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VPX/PMag setup

Driving '67

Well Known Member
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Hi,

Quick question. If I elect to run my backup power source for my PMag through the VPX, can I see and set up an alert that power is being drawn through the ECB? I?m thinking that it would be nice to know if the internal generator has failed during Flight.

My setup for my IO-390 is one traditional Mag & one PMag.

Thanks Jim
 
I?m not aware of an option for this. Do the new pmags default to internal power mainly? The older ones used ship power and only reverted to internal power if ship power failed.
 
Hi Jesse,

Reading the documentation for P114 series PMag, the internal generator is primary source, ships power backup assuming RPM is above cutover value.

Thanks Jim
 
While this might be an interesting feature to have, you don't have any warning that the PMags are running off ships power in the standard installation with switches or circuit breakers.

That's why testing the internal generator during you runup is an important step.
 
All the above is good and accurate advice.

There is no need to put your P-mags on your backup buss and you want the ability to drop powered to them individually during your run up to test their internal generators. This can be done with switches or by "pulling" a breaker on the VPX.
 
I think my question should be more oriented towards the VPX capabilities. Is the VPX able to measure and alert the pilot of current draw on a circuit? If so, can you set parameters when the message is generated?

Thanks Jim
 
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I think my question should be more oriented towards the VPX capabilities. Is the VPX able to measure and alert the pilot of current draw on a circuit? If so, can you set parameters when the message generated?

Thanks Jim

Yes it does measure current and can monitor if a circuit is drawing current or not. Very low current devices may or may not be easy to monitor. The alert is opposite what you are asking for I think. The alert warns the pilot if the circuit is turned on and not drawing current (example burned out light bulb).

You can turn this detection feature on or off on and individual circuit basis.

While you can visually see what current a circuit is drawing (display dependent), there is no alert that I am aware of that will let you know current IS being drawn.
 
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What I have done, though it is not supposed to be possible, is wire a GP input from my Dynon EMS220 to the power input pin of the PMag.

With a switch between the VP-X power pin and the PMag and that switch opened, if the PMag is running off its' internal alternator I still see voltage on the Dynon.

Apparently the PMag input is diode-isolated so its' internal alternator can't back-drive the electrical buss, but the Dynon doesn't lie. Which makes me wonder if it is like the VP-X, inasmuch that with the power pin selected off, you can still read buss voltage, until you put a load on it, then it drops to zero.

So, besides the 'power-check' during preflight, whereby ships power is cycled off then on, you can turn off ships power as part of your after-takeoff checks and the Dynon will flag an alarm if one of the PMags drops to zero volts in flight. Mind you, if it does that, you'll lose that side's ignition anyway so the slight roughness will alert you something is wrong, but the Dynon will tell you what is wrong.
 
...Apparently the PMag input is diode-isolated so its' internal alternator can't back-drive the electrical buss, but the Dynon doesn't lie. Which makes me wonder if it is like the VP-X, inasmuch that with the power pin selected off, you can still read buss voltage, until you put a load on it, then it drops to zero.
...

You are correct, the P-mags are supposed to be diode-isolated. If yours are backfeeding the electrical system, you should give Emag a call and see if this is "normal".
 
I don't understand the lure of the VPX. What makes it so desirable? Sure the diagnostic capabilities are neat, but what are the failure modes? I like the independence of CB's and switches, why would you want to run all of your devices though one computer box, that if it fails, what happens? CB's are isolated. I just don't know enough about the VP I guess..
 
You are correct, the P-mags are supposed to be diode-isolated. If yours are backfeeding the electrical system, you should give Emag a call and see if this is "normal".
But like the VP-X, a diode will show bus voltage when biased reverse if you connect a voltmeter but zero volts with a load on it, which makes me wonder if I connect a load across the Pmag power input whether the indicated voltage would drop to zero...something to try at the 100-hourly, methinks...

The VP-X manual has a note about this exact issue with a comment about not being disturbed by it.
 
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This is where their Pro version is more robust, have two independent CPUs and power buses.

I don't understand the lure of the VPX. What makes it so desirable? Sure the diagnostic capabilities are neat, but what are the failure modes? I like the independence of CB's and switches, why would you want to run all of your devices though one computer box, that if it fails, what happens? CB's are isolated. I just don't know enough about the VP I guess..
 
I'm not worried about the VP-X.
I know, just pointing out that simply because there is a diode between the PMag alternator and the buss doesn't necessarily mean there is a fault if you can sense voltage at the input pin. That it occurs on both PMags - and has done since installation - makes an independent, but identical failure unlikely...
 
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