Help Needed with Over Voltage

A couple weeks ago I finished a short flight and after when, I parked the airplane, I left the master switch on (duh). The next time I tried to fly, I noticed it was dead. I attempted to jump start the airplane, but thought about how fast that would charge my battery and decided it wasn't worth the risk. I had gotten as far as connecting the jumper cables. After I decided to squash the jump start idea, I noticed that my 60 ALT CB had popped. Weird.

So I charged the battery the proper way. 2 amps all the way up. All is well.

I flew the airplane a week later and all was well, until just prior to landing. I noticed that the ALT CB was popped again. So I turned off the ALT side of the master switch, and I turned off all electrical items, then turned back on the ALT side of the master switch. Volts were normal again @ 14v. So I turned electrical items back on one at a time so as not to overload the system. All seemed well again.

I tried to fly the airplane yesterday. Started it. Taxiing out I looked at the CB and all was well until I looked at the volt meter. The volt meter was showing 18 plus volts. I have a cigarette lighter volt meter for emergancy purposes (I carry one in every vehicle) and it displayed 18+ volts too. So I turned off the ALT side of the master switch and parked the plane. I plan to dive into it today.

Does anyone have any advice on where they'd start? To me it sounds like a voltage regulator, but I'm worried about the CB popping twice. The CB had never popped before... ever.

Details-
RV-4
0-320 D1A
60 amp alternator (internally regulated I believe)
Steam gauges
60 amp CB
2 sided master switch
Breaker switchs on each individual electrical item (and nothing else has popped)
Battery seems strong holding right now at 12.65 volts and cranks the engine good


Thanks for any ideas or help you can offer.

X

Yep - you need to repair/replace the alternator.

The other point is a 60 amp alternator needs at least a 70 amp breaker. Under high loads (low battery(s)) a 60 amp alternator can put out more than 60 amps. The 70 amp breaker mitigates such nusance trips.

Carl

Believe you said you thought your voltage regulator was internal. Check and verify that the regulator is in fact internal. If the voltage is correct and I believe it would be from the two voltage reading, I would replace the alternator before next flight. Otherwise replacement of that voltage regulator External would be in order.

...

Here Is the latest.

I found this smoked wire (resistor I think). The only thing I can read on it is 1562



Here is the back of the alternator. Can anyone tell me about this?



You can see the smoked wire on the back of the alternator in the pics.

not sure why that is there but it looks like a diode. They usually have a 1Nxxxx part number

A trip to an auto-electric shop will quickly tell you whether it's bad, and whether it's internally regulated (probably is, but you need to know for sure).

Agree that that is almost certainly a diode, and that there's no logical reason for it to be where it is. There are some overvoltage schemes that use a zener diode to trip overvoltage circuits, but they aren't wired like that.

Suggest following that blue>white wire that's bundled with the B lead (fat wire) to see where it goes, then report back. (unlikely that it goes to an external regulator, but it is possible.)

18V won't kill most modern avionics made to operate on both 14 & 28 volt systems, but it will eventually (or quickly) kill many items made to operate on 12-14 volts, including lighting, some radios, 12V power supplies for things like tablets, portable gps, etc. And there's always a chance that it could go higher in a failure situation. Overvoltage protection is usually a good idea for that reason.

Charlie

I bet that diode is there to raise the bus voltage. By having the diode there you lower the sensed field voltage by the forward voltage of the diode, then the alternator will regulate the bus voltage to that much higher. Once you have a high field current going into the alternator, like in case of a dead/dicharged battery, it will overheat that tiny diode.

Someone could have installed the diode to raise the bus voltage for a LiFePo4 battery.

Just short out that diode (connect blue wire to white wire) and try your alternator again.

Lenny

Lenny Iszak said:

I bet that diode is there to raise the bus voltage. By having the diode there you lower the sensed field voltage by the forward voltage of the diode, then the alternator will regulate the bus voltage to that much higher. Once you have a high field current going into the alternator, like in case of a dead/dicharged battery, it will overheat that tiny diode.

Someone could have installed the diode to raise the bus voltage for a LiFePo4 battery.

Just short out that diode (connect blue wire to white wire) and try your alternator again.

Lenny

Click to expand...

