Voltage variation

I still haven't had time to work with my A&P to remove my (newly)
flakey JPI EDM-700 unit. (As you may recall, some of the LED segments
that make up the number display are going in and out.)

Someone here suggested that it might be due to low voltage, possibly
indicating an alternator problem. I've started watching the voltage,
and have noticed something odd:

The JPI has its own built-in voltage indicator. It will usually read
13.4 or 13.5 volts. We also have a stand-alone, panel-mounted digital
voltage/amperage meter, mounted a few inches below the JPI. It
usually reads a few tenths of a volt different than the JPI. (I can't
remember if it's up or down...)

a) Is this normal? Why would they read different voltages?

b) What voltage is normal? Is 13.4 too low?

Thanks!
--
Jay Honeck
Iowa City, IA
Pathfinder N56993
www.AlexisParkInn.com
"Your Aviation Destination"

"Jay Honeck" wrote in message
ups.com...
I still haven't had time to work with my A&P to remove my (newly)
flakey JPI EDM-700 unit. (As you may recall, some of the LED segments
that make up the number display are going in and out.)

Someone here suggested that it might be due to low voltage, possibly
indicating an alternator problem. I've started watching the voltage,
and have noticed something odd:

The JPI has its own built-in voltage indicator. It will usually read
13.4 or 13.5 volts. We also have a stand-alone, panel-mounted digital
voltage/amperage meter, mounted a few inches below the JPI. It
usually reads a few tenths of a volt different than the JPI. (I can't
remember if it's up or down...)

a) Is this normal? Why would they read different voltages?

There are numerous reasons, which might be additive:

Gauge calibration.
Different wire paths to the gauges leading to different voltage drop before
the electrons get to the gauge..
Different wire sizes, leading to different voltage drop.
Corrosion in connections.
Loose connections.
Etc.


b) What voltage is normal? Is 13.4 too low?

13.4 is a *little* low, but that could easily be a gauge calibration issue
or a voltage regultor that needs a slight tweak.


Thanks!
--
Jay Honeck
Iowa City, IA
Pathfinder N56993
www.AlexisParkInn.com
"Your Aviation Destination"

Jay Honeck writes:


The JPI has its own built-in voltage indicator. It will usually read
13.4 or 13.5 volts. We also have a stand-alone, panel-mounted digital
voltage/amperage meter, mounted a few inches below the JPI. It
usually reads a few tenths of a volt different than the JPI. (I can't
remember if it's up or down...)

a) Is this normal? Why would they read different voltages?

Yes normal. Many reasons, all of the don't bother ilk.

b) What voltage is normal? Is 13.4 too low?

That is low. A charging L-A battery will be at/near 14.4v.
You'll have some drop between it & the buss, but...

....Under what load do yue see 14.4? How long after starting?

What is the voltage at moderate RPM with no loads,
& with the JPI on so you can see its reading?

With every frigging thing on, including the Jacuzzi and the landing
lights? [We'll skip cycling the gear....]

--
A host is a host from coast to
& no one will talk to a host that's close........[v].(301) 56-LINUX
Unless the host (that isn't close).........................pob 1433
is busy, hung or dead....................................20915-1433

b) What voltage is normal? Is 13.4 too low?

That is low. A charging L-A battery will be at/near 14.4v.
You'll have some drop between it & the buss, but...

...Under what load do yue see 14.4? How long after starting?

That's 13.4 or .5 in cruise flight, with two nav/coms and strobes on.
Probably turning 2300 RPM, say, ten minutes after launching.

What is the voltage at moderate RPM with no loads,
& with the JPI on so you can see its reading?
With every frigging thing on, including the Jacuzzi and the landing
lights? [We'll skip cycling the gear....]

Dunno. I'll have to record these, next time I fly.

The sealed battery is maybe a year old. Cranks just fine.
--
Jay Honeck
Iowa City, IA
Pathfinder N56993
www.AlexisParkInn.com
"Your Aviation Destination"

On Sun, 23 Sep 2007 13:31:10 -0700, Jay Honeck
wrote:

I still haven't had time to work with my A&P to remove my (newly)
flakey JPI EDM-700 unit. (As you may recall, some of the LED segments
that make up the number display are going in and out.)

Someone here suggested that it might be due to low voltage, possibly
indicating an alternator problem. I've started watching the voltage,
and have noticed something odd:

The JPI has its own built-in voltage indicator. It will usually read
13.4 or 13.5 volts. We also have a stand-alone, panel-mounted digital
voltage/amperage meter, mounted a few inches below the JPI. It
usually reads a few tenths of a volt different than the JPI. (I can't
remember if it's up or down...)

a) Is this normal? Why would they read different voltages?

b) What voltage is normal? Is 13.4 too low?

