Voltage variation

Roy Smith wrote:
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.

Quite possibly. However, if I _don't_ say that, somebody will try this
test with jumper cables for one meter and two 500 foot spools of 30 gauge
wire for the other meter, and then write me an indignant email when they
buy a new meter and its reading still doesn't match the other one.

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.

Precision is cheap. Accuracy costs a little more.

Matt Roberds

Anytime you are looking for electrical problems, you need to
keep in mind Ohm's Law. Voltage is equal to resistance times Current.
Reading voltages at various places in the airplane gives you little
information other than that the conductor is not entirely
disconnected, unless you remember that current flow affects voltage
drop.
I once forgot this principle and spent money that didn't need
spending. The strobe on the tail wasn't working, so I checked to see
that there was power available to it by pulling the feed wire off,
measuring the voltage there, and finding battery voltage. OK. Must be
the strobe's power supply shot. Bought another ($500) and installed
it. It didn't work, either. Then I remembered: E=IxR. With the strobe
turned on (but not working, of course) I took voltage measurements at
the bus (OK), at the circuit breaker's output input terminal (OK), at
the breaker's output terminal (not OK!) and realized that the breaker
was internally defective. There was full battery voltage there with
the power supply disconnected, because the meter requires only a few
microamps to drive it and the hundred ohms or so in the breaker's
corroded contacts wasn't enough to drop the voltage to it. Connecting
the power supply introduced a much lower resistance, increased the
current demand to two or three amps, and caused a huge voltage drop so
that the strobe was dead. The airplane needed a $15 breaker, not a
$500 power supply.

Dan

On Sep 24, 9:03 am, wrote:
Anytime you are looking for electrical problems, you need to
keep in mind Ohm's Law. Voltage is equal to resistance times Current.
Reading voltages at various places in the airplane gives you little
information other than that the conductor is not entirely
disconnected, unless you remember that current flow affects voltage
drop.
I once forgot this principle and spent money that didn't need
spending. The strobe on the tail wasn't working, so I checked to see
that there was power available to it by pulling the feed wire off,
measuring the voltage there, and finding battery voltage. OK. Must be
the strobe's power supply shot. Bought another ($500) and installed
it. It didn't work, either. Then I remembered: E=IxR. With the strobe
turned on (but not working, of course) I took voltage measurements at
the bus (OK), at the circuit breaker's output input terminal (OK), at
the breaker's output terminal (not OK!) and realized that the breaker
was internally defective. There was full battery voltage there with
the power supply disconnected, because the meter requires only a few
microamps to drive it and the hundred ohms or so in the breaker's
corroded contacts wasn't enough to drop the voltage to it. Connecting
the power supply introduced a much lower resistance, increased the
current demand to two or three amps, and caused a huge voltage drop so
that the strobe was dead. The airplane needed a $15 breaker, not a
$500 power supply.

Dan

I should add: Many batteries get replaced because the engine
doesn't crank very well. You need to take voltage drop measurements
across the master and starter solenoids while cranking; you'll often
find that their contacts are burned and introducing resistance with
the large current flow. Resistance checks mean nothing on these
things; even a quarter ohm will cost a bunch of voltage, and with
large current flows heat is generated, which increases the resistance
further. Same goes for cable and terminal crimps and connections.

Dan

writes:


I should add: Many batteries get replaced because the engine
doesn't crank very well. You need to take voltage drop measurements
across the master and starter solenoids while cranking; you'll often
find that their contacts are burned and introducing resistance with
the large current flow. Resistance checks mean nothing on these
things; even a quarter ohm will cost a bunch of voltage, and with
large current flows heat is generated, which increases the resistance
further. Same goes for cable and terminal crimps and connections.

Yes. I measure, for example:

a) On the battery term.
b) On the lug
c) On the wire in the lug.(That's why you have a sharp pointy probe.)
....

The hassle is, you have to do this under load, which usually means
"cranking" which means various dangerous parts are moving...


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