IO-540 P-lead resistors (repost)

[vlittle]

I'm struggling to get the correct resistor value for my IO-540-D4A5. Initially I used 30K, but I got spurious prop overspeed alarms, so I wired in a trimmer potentiometer, accessible in flight.

I can eliminate the spurious overspeed warnings, but now I get spurious dropouts in the rpm indication at low rpms (around 1500 and below).

The sweet spot is eluding me.

I have an electronic ignition on the right, the magneto is on the left.

It would be very helpful if you could publish the input structure for the EMS standard rpm input. This will help me establish the correct resistor value (or other circuit) to minimize this problem.

P-lead waveforms are dependent on the magneto, rpm and spark plugs. I'm using REM37BY, as recommended by AeroSport, but on not on the Lycoming list of approved plugs. Excessive ringing could make things worse, but I think an AGC circuit up front would take care of this. I presume that the SV does not use an AGC for these inputs, but it's easy to add one externally.

Thanks, Vern

 

[preid]

I have 100k resistors. I suffered the same issues with the provided 30k, the 100k resolved the issues. Have 2 mags however.

 

[jakej]

Vern
I had to recently go to 120K ( first time ever) on a customers Rv10, 100K was just a bit low to stop the spurious warnings at the takeoff rpm.
Good luck

Jake J

 

[vlittle]

Hey Jake, thanks.

I have a 1 Mohm pot in series with a 30K resistor, yet I can't find a sweet spot. If I eliminate the erratic overspeed indications, my mag check is erratic, and vice versa.

I think that the output of the mag varies with plug type and compression ratio as well as rpm. I'm going to regap the plugs and try again. By going to minimum gap, this will reduce the amplitude of the ringing on the p-lead waveform.

Vern

 

[jakej]

Vern

Another forum poster here, David, had similar issues & tried all sorts of 'fixes' including a check of his mags (I think only 1 of them was causing the fluctuations) , he changed out the mag for another new one & I think the problems went away. 
My suggestions however only relate to the fluctuations at takeoff rpm, in which case I personally could live with it, but you appear to have the issue at low rpm too - IMO it's not a big deal for the high rpm end, just saying ....

Jake J

 

[Carl_Froehlich]

I used the 30K resistors per the installation manual. No issues at 150 hours on a IO-540-D4A5.

 

[vlittle]

Vern

Another forum poster here, David, had similar issues & tried all sorts of 'fixes' including a check of his mags (I think only 1 of them was causing the fluctuations) , he changed out the mag for another new one & I think the problems went away. 
My suggestions however only relate to the fluctuations at takeoff rpm, in which case I personally could live with it, but you appear to have the issue at low rpm too - IMO it's not a big deal for the high rpm end, just saying ....

Jake J

I'm a perfectionist, and I'm looking for the root cause. I think I have a handle on the magneto waveform and the plug regapping may help. Higher compression may cause more waveform issues as well.

If I can't find the sweetspot, I have designed an external circuit that should help.

Vern

 

[dynonsupport]

Vern,
Sorry we missed you the first time around.

Most customers that need to change from the 30K seem to end up at 100K-150K. So there is a chance that with a 30K and a 1Mohm pot, you just have a hard time hitting the narrow range that works. You might want to consider a 100K in series with a 50K or 100K pot.

Something else funny is going on though. You have one electronic ignition, which I would expect to put out RPM all the time. The logic SkyView uses to display RPM is to display the highest RPM from the left and right. So in order to get a dropout, you would need the electronic ignition to be dropping as well.

You can see the RPM on both inputs individually on the SENSOR DEBUG DATA page in the setup menu. It would be interesting to look here and see which input is causing an issue.

Finally, you could just put in a sender instead of using the P lead pickoffs. The Vans VTACHGEN2 works well, or so does the solid state sensor that AFS sells.

 

[vlittle]

Thanks for the additional info.

I tried with a 100K pot + 30K, but it wasn't enough, so I switched to the 1M pot.
As for the electronic ignition, it's an Electroair. The only way to disable the unit is to turn the power off, which kills the rpm indication

I will check the sensor debug data for more insight into the problem. The AFS sensor looks like a good backup plan, although I have a device that I will test that may fix everything up without drilling another hole in the firewall.

Thanks, Vern

 

[mmarien]

Vern

If you have electronic ignition on the right, why don't you connect to that. They usually have an RPM signal wire for EFIS that give a signal regardless of the mag check.

 

[vlittle]

Vern

If you have electronic ignition on the right, why don't you connect to that. They usually have an RPM signal wire for EFIS that give a signal regardless of the mag check.

See post #8.

 

[rdowens]

Forgive me if you already know this...just trying to help...

You can hook up one side only. You don't need to hook up both mags. Pull the signal from the electronic ignition side because that will be a clean signal. You'll still get the mag drop when you check left and right because it's the engine RPM you're monitoring.