Thanks!



I have found that the JPI is very sensitive to voltage. I have a
persistent voltage drop in my system which is exacerbated by putting the
landing gear up or down. This causes a large current draw by the gear
pump motor, which due to resistance somewhere, causes a voltage spike
downwards. My JPI reboots as a result of this, and is the only actual
symptom. Other avionics are unaffected, so this problem is not severe,
but it does bug me.

This effect has been noted by many members of Cardinal Flyers Online
over the years, as I found when I was researching it.

I used to have an inflight voltage of under 13.0 when all electricals
were on, or maybe 13.3 with the "usual" load. I have had some work done
on my electrical system, including replacing old components and wiring,
which cleared the problem up for a time. Now my voltage is a steady
13.7, but I think this is still somewhat low, and the JPI reset problem
is starting to occur again.

In principle this is not hard to troubleshoot. With the engine running
and a full electrical load, simply measure the voltage starting at the
alternator and moving back to the avionics buss and battery, at all
possible points. At the alternator it should be over 14.0 I think. This
should isolate where there is a voltage drop, and therefore too much
resistance.

So I think 13.4 is too low, at least it is in my plane. I would like to
see 14.0. And you should determine exactly where the JPI and your panel
voltmeter are connected, because a few tenths of a volt difference is
certainly pointing to excess resistance between them.




randall g =%^) PPASEL+Night 1974 Cardinal RG
http://www.telemark.net/randallg
Lots of aerial photographs of British Columbia at:
http://www.telemark.net/randallg/photos.htm
Vancouver's famous Kat Kam: http://www.katkam.ca

Jay Honeck writes:

...Under what load do yue see 14.4? How long after starting?

That's 13.4 or .5 in cruise flight, with two nav/coms and strobes on.
Probably turning 2300 RPM, say, ten minutes after launching.

What is the voltage at moderate RPM with no loads,
& with the JPI on so you can see its reading?
With every frigging thing on, including the Jacuzzi and the landing
lights? [We'll skip cycling the gear....]

Dunno. I'll have to record these, next time I fly.

The sealed battery is maybe a year old. Cranks just fine.


OK, I asked "how long" because the voltage post-cranking may
be lower.

The real question is: what've the voltage at the battery terminals?
But that's hard to measure in flight...




--
A host is a host from coast to
& no one will talk to a host that's close........[v].(301) 56-LINUX
Unless the host (that isn't close).........................pob 1433
is busy, hung or dead....................................20915-1433

randall g writes:


In principle this is not hard to troubleshoot. With the engine running
and a full electrical load, simply measure the voltage starting at the
alternator and moving back to the avionics buss and battery, at all
possible points. At the alternator it should be over 14.0 I think. This
should isolate where there is a voltage drop, and therefore too much
resistance.


The problem is; on a car you have to dodge the radiator fan, but if I'd
ever worked on a plane (not me!!!! That would Be Wrong..) I'd wonder if
that big fan out front did not make life lots harder... and THAT's while
it is sitting still.

But a good test is, after the voltage is stable [ie the starting drain
has been recouped..] is to go from no load to fax load while looking at
the voltage.

What we are asking is: what's the voltage drop between the battery
and the avionics buss? If it is drop in the wiring, looking at no load
will show a voltage ~identical to that at the battery.



--
A host is a host from coast to
& no one will talk to a host that's close........[v].(301) 56-LINUX
Unless the host (that isn't close).........................pob 1433
is busy, hung or dead....................................20915-1433

Jay Honeck wrote:
The JPI has its own built-in voltage indicator. It will usually read
13.4 or 13.5 volts. We also have a stand-alone, panel-mounted digital
voltage/amperage meter, mounted a few inches below the JPI. It
usually reads a few tenths of a volt different than the JPI.

a) Is this normal? Why would they read different voltages?

Disclaimer: This is based on experience with ground vehicles and
equipment. I don't have an A&P; I don't even have a TG&Y. Your
mileage may vary.

Probably. The reasons have been well covered by other posters. If the
JPI is coming out soon, it might be interesting to pull the other meter
as well. Connect both of them to a 12 V battery sitting on a table -
with the same gauge and lengths of wire - and see what they read. If
they disagree under "ideal" conditions, they'll never agree in the
plane. (Alternatively, leave the other meter in the panel, but
disconnect the wires to the plane temporarily, and hook up wires from
the meter to a stand-alone battery.)

