MGL VEGA RPM-1

[Pcmaxy]

Has anyone installed a MGL VEGA RPM-1 tacho in their Jabiru?
I have a J230 today I had an avionics LAME install it and he couldn't get it to work correctly when connected to the standard Jabiru pickup sensor the instrument would fluctuate up to +/- 200 RPM at all throttle settings. He eventually connected it to one of the two coils which is less than ideal because I am unable to see the mag drop on the other coil.
Once connected to the coil the RPM indication was better but now over reads at take off and cruise RPM by about 400rpm, although it reads correctly at idle.
Do anyone have any suggestions to resolve this issue? Ideally I would like it to work as it should eg connected to the standard flywheel pickup with stable and correct rpm indication.

[Ugly Duckling]

A start would be to contact MGL https://www.mglavionics.co.za/

[rainier]

The Jabiru pickups are not really intended for this kind of RPM gauge as the signal from the pickup is very small.

You can make them work in many cases - make sure you have the filter set to off (internal dip-switch - see manual). The filter is there to remove noise from the signal and is designed to remove or reduce short, fast signals - exactly what you get from that pickup.
Then make sure you adjust the gap between pickup and flywheel as close as possible (and safe) - the reason is that the pickup is just a coil and relies on a changing magnetic field to produce a small signal. That signal amplitude falls rapidly with distance. Adjust the sensitivity of the gauge to maximum (on the rear of the RPM gauge).

You can use any of the two A/C leads going to your regulator as signal source - this tends to work for most but not always - it depends on the type of regulator fitted and the effects it has on that signal. One seemingly very good solution that one of our customers came up with long ago is to use a small 600 ohm 1:1 audio transformer which you can easily get from electronic shops or rs-components etc. Simply wire one side (primary and secondary are the same thing here) to the two A/C leads and the other side - one wire to ground and the other is your RPM. You now have a very nice A/C signal that is isolated from the stator coils and ground referenced.

Using the p-leads is also possible but yo should take care with this - the p-leads can produce voltages up to several hundred volts and that can damage things. It helps to have a scope to see what this signal looks like but effectively it is one giant pulse followed by a lot of smaller ones. It's the smaller ones that you need to get small enough so they don't get counted by the RPM gauge - the signal also changes with RPM - as RPM goes up the voltages also go up. In such applications you MUST install a dropping resistor in-line to your RPM gauge. For our instruments we usually end up somewhere around 20-30K or so - easiest way to get a good value is to use a 50K trimmer - figure out a setting that works over the RPM range - then measure the resistance of the trimmer using an ohm meter and choose the closest standard resistor value. You want a value that allows the gauge to see the big pulse but not see the smaller ones.
Disadvantage is that you can'y measure RPM on a mag you just shorted - however on a Jaburu it hardly matters - I presume you can hear the engine well and can compare the noise with that of the other mag-drop. In that case you still have a good estimate of the drop.

Apart from the above you can consider a plethora of other RPM pickups - we are just looking for any voltage-pulse source that is relatively clean, preferably has several volts amplitude and is repeatable.

Also note, apart from the noise filter mentioned above - there is a sensitivity adjust that works over a wide range - just stick a small screw-driver into the slot at the back of the instrument and adjust as needed.

On our larger systems you would use the RDAC - you connect your RPM sensor here. In addition to noise filter and sensitivity adjust you also get a high gain mode (which brings down minimum signal amplitude to about 1 volt) and you have a ballast resistor (220 ohms) that you can switch in. The latter is mostly used for all of the Rotax engines (including the two-strokes). This should never be switched on if you are using a p-lead for obvious reasons.

What is written above also applies for many other makes of universal RPM gauges and systems and may be useful.

[Jean Crous]

Why not use the alternator as a source for the pulse required ? Signals from there can be used independently from the ignition system. If one knows how many pulses per RPM the alternator gives,(the engine manual should say) then set the instrument pulse rate to the same ? This is how we do it on the Revmaster aircraft engines.
Jean

[rainier]

Why not use the alternator as a source for the pulse required ? Signals from there can be used independently from the ignition system. If one knows how many pulses per RPM the alternator gives,(the engine manual should say) then set the instrument pulse rate to the same ? This is how we do it on the Revmaster aircraft engines.
Jean

This is the suggestion I made using the regulator A/C input - however the success of this depends on the regulator and how it operates.
many of the more primitive types simply short the A/C input once the DC voltage reaches 14V or so. The idea here is that while this creates maximum current through the stator coils it also means (near) zero voltage so power is very small (power = voltage * current). So this regulates by switching the A/C on or off - it's either fully on or fully off and this state can change at any time during the sine wave from the coils.
Needless to say - this makes it tricky for using this as RPM source.
Most of the better regulators restart their "ON" state with each A/C wave start (as it transitions to zero) - the voltage is allowed to rise until it hits the DC voltage at 14V - if the DC voltage is below this then the regulator remains on for the full wave. This form, while it provides a changing A/C waveform depending on the state of charge of the battery generally is a usable source for the RPM gauge as you have a good bit of signal for every A/C period.
In both cases - the battery forms a vital component of the regulator - it's kind of like a big capacitor providing a stable DC voltage.
Some regulators will not work at all without a good battery and can cause significant damage to avionics and other systems supplied by your DC bus.
Worse, a failed regulator in combination with a weak battery (high internal resistance) is pretty much deadly to avionics. In our time we can attest to several cases where such an event has effectively taken out everything on a 12V or 24V bus - avionics, radios, transponders - you name it.

It is easy and cheap to protect your system against this - the best way is a unidirectional transorb specified to a voltage that is say about 50-100% above your highest expected DC voltage in normal operation - a circuit breaker should be placed between your power source and the transorb.
The unidirectional transorb is one of the best solutions as it reacts to negative voltages at any voltage and any over voltage by conduction (like a zener diode). It reacts very fast and the idea is that it will withstand the punishment until your breaker trips. If it does not the transorb will fail short circuit - and hopefully that will blow your main breaker or fuse.
We used to install transorbs in all our avionics in the past and they were really good - but unfortunately DO-160 latest editions does not allow this in an easy way anymore as avionics is no longer allowed to provide any form of voltage clamping on its supplies (the wording is a bit indirect but you can't do it anymore as there is no limit to the current provided by the overvoltage source - so theoretically you need to clamp the entire power of the universe). You need to use high voltage linear regulators (to about 80V in case your system is designed for 24/28V). You do not have to prove that your system can withstand higher voltages - but in our often poorly implemented power systems in aircraft in case of failure you can easily get voltages very significantly exceeding this.
Having said all this - I can't see anything in DO-160 that would prevent you from using a transorb external to any avionics - it's a bit of a grey area.

For 12V/14V systems - a 21V transorb is a good choice. For 24V/28V systems you should choose a 43V type. They come in different power absorption capabilities. We recommend 1.5K diodes in DO201 package - these are easy to work with in our environment. They are rated for 1.5KW peak pulse power. That's quite enough. It's cheap insurance.
You can get these from good electronics suppliers including on-line from Mouser or RS-Components, Digikey and others.

[Jean Crous]

Rainier , thank you for a very informative reply.
Jean.