Strange Plane Power Alt + EIS problem
 

Hi ya'll,

So last night I finally ripped out my old Van's 60Amp alternator - this was my second one and it had started to fail in the same way, after getting hot it wouldn't put out its rated output. I got tired of having to turn off recognition lights just to keep the alternator online. I had a good cooling duct and a heat shield between the exhaust stack and the alt, but still no joy at high loads.

SO - I've heard good things about the Plane Power automotive conversion alternator. I installed it last night - really nice bracket and seems well made. Fits perfectly out of the box. By the time I was done installing it was dark, so I figured on a test flight this morning. Started the plane and noticed a really strange problem: the EIS was showing that the bus voltage was 14.1V (yay!), but the CHTs/EGTs/MP were all being reported as REALLY high. Like 900 deg CHTs for a just started engine. :D

I was really puzzled - if I started the engine with the alternator offline all the readings would be correct, but once I turned on the field current the EIS would go crazy. I put the plane back in the hangar and started driving home (too friggen hot to work on the plane, must be nearly 90F out :p ). I've taken a fair number of EE classes, but I was puzzled and was planning to post a sad message to this site asking for advice. I was going to post here because I wanted to fly and figured Plane Power and GRT must be closed or on the way to OSH.

However, on the drive home I think I solved my own problem so I post this in case it happens to anyone else. The problem must lie with a subtle difference in how the alternator fail signal is handled between the two alternators:

In the Van's alternator the alternator fail signal is implemented via a transistor that pulls the fail signal to ground when it detects a fault. (I verified this with a multimeter during the initial install) Therefore I wired it to a spare EIS input, but since the EIS wants inputs between 5V and 0V I used a pull up resistor to the 4.8V EIS output voltage. It worked great for the 120ish hours of van's alternator flying - if the alternator was working the signal would float at 4.8V, if the alternator detected a fault it would pull down to 0V.

Though the wiring schematic for the plane power alternator is identical for this signal (the sample schematic shows a 12V bulb between 12V and the alt fail output), I think it must implemented differently. The Plane Power alternator DRIVES 12V out when good and pulls down to ground when bad. This works fine with an incandescent bulb, but not good for analog to digital (ADCs) converter expecting a max of 5V. Most multichannel ADCs will cope with 12V input without dying, but the high voltage will cause very large errors for the other inputs shared on that chip. Thus, I think this solves the mystery of the 'EIS acts crazy' with the Plane Power alternator.

I post this for two reasons:

* If correct, it might help someone else. (I'll test this theory tonight and update the post if I've made a mistake of some sort)

* If someone knows of a better idea, please send me a note. It might save me a bunch of fuzting later this evening. :rolleyes:

Those crazy alternators
 

A few comments. I am glad you got the plane power sounds like a winner. I do have a ND alternator but not one that Van sells. I am disheartened at all the issues people have with the ND's Van has sold. They are giving ND alternators a bad name, which are usually very reliable. I have a theory was the vendor Van purchased from. Van's new ND alt vendor is going to be better I think. Never the less sorry you had problems and think the plane power should be a good one for you.

All ND alternators "draw down" or go to ground (through a transistor) to illuminate the fault/ no charge warning light, the Plane Power is different? I would think they work the same since the regulator is essentially the same. You also point out the warning light is wired the same? Something does not match. May be I am missing it.

The EIS engine monitor will fry with more then 5 v. I think your idea of connecting the alternator warning direct to the EIS, is very clever. I would not have thought of it. However I would not recommend doing it, just because it's a discrete signal at 12V. A discrete light is perfect. I get that you added a drop down resistor, but just think a discrete light for this one function is safer, at least safer for the EIS, just incase the alternator goes crazy. The EIS is expensive and a light bulb is cheap.

Hi George,

Thank you for your good ideas. Some comments below...

I would have thought this also, I'll measure it tonight :cool: . I think the regulators are in fact different. I think the ND used by van's chose to use an open collector output, but the variant used by PP is some sort of push-pull output. Both work identically if you assume 12V+ on the other side of a bulb.


Acutally this is not true, after my post I checked the manual. I'll find the page in the manual, but the EIS docs are clear to state that if you accidentally hook 12V to one of the 5V inputs the device will be unharmed, however other signals will show a VERY high error. i.e. exactly what I saw.