Lenny, I am afraid none of this makes sense to me. It looks like the diode is going from power to ground. Like to prevent a shutdown spike. If it is fried, then either it was bad, or there is a bad diode within the alt and producing AC content in the output. .

And - why would anyone want higher voltage for a LiFePO4 battery. 14.2 is at the high end of its charge recommendation. At least based on A123 cell literature. What did I miss here?

The recommendation to remove an take to A'zone, or the like, seems like a good fact finding method to advance the knowledge.

Please do not short out that diode. The diode is between the alternator output and ground. The blue wire goes to the bolt on the case of the alternator. (ground). I personally don't think that is the best place to ground the field. I would run it back to a ground on the firewall.

Jay

I had to search for a while but I was able to find the diode that was used. I have to admit that I'm still a little confused as to the reason for the diode. I'm not sure but I think I might actually have an automotive alternator.

rv7charlie said:

A trip to an auto-electric shop will quickly tell you whether it's bad, and whether it's internally regulated (probably is, but you need to know for sure).

Agree that that is almost certainly a diode, and that there's no logical reason for it to be where it is. There are some overvoltage schemes that use a zener diode to trip overvoltage circuits, but they aren't wired like that.


Charlie

Click to expand...

Like Charlie said - pull the alternator and take it to your local auto parts place. Many can test them. I guess you can repair it, but at less than $100 just get a new one while you are there. Have them put your old pulley on the new unit.

The diode is just wrong.

Recommend you download and review the AeroElectric manual on how to wire an internally regulated alternator.

Carl

Oooh, i'm an idiot, I thought the diode was originally in the red sleeve. It is connected between the alternator output and ground. Yeah, don't short that out
And you are right there's no reason why that diode should be there.

Lenny

Diode

Xkuzme1 said:

Here Is the latest.

I found this smoked wire (resistor I think). The only thing I can read on it is 1562

Here is the back of the alternator. Can anyone tell me about this?

You can see the smoked wire on the back of the alternator in the pics.

Click to expand...

It would appear that the burnt diode was a zenor diode to clip the "B" lead voltage if was raised above the zenor diode breakover rating. If it was a 15V zenor, the 18 volt alternator output would definately fry this diode. This zenor diode (or conventional diode) is not required on any alternator system that I know of.

So my next question is what is under the red sleeve (field wire)? Is it another diode? Is it a resistor? Or just a connection? You shouldn't need another diode drop in the field (voltage sense) wire unless there is some other ground voltage drop between the engine and the battery ground. Any resistance in the field circuit WILL cause the alternator output voltage to rise above it's internal voltage reference (usually 14.3 Volts DC). This is because the field wire voltage (at the alternator input) is where the aircraft bus voltage level is sensed. If there is a say 4 volt DC voltage drop in the field line (relative to the battery voltage) then the alternator will output 14.3 VDC plus 4 VDC, or 18.3 VDC on the "B" lead that goes to the battery (causing an over voltage situation).

Cessna split master type switches used to fail in this manner..... Because of the relatively low switching current, the field portion of the switch would fail with a higher than normal resistance, causing a voltage drop in the field sense input to the regulator of the alternator. The result would be a buss overvoltage condition that usually did not pop the field circuit breaker (which is the normal OV protection method). Instead, the "B" lead circuit breaker (50-70 amp) would pop due to excessive currents into the battery.

It looks like someone with a misunderstanding of how electrical systems work wired it up.

To have a transient suppression diode on the B lead of an alternator is just wrong. The only possible location to put one would be on the Field wire to serve as a way of disconnecting the alternator in an overvoltage by tripping the Field breaker.

Even so, a better way is to use the B&C crowbar overvoltage protection module.

It looks like your alternator is kaput. Replace it with a new one... search the archives for lots of advice on this.

Fred.Stucklen said:

It would appear that the burnt diode was a zenor diode to clip the "B" lead voltage if was raised above the zenor diode breakover rating. If it was a 15V zenor, the 18 volt alternator output would definately fry this diode. This zenor diode (or conventional diode) is not required on any alternator system that I know of.

Click to expand...

Fred, once upon a time, it was said that an alternator would emit a voltage spike on shut down. I was trained to shut down the master then kill the engine for that reason on a circa '73 C182.

Could this be the intended function of that diode? Modern alternators don't do that.

