fzzzzt, popped alternator breaker C-172M

Ok, here is one for you.

The other night, about the time I landed, I hear a pop and remember seeing a visible flash. I see the amp gauge has a big discharge.

After I taxied off, I try resetting the alt half of the master. The high volt light is dimly lit and glows brighter when I turn off
half the master but that doesn't fix anything. I find that a breaker is popped, so I reset it and everybody is happy again. It is
getting pretty late so I put the machine away.

Any ideas? Everything that was on, kept working. Now I don't know know if it was the field or output breaker but I am guessing by
the flash of light that it was the main.

Pretty hard to trouble shoot a problem that isn't there now, but I am at least going to take a look at the back of the panel with a
mirror for signs of arcing just in case something is loose and bouncing around. (interestingly this happened during one of my better
landings)

Where I was raised, blown breakers aren't something to be ignored.

Mike Z.

You need to replace the main 60 amp breaker. This is a thermal type
breaker and as it gets older the contacts build up oxidation and self
heat more reducing the breaker rating.
You are lucky!! When my 60 amp breaker popped one night the load
dump fried the alternator field, the voltage regulator and the over
voltage lamp.
I measured the 60A breaker and it would pop at 35 amps.
Cessna wants lots of money for the 60 amp breaker but it still is lots
cheaper than a new alternator and regulator.
On a C172M the 5 amp field breaker will NOT protect the alternator
field winding since it is not in series with the alternator field. It
protects the over voltage relay that draws only 0.1 amps.

On Mon, 03 Nov 2003 21:08:22 GMT, "Mike Z."
wrote:

Ok, here is one for you.

The other night, about the time I landed, I hear a pop and remember seeing a visible flash. I see the amp gauge has a big discharge.

After I taxied off, I try resetting the alt half of the master. The high volt light is dimly lit and glows brighter when I turn off
half the master but that doesn't fix anything. I find that a breaker is popped, so I reset it and everybody is happy again. It is
getting pretty late so I put the machine away.

Any ideas? Everything that was on, kept working. Now I don't know know if it was the field or output breaker but I am guessing by
the flash of light that it was the main.

Pretty hard to trouble shoot a problem that isn't there now, but I am at least going to take a look at the back of the panel with a
mirror for signs of arcing just in case something is loose and bouncing around. (interestingly this happened during one of my better
landings)

Where I was raised, blown breakers aren't something to be ignored.

Mike Z.

Thanks.

Mechanic is ordering a new breaker from Edmo.

He of course asked if I knew for sure if that was it. "No, but everything is working and you have to start somewhere."

Hopefully get the new one installed before it pops again and blows something else up.

Mike Z


wrote in message ...
You need to replace the main 60 amp breaker. This is a thermal type
breaker and as it gets older the contacts build up oxidation and self
heat more reducing the breaker rating.
You are lucky!! When my 60 amp breaker popped one night the load
dump fried the alternator field, the voltage regulator and the over
voltage lamp.
I measured the 60A breaker and it would pop at 35 amps.
Cessna wants lots of money for the 60 amp breaker but it still is lots
cheaper than a new alternator and regulator.
On a C172M the 5 amp field breaker will NOT protect the alternator
field winding since it is not in series with the alternator field. It
protects the over voltage relay that draws only 0.1 amps.

While I agree with the probably diagnosis that it's most likely
the breaker, it disturbs me to see yet another person blindly replacing
components. My mechanic is that way, replacing/overhauling things that
are not necessarily bad, just because if they were it could cause the
problem. From what I've seen, there's a pretty big lack of
troubleshooting ability from a lot of people who are just *used* to
spending lots of money on possibly unnecessary parts.

That said, sounds like it probably is your breaker... hopefully
that'll fix it. I'd pull out an ammeter and power supply (or battery with
different loads) and test for current tripping point. I'd imagine the
main breaker is $100 or more.

YMMV
-Cory

Mike Z. wrote:
: Thanks.

: Mechanic is ordering a new breaker from Edmo.

: He of course asked if I knew for sure if that was it. "No, but
everything is working and you have to start somewhere."

: Hopefully get the new one installed before it pops again and blows
something else up.

: Mike Z


--
************************************************** ***********************
* The prime directive of Linux: *
* - learn what you don't know, *
* - teach what you do. *
* (Just my 20 USm$) *
************************************************** ***********************

wrote in message ...
You need to replace the main 60 amp breaker. This is a thermal type
breaker and as it gets older the contacts build up oxidation and self
heat more reducing the breaker rating.
You are lucky!! When my 60 amp breaker popped one night the load
dump fried the alternator field, the voltage regulator and the over
voltage lamp.
I measured the 60A breaker and it would pop at 35 amps.
Cessna wants lots of money for the 60 amp breaker but it still is lots
cheaper than a new alternator and regulator.
On a C172M the 5 amp field breaker will NOT protect the alternator
field winding since it is not in series with the alternator field. It
protects the over voltage relay that draws only 0.1 amps.

The 60 amp breaker can be bought from someone other than
Cessna, like Aviall, for a lot less cash.
The 5 amp breaker in the 172M feeds the regulator and
therefore the field. We have two of these airplanes. The overvolt
sensor disrupts the feed if the voltage gets too high.
A good way to burn out an alternator field is to turn the
master on when preflighting, then forget to turn it off. The field is
being fed full voltage (since the alternator isn't producing anything)
and gets really hot (since the alternator isn't turning and drawing
cooling air through itself).
If the original poster's breaker popped during a landing, I
might expect to find some loose bit of hardware dancing around behind
the panel and tangling with the bus bar. Wouldn't be the first time.