For the Van's alternator (at least the version I measured) the fail signal is not a descrete 12V signal, it is just an open collector path to ground. By using a pull-up resistor you can use the signal with any sort of input.

I'll post details tonight from my PP testing.

Nippon Denso Alternator Schematic
 

One of the problems with the supposed unreliability of the Nippon Denso alternators is that the Van's electrical plans (at least of a couple of years ago) were not correct for the alternators that guys were buying as they showed the 'L' line to be grounded.

From the ND service manual (sorry, I don't have the exact alternator number handy): The 'L' line not only pulls down to ground (using an NPN transistor for you techies) to turn on the alternator fail light but also sources 12v to drive an electric choke (for an automotive carburator) when the alternator is producing voltage, using a PNP transistor on the same line (in a complimentary, totem pole configuration). The 12volts you are seeing on this line when the alternator is running leads me to think you have one similar to this. Grounding the L line like the plans I saw called for would fry the PNP driver. If it was the only thing that popped in the regulator, chances are the rest of the thing would work. If it took other components along with it, the regulator would fail. One guy went through several alternators, insisting that he had it wired right, and did but the schematic in his plans was wrong for his particular alternator model. When we wired it up like the skiz below, all was fine and he's got several hundred hours on it now. Another buddy just passed 100hrs on the same ND model w/no problems.

About connecting the 'L' line to the EIS... it sounds like its input voltage is protected for the full charging voltage of the system (~14v) but its designed to sense over a smaller range. Overdriving it may not break it but it will throw the readings off, as you have seen. Its also unwise to ask a protection circuit to handle continuous overloads. You will need to isolate that signal from the EIS, maybe a small 12V relay with the coil connected where the choke heater is and letting the contacts drive the EIS input. An opto coupler or NPN transistor would work well too if you are electronically inclined. If you use a relay, be sure to put a diode across the coil, band end to the 'L' line. If you want to go the electronic route, I can steer you onto what you need.

While we are at it:
the B line is the high current battery charging output.
the IG line applies power to the field through the regulator. Open this and the alternator dies.
the S line is the sense input for the regulator. Its designed to measure the voltage at the battery without pulling any current through it so there is no voltage drop in the sensing. In this way, the alternator can compensate for the voltage drop at high currents in the B line by driving it at a bit higher voltage at the alternator to maintain the battery charging voltage at the correct value. Several circuits I have seen, including the offending plans, simply tie this wire to the B terminal at the alternator which is incorrect. The regulator will be sensing before the voltage drop in the B wire with the result that the actual charging voltage at the battery will be lower than it should be under high current loads.

Hope this helps.

I thought the same thing - in fact, I just got back from Fry's picking up an appropriate optoisolator. We'll see how it does tonight.

An update: Sure enough the PP alt sources 12V when operating (thanks for the details Lucky) - this is different from the Van's alt. So, I installed an optoisolator and appropriate bias resistors and now all is well.

If I had been doing a 'from scratch' install when I originally hooked up the fail indication, I probably would have gone the bulb route. But I already had the EIS and didn't want to deal with drilling/labeling a new hole on my panel. (me = lazy ;) )

Boy, I'm not looking forward to this electrical stuff ... I'm going thru the Aeroelectric book now but all this "optoisolator" and "bias resistor" stuff sounds like voodoo to me.
When I was a kid I was interested in this stuff, used to make my own circuit boards, etc. but for some reason it seems more difficult now. Anyone have any suggestions for other resources to read for more info?

Thanks!

T.

Actually, I think you've found the place right here
 

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The best thing is its all RV / airplane specific.

Just ask. The info you need is right here... in the vast collective consciousness that is VAF.

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Thanks good to know
 

That's good to know, I guess I'm mistaken about damage to the EIS w/ high volts into the AUX input; I thought the manual warned of damage, but it's been a long while since I looked at the manual.

Hummm I might consider doing the same to save another hole in the panel, especially with a "optoisolator". :D Did you really use a optoisolator? If so how did you wire that in? What aux scale factor and offset did you use?

PS don't feel bad TShort, I had to look up "optoisolator" also. My suggestion is google and Wikipedia.

What is the Electric Choke Heating Coil you show in the diagram?