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HDX - EMS 220 custom sensor files

keitht

Well Known Member
Have been considering a revision to my sensor mapping and creating a custom file for a second current shunt amplifier. I have a two alternator, one battery, main bus and essential bus configuration ( Bob Nuckolls Z-12 configuration) the main bus load is monitored by a CS-02 connected to an enhanced input, the main 60 amp alternator current shunt connected to the current sensor input, two capacitive fuel sensors and the fuel pressure sensor connected to the remaining enhanced inputs so there is limited options for connecting a current shunt for the second 30 amp alternator. My thought was to build a instrumentation amplifier powered from the EM -220 +5volts supply scaled 0-5 volts for 30 amps connected to the second Voltage sense input (pin2) and custom scaling. Dynon tech support suggested moving the fuel pressure sensor from an enhanced input ( presumably to a type A input because that is all that is available), hooking the instrumentation buffer amp to the enhanced input and writing a text script (whiget) to define the parameters for the new input. The wiki on the Dynon site provided details and warnings about some details about doing that. I was wondering if any brave souls had experience with this process and advice on avoiding having to send the HDX back to Dynon to reprogram after making fatal errors in the text files.

KT
 
Another way to do is to have one shunt that can read amperage in/out of the battery only. I have it this way in my airplane with dual alternators and one battery. The two alternator B leads each have their own ANL fuse to protect the ships wiring and battery in the event of a shorted alternator, then they come together onto one leg of the shunt with the battery line on the other leg. This way you say amperage charge/discharge from the battery - but not the alternators. You can tell at a glance with your voltage/amperage on the EFIS if your alternators are carrying to load, or the battery. In practice I run one alternator or the other, and switch them every so often to make sure they are both working (and check them on runup). With this setup if my primary alternator goes TU, the rising discharge and dropping voltage from the battery will trigger my EFIS alarm and draw my attention to it - at which point I will bring the other alternator online and start with Plan B.

I'm not a fan of electrical setups where a failure is automatically picked up by the backup system without alerting the pilot. My aircraft is electrically dependent and if I have a failure I want to know about it - and then I will activate the backup system and decide what to do from there.
 
Combining alternators on one shunt

Greg, did think about that solution but had the concern that the shunt rating needed to be sized for the combined output of both alternator if both field switches got turned on together. The solution to that would be to change the field switches to a single two pole three position with center off. This has the disadvantage of only being able to have one alternator active. I have ANL fuses in the output lines for both alternators and then through the shunts to the battery common point so I could take a current measurement across the alternator side of both shunts in series. This would work if the scaling for both shunts is the same and only for the case where one alternator is active. For the case of both alternators active the reading would be undefined depending on the relative voltage output of each alternator. I have dual P Mag ignition so have some independence to keep the fan turning but dual HDX all glass displays with no back up steam gages so having knowledge of what is providing (and what is capable of providing) electrical power is important. Seems there is no obvious easy answer.

KT
 
Greg, did think about that solution but had the concern that the shunt rating needed to be sized for the combined output of both alternator if both field switches got turned on together. The solution to that would be to change the field switches to a single two pole three position with center off. This has the disadvantage of only being able to have one alternator active. I have ANL fuses in the output lines for both alternators and then through the shunts to the battery common point so I could take a current measurement across the alternator side of both shunts in series. This would work if the scaling for both shunts is the same and only for the case where one alternator is active. For the case of both alternators active the reading would be undefined depending on the relative voltage output of each alternator. I have dual P Mag ignition so have some independence to keep the fan turning but dual HDX all glass displays with no back up steam gages so having knowledge of what is providing (and what is capable of providing) electrical power is important. Seems there is no obvious easy answer.

KT

The shunt doesn't need to handle the combined output of both alternators - it will only see the output going into the battery (charging the battery) which will be limited by the resistance across the battery and the charging voltage. For a lead acid battery, the most it will usually take is in the 20 amp range, while a lithium battery might take 35-40 amps initially after start and tapering off quickly.

I run both alternators occasionally as well - with separate field switches - the alternators will not output more current than is needed to maintain the voltage regulator setpoints, and as that voltage rises the regulator drops the alternator field current to bring it back down. So if you have for example 15 amps of active current needed to keep everything running, and the battery wants 10 for charging to bring it up to full charge, then you need 25 amps from the alternator(s). 24 amps would result in a slightly low bus voltage, and the regulator ramps up the field current, while 26 would be slightly high and the regulator ramps it down. It doesn't matter if you have one alternator or two online, they will simply share the load and respond to the bus voltage.

In practice, what I do is keep one alternator on all the time, occasionally switching from one to the other to keep them both exercised. My bus voltage rides at a normal 14.4, and my amps rides at a normal zero after the lithium battery is recharged after engine start (a couple minutes). I have alarms set on my EFIS both for low voltage (at 13.5 if memory serves) and high current (anything over 15 amps, I need 19 amps to keep everything running) flowing OUT of the battery - both of those would alert if my alternator took a vacation, at which time I would simply shut off the field coil on the active alternator and bring the other one online. My standard runup routine includes checking both alternators for output, and checking the alarms for function by taking both alternators offline.

Some folks will tell you that switching alternators on/off with the engine running is a sure way to kill the alternator - and that was true decades ago - but the modern alternators ramp the field coil amperage up slowly enough that you simply can't/don't shock them to that degree. I've got 700 hours of flying on these original pair of alternators (Plane Power primary and B&C secondary) with no problems at all, and I switch them on/off every flight with the engine running.
 
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Sensor widget files

The original point of my post was to solicit information about redefining sensor files by writing text files for the HDX/EMS-220 firmware. I would like to drag the conversation back in that direction. The HDX system capabilities appear to be really powerful and flexible (even if the number of enhanced inputs is less than I would like) so being able to understand how to program those features from those that have experience is the focus of my efforts.

KT
 
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