>Saw your remark about testing for a shorted diode in an
>alternator and wonder how you would do it.
When an alternator's diodes are intact, the "ripple"
voltage riding on the alternator's output is only about
5% peak-to-peak of the 14 volt nominal or something on
the order of 700 millivolts. When a diode opens or
shorts, the ripple voltage raises markedly. Over the
years, several products have been offered as "alternator
failure" lights that monitor ripple voltage on the system
and illuminate a warning lamp if ripple voltage becomes
markedly higher.
If you own a digital mulimeter, you can set the device
to read AC volts and connect it to the bus of your airplane.
Turn on all the loads you can at cruise and note the AC
volts value at some time when you're reasonably certain that
all is well with your alternator. At some time in the future,
you want to confirm the health of your altenator, repeat
the load/rpm condition. A bad diode will more than triple
the AC ripple voltage reading.
My personal favorite for alternator health testing involves
a portable load bank of fat power resistors or a combination
of light bulbs and the necessary switches to add or remove
load in usefully small steps. I've built a simple loadbank
using halogen headlamp bulbs in an approx 5" rectangular
tube made of plywood. With a 12VDC muffin fan in each end,
there was plenty of air movement to keep things cool. Eight
H4 (100W) halogen bulbs and an H3 (55W) lamp would allow me
to create a load of over 60 amps in steps of about 4 amps.
Suitable wires are used to attach this fixture right to the
back of the alternator leaving the normal aircraft wiring
in place. Oh yeah, I mounted a DC voltmeter and an ammeter
on my little plywood box to watch what happend when I
added load. The voltmeter had a switch that would let me
go back and forth between looking at the alternator output
voltage and alternator field voltage. Make the wires long
enough that you don't have to stand right behind the prop
during engine runup.
Run the engine up and MINIMIZE aircraft systems loads.
Increase the load in smallest steps noting when the voltage
falls below 13.0 volts. The voltage should not sag this
low until you've added a load very close to the alternator's
rated output. If you have a bad diode, you generally can't
get much more than 1/2 the alternator's rated output.
This simple piece of test equipment is better than an
alternator test stand examination of your hardware.
You can do a complete diagnostic of the alternator system
BEFORE putting a wrench to the airplane. Any mechanic who
doesn't probe the field voltage of an alternator while
sujecting it to some considered loads and RPMs does
not understand how the thing works. He/she may make an
educated guess as to the cause of your problem but for
my money, I wouldn't let them get out the toolbox until
they explain exactly what's wrong and why they think so.
Far too many airplanes leave the shop with parts they
didn't need. The equipment I've described above combined
with techniques describe here and in the Appendix to
our book (http://www.aeroelectric.com/errata/z.pdf)
can make the task easier and effective.
>Also would a decent dvm be ok or do you need an "iron vane" type meter?
I prefer analog meters . . . but they don't need to be
iron vane. Analog meters can offer a sense of dynamics
you cannot see well with digital meters. Our ammeter/
voltmeter combo would be a good candidate for building
diagnostic instrumentation right into a home made
loadbank.
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