SkyView EMS Tach Inputs

[kurtfly]

Dynon,

I am wondering what the input impedance is on the "Low Voltage" tach inputs on the SV-EMS. I have dual P-Mags with SV connected to the Tach signals which outputs 12 V signal. When I connect to the SV the Tach signal drops to ~5V. The P-Mag has an output pull up of 4.8K ohm to 12V. This calculates the SV input impedance to ~3.5K ohm. Is this correct? Seems very low?
The SV tach indications works OK; however, I want to add an Electronic Ignition Commander to my aircraft which needs the tach signals also, but must be >8V. Will moving my SV tach to the high voltage inputs increase the input impedance? If so to what?

Thanks,
Kurt

 

[Raymo]

Do you have the Zener diode installed that came with the P-Mags? If so, it would explain the voltage drop. You should not need it if using the standard RPM inputs (pins 32, 33). If installed, you would be using pins 34 and 35 for the low voltage option.

 

[kurtfly]

No external zener installed.  There is one internal, 12V.  Direct wiring to the SV EMS.  I am surprised the SV has such a low input impedance.   I was expecting at least 1 M Ohm.  Before I dig into behind the panel to change to the High V inputs or build a Tach distribution amplifier would like to get Dynon's thinking. I am thinking the input may be an opti-isolator. The P-Mags Tach current limiting is kicking in. Would like to know what the High V input circuit is. It will work if I can keep Tach signal above 10 V.

 

[dynonsupport]

Time to get technical!

The low voltage input has 25K of impedance to ground when the voltage is between 0V and 4V. Above 4V, it has 1K ohm of impedance. This is why you're seeing it limit where it is. We never designed the low voltage input to be a high impedance load to the signal, just one that could handle a signal in the 0-12V range with an assumption the source impedance wasn't significant. The loads on this input are there for a variety of reasons including ESD protection and large overvoltage survival.

You should only need the low voltage inputs if your signal never goes above 10V. If it goes above 10V, the high voltage will work.

The high voltage inputs are 60K to ground up to about 9V, and then 35K to 3.3V. This appears it will work fine with the P-Mags and EI commander in parallel unless the EIC loads significantly.

Your final option would just be to put a resistor in series to the low V input. We only need a peak of 2V on this input to trigger.

 

[kurtfly]

Thank you! That makes sense. I will move my tach signals to the Hi V inputs...

KR

 

[scheevel]

I have a related question. I am installing an electronic ignition module from Rotec. I have not been able to get a response from them to my quesitons. The ICU that they use appears to be similar to a BOSCH unit with a hall sensor trigger. It has a tach output, but no indication if this is sending the hall triggering voltage as a pulse or something higher. If you have dealt with this type of ICU tacho output in the past can you tell me if it connects to the high voltage or low voltage EMS input. Right now, I have it connected to the high voltage input. Thanks for any help you can render.

Jay