RPM pickup from Alternator

[rizzz]

Hi all,

Perhaps someone with a bit more knowledge of electronics can help me with this:

I have an (older) MGL Stratomaster E1 and I'm trying to pick up my RPM signal from my permanent magnet alternator which should generate 6 pulses per revolution.
It seems to work fine initially but as the engine gets hotter and/or I crank up the power (over 2000rpm) the readings start to drop away as if the engine was slowing down, even though the engine remains at the same power or is speeding up (I can clearly see/hear it does).
As if the signal is not clean/fading after a while.

The E1 manual suggests installing a ballast resistor is needed "in some cases". Would this be such a case? (the manual is pretty vague on this)
If so, what kind of resistor, how many Ohms?

Cheers,
Michael

[tonyr]

Hey Michael,

My Dynon uses a feed from the Jab alternator via a 1/4Watt 33K resistor to limit the signal, otherwise its unreliable and goes a bit crazy at higher revs.
Sounds like the E1 has a similar problem. Resistors are cheap!

Cheers
Tony

[rizzz]

Hey Michael,

My Dynon uses a feed from the Jab alternator via a 1/4Watt 33K resistor to limit the signal, otherwise its unreliable and goes a bit crazy at higher revs.
Sounds like the E1 has a similar problem. Resistors are cheap!

Cheers
Tony

Thanks Tony,
Is this resistor installed as in the drawing below?


Cheers,
Michael

[tonyr]

mmm MGL have the resistor in parallel with the input.
They don't show it coming off the alternator though, just an rpm pickup? Perhaps from ignition circuit?

The Dynon has the resistor in series with the input for an alternator feed, to limit the voltage levels into the input line. Can be up to 40VAC out of the Jab alternator, normally around 20VAC.

edit: I just had a look at the E1 manual, I would not use the supplied 220 ohm ballast resistor across the alternator output, you will fry it!
Try a large value resistor like I suggested in series with the alternator pickup. It wont hurt anything.
(which is how they do a Bendix mag system)

[rizzz]

Ok, thanks Tony.

[tonyr]

Does your E1 have an RPM sensitivity adjustment screw on the back?
You could use a 10k resistor as they suggest in the manual and use the sensitivity adjustment to fine tune.

[rizzz]

Does your E1 have an RPM sensitivity adjustment screw on the back?
You could use a 10k resistor as they suggest in the manual and use the sensitivity adjustment to fine tune.

No that's on the newer E1, mine is still the old "Stratomaster Maxi Single", you can find the manual here:
http://www.mglavionics.co.za/Docs/e1.pdf

[ededoad]

I got my E1 rpm to work using a hall effect sensor wired as shown in the E1 manual. The engine I am using has a Tach drive and I used a speedometer sensor driven off of that. Trying to get a reading from the magneto was useless. Rpm indication was all over the place.
This post is probably no help for the Jab.
Regards

[DCASonex]

Actually, your post is relevant to Jabiru, and CAMit engines. The usual alternator pickup is not applicable if using one of the new external alternators, and a hall effect works well. CAMit now sells a custom made one that should fit both CAMit and Jabiru engines.

David A.

[rizzz]

After a lot of trial and error and many emails back a forth with Matt at MGL I finally got this to work properly.
Just thought I’d share how it was done as this might be useful to others, I have learned I am not the only one trying to get proper RPM readings from the old MGL E1. Many have given up and used and used another instrument in the end to get their tach readings.

The problem is that the older E1 (I don’t know about the new E1) needs a clean square signal as RPM input, like what you would get from a hall effect sensor. This is what I learned from Matt.
Trying to read the peaks directly off one of the alternator AC inputs just isn’t good enough for this instrument, it “works” sort off but the readings tend to be erratic and become very inaccurate at stages (at least in my case). Resistors in series/parallel, capacitors, insulating the wires, … does not fix the issue (I’ve tried it all).

The other thing I tried was a tooth counter sensor (also from MGL) above the starter ring gear which should give me a clean square pulse per tooth but this did not work at all at higher rpm! Only worked at starter RPM but once the engine kicked in the signal went away.
Matt told me this was not a good idea as my ring gear has 98 teeth, which is way too much for this sensor/instrument (not sure which is the problem here, I did not question further).

So in the end I solved the problem using an optocoupler on BOTH AC outputs from the alternator like so (this was my dad’s idea, thanks dad!):

As you can see in the simulation above this converts the AC sine wave (green) from the alternator into a nice clean square signal (blue) on the output side of the optocoupler.
I simulated this at values anywhere between 10V and 70V coming from the alternator which should be about the correct range and it works perfectly through the entire range using the resistors as per the diagram above (make sure your resistor on the input side is at least 2W, the ather one can be 0.5 or less, the diode is a 1N4007).
Matt at MGL confirmed as well this solution would probably work.

So after all that I built the circuit on a little breadboard and yesterday I tested it for real on the airplane.

Works perfectly! It really does! I’m stoked! No need for expensive hall effect sensors etc.

Now I just need to make a proper circuit board and put it all in a little box that can be attached to the firewall.