PMag issue

[GalinHdz]

After much thought I think if the diode inside the Pmag goes T.U. I will be more concerned with the 60+ volts feeding back to the main buss frying some sensitive flight equipment or an umpteen thousand dollar radio. At that point a few hundred dollar EMS module would be the least of my worries.

My 2cents.

[N941WR]

Originally Posted by GalinHdz

After much thought I think if the diode inside the Pmag goes T.U. I will be more concerned with the 60+ volts feeding back to the main buss frying some sensitive flight equipment or an umpteen thousand dollar radio. At that point a few hundred dollar EMS module would be the least of my worries.

My 2cents.

While the voltage is high, the amperage is low. You can always add a diode to the power line, just incase.

In all the P-mag issues I've looked into, I'm yet to hear of an internal diode failure. Ours doesn't count because of the abuse / torture we have put them through.

One other thought, if a P-mag tried to back feed your electrical system, it would probably blow the breaker.

[KRviator]

Originally Posted by N941WR

I am not sure this is correct.

During the development of the EICommander we built a test rig that allows us to spin two P-mags simultaneously with an electric motor.

After reading this post we spun up our test P-mags and measured the voltage across the power and ground leads. The measurement on one was in millivolts. The other one has a bad diode and put out over 24 volts. (We have known of this problem for some time and since we don't fly with these P-mags and because we may have been the cause of this failure, we haven't sent it in for repair.)

After checking with Emag, they confirmed that the P-mags have a diode to prevent them from back powering your electronics.

I can now personally confirm this is indeed the case, at least with my two P-Mags. I can deselect ships power to both P-Mags above 750 (Left) and 760 (Right) RPM and the Dynon reports that they are still powered on. The benefit of this is as I postulated earlier, as part of your after-takeoff checks, turn off ships power, and the Dynon will give me a Warning audio alert and message if one of the internal alternators fails.

How much current they are able to put out I'm not sure, but I suspect it to be very similar to the note in the VP-X operating manual, whereby a power pin that is actually "off" shows bus voltage with no load, however, if you try to actually draw any current from it, it shows 0 volts.

[N941WR]

Originally Posted by KRviator

...The benefit of this is as I postulated earlier, as part of your after-takeoff checks, turn off ships power, and the Dynon will give me a Warning audio alert and message if one of the internal alternators fails.

...

Wouldn't you really want to know this before you took off?

I don't have a VPX, so I'm a little ignorant about their operation but can't you perform the same test on the ground?

When I had one of my internal generators fail, I discovered it on the ground by switching off ship's power during my run-up.

[KRviator]

Originally Posted by N941WR

Wouldn't you really want to know this before you took off?

Certainly, and that is part of my runup checks.

However, by isolating ships power after takeoff, I now have ongoing monitoring - and datalogging - of the state of the internal alternators, and instantaneous audible and visual warning if one should fail in flight. Then it is simple to turn that side off, restore ships power, and regain that ignition, but the point here is, I now know I am operating off ships power on that side and can plan accordingly.

In the event of an accident, there is evidence not only that either or both ignitions were on/off, but that they were actually being powered.

In the screenshot below, the "x IGN" sensors are simply wired to ground through one side of a DPDT switch, the other side grounding the Pmag's "P-lead". The "x IGN P" proves there is voltage at the Pmag, either internal or external, independent of switch position.

[N941WR]

Very nice set up. Bummer it only works with the VPX. It drives me crazy that the P-mags don't put out a condition flag indicating where it is being powered from so we could report it on the EICommander.

[KRviator]

Originally Posted by N941WR

Very nice set up. Bummer it only works with the VPX. It drives me crazy that the P-mags don't put out a condition flag indicating where it is being powered from so we could report it on the EICommander.

I think you've misunderstood my 'Stralian English!

My setup will work with any EFIS/EMS that supports a voltage input. For the Dynon, I ran two wires to the P-mag terminal screws from pins 4 & 23 on the EMS-220, then simply configured them as 0-2.5V = "OFF" and 2.5V + = "ON". The EMS can't tell, and doesn't care, where the voltage is coming from, but if I have my "P MAG POWER" switches off, and still have green dots, then I am assured all is right with the world.

Had I actually put a little more thought into it, I wouldn't have used 5V inputs, I'd have used the 12V inputs so I could actually measure the voltage, not just have it as ON/OFF. Even though I am jamming 12VDC into a 5V EMS pin, Dynon has assured that the pins are actually rated to sink 12V, it's just that anything over and above 5V shows as 5V.

The VP-X comment was merely my hypothesis as to why I can actually read the internal voltage when it is supposed to be isolated from back-powering the bus, thinking it may be similar in setup to the VP-X.

With the VP-x there is a note in the installation manual that warns you not to be concerned if you see bus voltage on a power pin that is supposed to be "off", as it is a quirk of solid-state electronics. There is no current behind it, and if you attach a load in place of a voltmeter, it will show 0 volts.