If the meters don't agree, the next question is "How do you know which
one is right?" The absolutely correct answer is to measure a known
standard voltage, and the pretty good answer is to compare to a
known-accurate meter. The trick is to find either of these things.
I guarantee that one (probably both) of them exist up in Cedar Rapids
at that little radio company up there. Getting to _use_ them is
another story. A new or nearly-nuke Fluke DMM with a fresh battery is
probably the easiest thing to get your hands on that has a decent chance
of being accurate.

"A man with a watch knows what time it is. A man with two watches is
never sure." -- Segal's Law, as quoted at
http://tycho.usno.navy.mil/clocks.html

b) What voltage is normal? Is 13.4 too low?

It depends. If you're measuring at the battery terminals, anything
below about 12.6 or 12.7 V with the engine running at cruise speed
is too low - this means the alternator/generator is most likely not
charging the battery. Anything above about 15.0 V measured under the
same conditions is too high. Anything in between could be considered
"normal". Some numbers that get thrown around for a nominal system
voltage for a 12 V battery include 13.8 V and 14.0 V. I have a mid-60s
King nav/com (hollow state!) that lists 13.75 V on the name plate.

If you want to get specific, AC 43.13-1A Change 3, paragraph 442, says
"The voltage drop in the main power wires from the generation source or
the battery to the bus should not exceed 2 percent of the regulated
voltage, when the generator is carrying rated current or the battery is
being discharged at the 5-minute rate." There is also a table that
shows the "maximum acceptable voltage drop in the load circuits between
the bus and the utilization equipment." For a 14 V nominal system, this
is 0.5 V for continuous operation and 1 V for intermittent operation.
In other words, if your generator or alternator is designed for a
regulated voltage of 14.0 V, the minimum bus voltage would be 13.72 V,
and the minimum voltage you would expect at a continuously-powered piece
of equipment would be 13.22 V. If the regulated voltage is 13.8 V,
these change to 13.52 V at the bus and 13.02 V at the equipment.

In practical terms, if you want to power something from a 12 V nominal
vehicle electrical system, it should probably _work_ on anything from
about 11.0 V to 15.0 V, _survive_ anything from 10.99 V down to zero,
and not explode immediately if fed more than 15.0 V. This is somewhat
dependent on how much current the thing draws - it's a little easier
to make a low-current device work over a large range of input voltages
than a high-current one. It also depends on intended application. It's
OK if your in-flight DVD player cuts out at 12.0 V; your landing light
should still work down to 9.0 or 10.0 V in case you have to land at
night with a dead alternator and discharged battery.

Disclaimer: This is based on experience with ground vehicles and
equipment. I don't have an A&P; I don't even have a TG&Y. Your
mileage may vary.

Matt Roberds

In article ,
wrote:
If the JPI is coming out soon, it might be interesting to pull the other
meter as well. Connect both of them to a 12 V battery sitting on a
table - with the same gauge and lengths of wire - and see what they
read.

The bench test sounds fine, but the same gauge and lengths of wire part is
kind of silly. If these things are pulling more than a 100 mA, I'd be
surprised. Worrying about the voltage drop along a couple of feet of any
kind of wire you're likely to have laying around is just silly.

#18 copper wire is 7 mili-ohm/ft. If this thing were drawing 100 mA, there
would be a 0.0007 V drop along the wire. The problem reported was one
reading being off by several 1/10ths of a volt. Don't worry about matching
wire gauges and lengths on the bench.

As for a reference voltage source, any reasonable DVM you can buy at Radio
Shack fo $20 will be more than accurate enough for this kind of work.

Jay Honeck wrote:

I still haven't had time to work with my A&P to remove my (newly)
flakey JPI EDM-700 unit. (As you may recall, some of the LED segments
that make up the number display are going in and out.)

Someone here suggested that it might be due to low voltage, possibly
indicating an alternator problem. I've started watching the voltage,
and have noticed something odd:

The JPI has its own built-in voltage indicator. It will usually read
13.4 or 13.5 volts. We also have a stand-alone, panel-mounted digital
voltage/amperage meter, mounted a few inches below the JPI. It
usually reads a few tenths of a volt different than the JPI. (I can't
remember if it's up or down...)

Your Lowrance 2000C has a voltage indicator that can be configured on a
screen, it will replace one of your other displayed options...