Noise Problem. Both Comms Breaking Squelch

nrp wrote:
Another long shot is might the master switch or master relay be
intermittent? Normally I'd think any inductive kickbacks from motors
etc would be ballasted by the battery - as long as it stays connected
to the main bus. But if there is anything intermittent in the master
contactor system, any inductive load will kick into whatever
electronics are down-circuit from the intermittent.

A classic example of this is if a master contactor should stutter
(like from a weak battery) when the starter is engaged, the collapsing
field from the starter could generate enormous voltage spikes which
could wipe out any electronics that are also turned on. This is
probably why we are told to turn off radios etc before starting.

May not be applicable, but I have a '65 Cessna that has a single master
switch (DPST). It does connect the alternator and pull in the master
solenoid. I had ammeter swings and they would get so bad as to shut down
the system. I guess it was making the OVR trip. I found quite by
accident that it was the bad (design) master switch on the alternator
side. Replaced with a really good (designed) switch and I have had no
more problems.

--

Regards, Ross
C-172F 180HP
KSWI

Peter writes:


MikeMl wrote

I personally think this is a DUMB design that causes many more problems
than it prevents.

Overvoltage crowbars are used on switching power supplies which have
instant acting short circuit protection features and whose output
power is limited by the magnetic components anyway.
.....
If one was going to do an overvoltage protector for an aeroplane, the
way to do it is to put something in series with the alternator field
winding (i.e. in series with the existing voltage regulator) which
goes open circuit when the bus voltage reaches say 32V. That will kill
the alternator output very fast.



Err, that field has an inductance of something on the order of
1H. Opening the excitation to it will do what, in the short
term? Crowbarring that field to ground & clearing the 5-10 amp field
breaker sounds like a good idea to me, given the price of avionics in a
aircraft, and the proprensity for people/Murphy to do bad things....

[I've seen car owners yank the battery cable off while the alternator is
going full tilt; "The battery only starts the car; the alternator runs
it..." The result was a 65V+ "load dump" hundreds of ms long into the
car...fried computers, dead stereos, you name it.]

Falsing is a problem with any protective system design. How fast is too
fast? How slow is too slow to save the consumers from overvoltage? It
sounds like the OVP is a shade oversensitive, but I also wonder about the
spikes that trip it. I'd first check grounds: the alternator ground, the
battery ground, the regulator ground, the engine-frame jumper.....


I have NO clue how this related to noise breaking the squelch....

David Lesher wrote:

Err, that field has an inductance of something on the order of
1H. Opening the excitation to it will do what, in the short
term? Crowbarring that field to ground & clearing the 5-10 amp field
breaker sounds like a good idea to me, given the price of avionics in a
aircraft, and the proprensity for people/Murphy to do bad things....

[I've seen car owners yank the battery cable off while the alternator is
going full tilt; "The battery only starts the car; the alternator runs
it..." The result was a 65V+ "load dump" hundreds of ms long into the
car...fried computers, dead stereos, you name it.]


I dont have an internal schematic of the LR3C to know exactly what the
crowbar does. Two possibilities:

One is that the crowbar is upstream of the regulator so it just blows
the Field Breaker, effectively just removing power from the
regulator/field, leaving whatever current is already flowing in the
field inductance to dissipate itself in its own coil resistance. There
is almost certainly a catch (snubber) diode across the field winding
oriented such that the field current decays with a time constant of RL,
where R is the coil resistance, and L is the coil inductance.

Two is that the crowbar is downstream of the regulator (directly across
the field winding). Throwing a dead short across the coil does not
change the time constant mentioned above. The current will still
continue to flow with a time constant of RL. The only way to "shut off"
the current faster is to instantaneously reverse the voltage applied to
the field winding, which a simple crowbar does not do.

Neither method (unless the LR3C is a lot more complicated than I think
it is) prevents a "load dump" as David Lesher describes it! This is why
I think it is a poor design. It is no better than a more conventional
OVP module such as used in Cessnas and Pipers which simply breaks the
connection between the Field Breaker and the input to the regulator.

