1) what does the signal look like (square wave, sawtooth, sine, etc)



2) What is the typical amplitude



3) why is the p-lead a typical source of noise in the RF range



4) how is the signal produced when the original intent of the p-lead was as a way to ground the magneto and shut down the engine.



5) how do p-lead tachometers typically prevent from a failure that would ground the p-lead.



Thanks,



Craig





Read this topic online here:



[url=http://forums.matronics.com/viewtopic.php?p 63104#263104]http://forums.matronics.com/viewtopic.php?p 63104#263104[/url]

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ChangDriver

Joined: 15 Sep 2007
Posts: 266

Posted: Tue Sep 15, 2009 6:19 pm Post subject: Re: P-lead as a source for a tachometer

 	  Quote: 	
		  Just curious, how will you check for rpm drop between mags if the tach goes dead when one p lead is grounded?



Rick Girard 	




Rick:



Would basically digitize both p-lead signals and logical OR them together.  This way when left p-lead is grounded, the right p-lead would pass and vice versa.  Both signals should be in phase with one another so when both are on, a signal passes.  This also opens up the possibility of adding a panel light to let you know if one mag fails in flight.



Craig

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