Dan

The 5 amp field breaker in the 1976 Cessna 172M does NOT have the
alternator field current flow through it. It has only the field relay
coil current flow through it which is about 1/10 of an amp.
The alternator field current comes from the output terminal of the
alternator through the field relay and the switching transistor in the
regulator. There is NO fuse or breaker in this circuit unless you
call the 60 amp alternator breaker the field breaker.
Look it up, you will be surprised. The schematic of the regulator is
shown in the Cessna "Alternator charging Systems"Manual. The
airframe wiring is shown in the airframe service manual.
Cessna made a mistake in my opinion.
Ask your self what will happen if the 60 amp breaker opens under
nearly full load. Will the field switching transistor that normally
operates at 14 volts be able to take the 90+ volts without shorting?
Will the over voltage relay be able to break the 90+ volts DC on a
relay that has contact spacing designed to break 14 volts?
If the above two answers are no then you have just applied 90 volts or
more to the field winding and the regulator. I can tell you this it
does not take long to fry the regulator and the field into a crispy
critter once this occurs. I know for sure that if this happens just
after take off you will not be able to come back and land before the
regulator and the alternator is fried.
The 14 volt over voltage light is really bright with 90 volts applied,
but not for long!

When I was looking for the 60 amp breaker the largest I could find in
that case size was 50 amps so I had to buy the Cessna over priced
part. This circuit really needs a 70 amp breaker since the 60 amp
rated alternator will generate more than 60 amps when the windings are
cold.

You are correct that if the breaker popped during landing then there
may be something loose in the wiring or a sharp metal edge cutting
through a wire's insulation.
John Frerichs

On 4 Nov 2003 17:55:59 -0800, (Dan Thomas)
wrote:

wrote in message ...
You need to replace the main 60 amp breaker. This is a thermal type
breaker and as it gets older the contacts build up oxidation and self
heat more reducing the breaker rating.
You are lucky!! When my 60 amp breaker popped one night the load
dump fried the alternator field, the voltage regulator and the over
voltage lamp.
I measured the 60A breaker and it would pop at 35 amps.
Cessna wants lots of money for the 60 amp breaker but it still is lots
cheaper than a new alternator and regulator.
On a C172M the 5 amp field breaker will NOT protect the alternator
field winding since it is not in series with the alternator field. It
protects the over voltage relay that draws only 0.1 amps.

The 60 amp breaker can be bought from someone other than
Cessna, like Aviall, for a lot less cash.
The 5 amp breaker in the 172M feeds the regulator and
therefore the field. We have two of these airplanes. The overvolt
sensor disrupts the feed if the voltage gets too high.
A good way to burn out an alternator field is to turn the
master on when preflighting, then forget to turn it off. The field is
being fed full voltage (since the alternator isn't producing anything)
and gets really hot (since the alternator isn't turning and drawing
cooling air through itself).
If the original poster's breaker popped during a landing, I
might expect to find some loose bit of hardware dancing around behind
the panel and tangling with the bus bar. Wouldn't be the first time.

Dan

wrote in message ...
The 5 amp field breaker in the 1976 Cessna 172M does NOT have the
alternator field current flow through it. It has only the field relay
coil current flow through it which is about 1/10 of an amp.
The alternator field current comes from the output terminal of the
alternator through the field relay and the switching transistor in the
regulator. There is NO fuse or breaker in this circuit unless you
call the 60 amp alternator breaker the field breaker.
Look it up, you will be surprised. The schematic of the regulator is
shown in the Cessna "Alternator charging Systems"Manual. The
airframe wiring is shown in the airframe service manual.
Cessna made a mistake in my opinion.

I looked it up, and you are right. My mistake. A failed field
breaker will only open the field relay, disconnecting the alternator
output from the field. I guess the 5-amp breaker is a throwback to the
generator days, when some had the regulator output run through a
breaker or fuse on the way to the field.
I agree with the 60-amp output breaker opening causing big
problems. We've duplicated that here in the shop on a demonstration
circuit. The airplane would be safer if it was wired to disconnect the
field when the output breaker pops.

Dan

Mike,

Had the same thing happen to me on a 172M - the circuit breaker for the ALT
went south. Once it was replaced, no further problems. Incidently, I was on
a downwind leg when it went out - made for an interesting landing - safe but
interesting :-).

Harry
PP-ASEL

"Mike Z." wrote in message
ink.net...
Ok, here is one for you.

The other night, about the time I landed, I hear a pop and remember seeing
a visible flash. I see the amp gauge has a big discharge.

After I taxied off, I try resetting the alt half of the master. The high
volt light is dimly lit and glows brighter when I turn off
half the master but that doesn't fix anything. I find that a breaker is
popped, so I reset it and everybody is happy again. It is
getting pretty late so I put the machine away.

Any ideas? Everything that was on, kept working. Now I don't know know if
it was the field or output breaker but I am guessing by
the flash of light that it was the main.

Pretty hard to trouble shoot a problem that isn't there now, but I am at
least going to take a look at the back of the panel with a
mirror for signs of arcing just in case something is loose and bouncing
around. (interestingly this happened during one of my better
landings)

Where I was raised, blown breakers aren't something to be ignored.

Mike Z.

Thanks Harry,

I flew last night with no problem but I will feel better after the new breaker is installed.

After seeing the damage reports from other folks blowing the output breaker, I am not really willing to wait and see, although
waiting until it is really broke or doing some honest test and troubleshoot would be my normal reaction.

Mike Z


"Harry Gordon" wrote in message ...
Mike,

Had the same thing happen to me on a 172M - the circuit breaker for the ALT
went south. Once it was replaced, no further problems. Incidently, I was on
a downwind leg when it went out - made for an interesting landing - safe but
interesting :-).

Harry
PP-ASEL