Advice on periodic voltage fluctuation - C172

Recently I have noted an in-flight system voltage fluctuation in my
C-172K. The fluctuations are quasi-periodic, with an amplitude of about
0.4 volts and a period that varies between about 1.6 and 1.8 minutes.
The varations are contained within a band between 14.0/13.9 and
13.6/13.5 volts (the monitoring system quantizes to +/- 0.1 volts).

I have posted a graph of these observations for two 1.5 hour flights at
http://home.comcast.net/~argon39/Vol...luctuation.pdf

The battery is less than a year old (conventional Pb-acid Concorde),
the voltage regulator is a quite new Zeftronics unit. I replaced the
regulator about a year ago when I first noticed fluctuations (the
replaced regulator was a 1969 vintage electromechanical unit that was
original equipment).

I know of no quasi-periodic electrical load that has a similar duty
cycle, and thus the observations would seem to point to something in
the alternator (brushes and diodes in the alternator have not been
worked on in quite some time).

Any ideas about what might produce these quasi-periodic changes? Thanks.

Us Cherokee types also have an overvoltage regulator to deal with.

One other simple cause may be the master switch. The portion that
connects alternator to battery carries a fair amount of current and the
contacts get pitted and corroded over time. A bad connection can
result in voltage fluctuations.

I thought of that too since I had a problem and replaced the
Masterswitch. I do not have the split one. But is seemed the oscillation
was too slow. Mine you could see on the ammeter like it was waveing to
you. I also cleaned every ground I could get my hands on.


Regards, Ross
C-172F 180HP
KSWI


Paul kgyy wrote:
Us Cherokee types also have an overvoltage regulator to deal with.

One other simple cause may be the master switch. The portion that
connects alternator to battery carries a fair amount of current and the
contacts get pitted and corroded over time. A bad connection can
result in voltage fluctuations.

On 2 Aug 2005 07:59:29 -0700, "argon39" wrote:

Recently I have noted an in-flight system voltage fluctuation in my
C-172K. The fluctuations are quasi-periodic, with an amplitude of about
0.4 volts and a period that varies between about 1.6 and 1.8 minutes.
The varations are contained within a band between 14.0/13.9 and
13.6/13.5 volts (the monitoring system quantizes to +/- 0.1 volts).

I have posted a graph of these observations for two 1.5 hour flights at
http://home.comcast.net/~argon39/Vol...luctuation.pdf

On my PA28-180, I had a somewhat similar problem which I diagnosed to
be a bad circuit breaker causing varying voltage (IR) drops in the
voltage regulation circuit. The circuit breaker was exhibiting slight
and varying resistances when force was exerted on the case of the
breaker. Aiframe vibration and turbulence was apparently enough to
change the force on the breaker, which randomly varied the resistance.

Your .pdf data looks like it is very badly aliased because of the
apparently low sample rate such that it really should not be used to
make any frequency decisions. Sorta the same as the wheels on the
stagecoach wagons going backwards in old western movies.

Do you see ammeter or voltmeter fluctuations? Might there be a halogen
beacon operating at the meter frequency?

Your .pdf data looks like it is very badly aliased

Good point, these data are at or near the Nyquist. I'll set the JPI
recorder to 2s intervals and collect some data this way to see how
different it looks.

Do you see ammeter or voltmeter fluctuations? Might there be a halogen
beacon operating at the meter frequency?

No halogen beacon, just an old-fashioned incandescent blinker at about
a 3s duty cycle. No strobes, either. The JPI is my only voltmeter.
Ammeter bounces around a bit in concert with the beacon. Other current
draws are the usual suspects, 2 Garmin radios, Garmin transponder and
audio panel. These were both ay flights so only the beacon was on all
the time and taxi/landing lights on for takeoff and landing.

argon39 wrote:

Ammeter bounces around a bit in concert with the beacon. Other current
draws are the usual suspects, 2 Garmin radios, Garmin transponder and
audio panel.

Just a wild guess, but maybe the heater in the transponder encoder cycles at
that frequency?

George Patterson
Give a person a fish and you feed him for a day; teach a person to
use the Internet and he won't bother you for weeks.

The HEATER in a transponder??????

If you are thinking of the heater in the altitude encoder (a) they are small
watt units and (b) most of them are analog.

Jim



"George Patterson" wrote in message
news:jk5Ie.6765$2y2.6178@trndny02...
argon39 wrote:

Ammeter bounces around a bit in concert with the beacon. Other current
draws are the usual suspects, 2 Garmin radios, Garmin transponder and
audio panel.

Just a wild guess, but maybe the heater in the transponder encoder cycles
at that frequency?

George Patterson
Give a person a fish and you feed him for a day; teach a person to
use the Internet and he won't bother you for weeks.

RST Engineering wrote:

If you are thinking of the heater in the altitude encoder .....

That's what I said.

George Patterson
Give a person a fish and you feed him for a day; teach a person to
use the Internet and he won't bother you for weeks.