My field is alive during start and shut down, soft and easy ramp up and ramp down of current, just like the tens millions of vehicles on the road today......think there are no sensitive electronics in todays vehicles?

That "diode" sounds like a load dump spike suppressor. It would be a transient suppressor rated nominally above the nominal voltage output of the alternator.

Perehelion Designs is one source for these - I have one on my Rocket.

A load dump spike is caused by suddenly removing a heavy load from the alternator.

Yeah, the split switch is a holdover from antealternator, antesilicon days.

You won't find a CB in the alternator B lead of any modern road vehicle. There is typically a 'fusible link' that's 4 numerical wire sizes smaller than that actual lead, and ~4-6" long, at the *bus end* of the wire. It is there to protect the B lead from the *battery*; not the alternator. The B lead should be sized to handle the max output of the alternator (~10-20% above the rated capacity), so the alternator could never damage the wire.

(If that diode was indeed a zener, it would have blown so fast that the B lead CB would likely have never even seen the load spike.)

Sorry, kinda veering off topic, but hopefully the OP can get the wiring sorted out and perhaps upgrade the system. BTW, the B lead in the photo looks like ~#10 or so; a bit small for a 60A alternator unless it's a really short wire.

Charlie

Xkuzme1 said:

... I had gotten as far as connecting the jumper cables. After I decided to squash the jump start idea, I noticed that my 60 ALT CB had popped...

Click to expand...

Sounds like you connected the jump start battery backwards. The clue was the 60A breaker popping without the engine running. That also explains the little diode burning up.

I'd suspect the regulator and/or other small sense diodes in the path between the alternator and regulator are fried too, causing the overvoltage.

Heinrich

Typically diodes are used on starter and master realays to prevent voltage spikes when the relays are shut down ,if diodes are bad or missing it will often trip the field CB

But, he said the 60A B lead breaker tripped.

Diode

What you state is fact, IF there is a solenoid in the "B" lead, and it's opened while the alternator is generating a high output current. The alternator inductance will cause a spike if the "B" lead current is suddenly unloaded. But this isn't the case in this situation that's been illustrated here. The diode that burnt out was in the correct place to prevent the spike, but the solenoid isn't in the "B" lead.

While the same alternator inductance will indeed generate a voltage spike, it is absorbed by the battery (a VERY big capacitor) that is attached to the "B" lead.

I ask again, what is under the red sleeve (field wire)? Is it another diode? Is it a resistor? Or just a connection? If it is a component, it shouldn't be there.....

I suspect the burnt out diode was a zenor diode to prevent an over voltage situation, but was not rated to handle that situation when it did occur. This design approach just doesn't work as there is nothing to stop the failed alternator from a continuous over voltage output. This kind of energy is not easily absorbed by conventional components.

BillL said:

Fred, once upon a time, it was said that an alternator would emit a voltage spike on shut down. I was trained to shut down the master then kill the engine for that reason on a circa '73 C182.

Could this be the intended function of that diode? Modern alternators don't do that.

Click to expand...

Ok. So here is the final outcome.

The alternator has far more plugs than the standard one wire alternator. That coupled with the fact that I didn't see anything on my firewall that looked like a standard(ish) voltage regulator, led me to believe that the alternator was internally regulated.

I took the alternator to every auto parts store in town. Nobody could test it without a part number and there was no part number stamped anywhere on the alternator.

Defeated I spent a few hours looking at alternators online. No luck. Then I had the idea of googling the names of the extra plugs on the back of the alternator (D-, D+, and DF). Immediately an exact picture of the alternator popped up. It is from a Porsche. Bingo!

Now that I had found the correct alternator, I noticed it was not internally regulated. So I went back to the airplane and started picking around. What appeared to be a noise filter, and looks like no other voltage regulator that I had ever seen before, was indeed the voltage regulator. I looked at pictures of Porsche regulators and lots of them looked similar, but none were a dead ringer. So I bought the closest thing I could find and wired it up. Works like a charm.

I haven't yet reinstalled the diode because I havnt received it in the mail. I am still slightly confused at the exact reason for it, but I do plan to reinstall it.

Just in case ir can help someone else:

Alternator: Bosche AL75X ($65 from Napa)
Voltage Regulator : ACDELCO PRO E695 (not the napa part number)

I bought an extra alternator and extra voltage regulator. Total cost... $90.

Thanks again for all of your help.

X