I have a sensor in a Slick on one side that gives a clean signal. I pull from that and completely ignore the other mag. It works exactly as I describe.

It's worth a try. It may be that your electronic ignition side will work like my sensor, or maybe it won't. It's easy enough to give it a go--you can hook it up that way in 10 minutes, I'd bet.

-Randy

 

[vlittle]

Electroair does not put out a tach pulse when it's turned off for the mag check. The only source for RPM at this point is the magneto. Setting the resistor value for reliable low-rpm indication does not work at high rpm due to the false high rpm readings... and vice-versa

Using the AFS mag sensor is a reasonable alternative, and it looks easy to install.

However, I will be testing another solution next week, involving a signal conditioning circuit for the p-lead. This is the easiest solution because it does not require another sensor on the hot side (that can fail) plus new wiring. I'll report back.

 

[vlittle]

Re: IO-540 P-lead resistorS---SUCCESS!

Well, I did what I do, and that's design interesting electronic gizmos. I solved the problem for my p-lead tachometer interface.

It required external circuitry (see attached) that conditions the p-lead pulses and presents a digital pulse to the rpm inputs on the EMS. The heart of the circuit is the peak detector that measures the amplitude of the p-lead pulses as they vary with engine operation. Then, these pulses are sliced at a fixed adjustable % threshold to provide a tach pulse.

If the circuit is powered by the 5 volt EMS power output, connect the TACH pulse to the low voltage RPM input. If the circuit is powered by 12 volt EMS power output, connect the TACH pulse to the standard RPM input. The circuit will work the same with either power source.

Do not connect to aircraft power, this circuit does not have protection from the more hostile environment, and must work when the master bus is off (EFIS battery power).

I have a design for a circuit board that has all of this circuitry on it, plus some extra protection stuff. It's probably easier and cheaper to buy the AFS magneto sensor, however. I did this design because it was easier to incorporate into my aircraft and did not require another hole in the firewall.

Note: I found that a 25% slicer threshold worked for me.

 

[vlittle]

Re: IO-540 P-lead resistorS---SUCCESS!

Well, I did what I do, and that's design interesting electronic gizmos.  I solved the problem for my p-lead tachometer interface.

It required external circuitry (see attached) that conditions the p-lead pulses and presents a digital pulse to the rpm inputs on the EMS.  The heart of the circuit is the peak detector that measures the amplitude of the p-lead pulses as they vary with engine operation.  Then, these pulses are sliced at a fixed adjustable % threshold to provide a tach pulse.

If the circuit is powered by the 5 volt EMS power output, connect the TACH pulse to the low voltage RPM input.  If the circuit is powered by 12 volt EMS power output, connect the TACH pulse to the standard RPM input.  The circuit will work the same with either power source.

Do not connect to aircraft power, this circuit does not have protection from the more hostile environment, and must work when the master bus is off (EFIS battery power).

I have a design for a circuit board that has all of this circuitry on it, plus some extra protection stuff.  It's probably easier and cheaper to buy the AFS magneto sensor, however.  I did this design because it was easier to incorporate into my aircraft and did not require another hole in the firewall.

Note:  I found that a 25% slicer threshold worked for me.

Update: Did some circuit optimization and testing with a 5V power supply. Amended design attached.

 

[vlittle]

Here's some in-flight information on the expected magneto waveforms.

The first trace (magneto-1) shows a 2400 rpm primary trace on the top, showing both positive and negative firing events.  The pulses have a large damped sinusoidal component that needs filtering.  The bottom trace is the filtered and clamped voltage on the input of the opamp.  It has a peak amplitude of about 0.5 volts, meaning that my circuit could be further optimized to bring this level up to about 2 to 3 volts by lowering the input resistor to 30 Kohms or so.  The hysterisis feedback resistor should then be reduce to about 100 Kohms.

The second trace (magneto-2) is an expansion of one pulse, showing the damped sinusoids on the top trace and the filtered result.  This shows that further filtering may help... increasing the 0.47 uF peak detector capacitor to 1 uF would be beneficial.

Enjoy.

Vern

 

[ricardo_godoy]

installed a skyview in my aircraft, which has a Continental engine io550B, and am having trouble with the indication of rpm, which has readings of 500 rpm more. what do I do? already put a resistor of 150 k?

 

[ricardo_godoy]

put the resistors 150 rpm reading kea remains above normal .... 600-900 rpm more, compared with the tachometer that was already installed. what should I do?? because some other information depending on it,,,, :-/ :-/ :-/ :-/ :-/ :-/

 

[Dynon]

godoy - Does your tachometer ALWAYS read high? If so, you may just need to increase your pulses per revolution setting. What is it currently set to?

 

[ricardo_godoy]

good day, in comparison with the tach Horizon P-1000 installed, the difference is between 600 and 900 rpm most ... always .. I can make this correction in own system setup?