kevinh
Well Known Member
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.
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 ). 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.
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.
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 ). 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.