Actually, I have been thinking about Steve's problem some more, and the
transient which is tripping the crowbar could well be coming from a
"load dump lite". He mentioned that his hydraulic pump is his biggest
single load, ~35A. If his other loads are about 20A, then with pump
running, the alternator is cranking out close to 55A. At the instant the
hydraulic pump turns off, the field current is like 2A. Due to the
high inductance of the field winding, it takes about a 1/4 second for
the field current to decay back down to the less than the 1A it takes to
produce an alternator output of 20A, during which the bus voltage spikes
up, held in check only by the impedance of the battery.

This could well be the event that triggers the OVP crowbar! I have seen
similar OVP triggering in a Cessna 210, where the hydraulic gear pump
motor cycles spontaneously as the pressure in the system leaks down.

I called B&C today to ask about this, and the only suggestion was to
connect pin 3 (sense input) of the LR3C as close to the battery as
possible. In other words, the battery is the spike filter of last
resort, so if there is a lot of wire (resistance) between pin 3 and the
battery, then the impedance along the wire (and in the master relay)
could allow pin 3 to see a higher voltage during the transient.

B&C does not make a 28V ACU without OVP. Pin 3 on the LR3C is used as
the sense input for both the OVP circuit and the regulator. B&C's owner
could not tell me what the effective input resistance looking into Pin 3
is. If it were high enough, you could put a series R, shunt C filter in
this wire to prevent the OVP part of the circuit from triggering when it
shouldn't. However, this could also effect the control loop dynamics of
the voltage regulator. I would try 100 Ohms in series with Pin 3 and
1000 uF (+ end to pin3 and - end to pin 7), time constant of 0.1 sec.
You might have to tweak the voltage setting after adding the filter.

The other possibility is to open the box, and put an appropriate filter
between pin 3 and the OVP circuit such that it doesn't effect what the
VR sees.

MikeMl writes:


[I've seen car owners yank the battery cable off while the alternator is
going full tilt; "The battery only starts the car; the alternator runs
it..." The result was a 65V+ "load dump" hundreds of ms long into the
car...fried computers, dead stereos, you name it.]


I dont have an internal schematic of the LR3C to know exactly what the
crowbar does. Two possibilities:

One is that the crowbar is upstream of the regulator so it just blows
the Field Breaker, effectively just removing power from the
regulator/field, leaving whatever current is already flowing in the
field inductance to dissipate itself in its own coil resistance. There
is almost certainly a catch (snubber) diode across the field winding
oriented such that the field current decays with a time constant of RL,
where R is the coil resistance, and L is the coil inductance.

I'd hope.. but is there one??

Two is that the crowbar is downstream of the regulator (directly across
the field winding).

Doesn't this assume the regulator is high-side; with the bottom of the
field grounded? I thought the consensus was they were low-side, al-la GM;
i.e. the top end of the field gets fed with +13V from the aux diodes or
battery; and the regulator is in the leg to ground.

Neither method (unless the LR3C is a lot more complicated than I think
it is) prevents a "load dump" as David Lesher describes it! This is why
I think it is a poor design. It is no better than a more conventional
OVP module such as used in Cessnas and Pipers which simply breaks the
connection between the Field Breaker and the input to the regulator.

The issue there is opening that will need a fast, high voltage device
that you can turn off. The SCR crowbar gets turned on to protect,
an easier task.

The issue with load dumps is not just the spike peak voltage; it's
how long they last...i.e. total energy. The no-battery stunt was so
harmful because the Xl of the field ensured it could not halt alternator
quickly. So it wasn't a few ms of spike, it was many hundreds of ms
worth...



Actually, I have been thinking about Steve's problem some more, and the
transient which is tripping the crowbar could well be coming from a
"load dump lite".

I don't recall the OP's remarks; did he indicate the pump cycling
was tripping the breaker? Or was it an unknown?

I agree the pump cycling could be an issue; your savior is often the
low impedance of the battery eating those spikes. If it's not, the
spikes may have gotten bigger, or the Xc of the battery lower...

One spike solution is large MOV's across the spike creator. The gotcha
there is: MOV's actually are sacrificial, as small spikes erode them
over time; large ones will cause them to explode... [If you ever shake
a "surge protected outlet strip" and it rattles; that's the ex-MOV
inside...]


I called B&C today to ask about this, and the only suggestion was to
connect pin 3 (sense input) of the LR3C as close to the battery as
possible. In other words, the battery is the spike filter of last
resort, so if there is a lot of wire (resistance) between pin 3 and the
battery, then the impedance along the wire (and in the master relay)
could allow pin 3 to see a higher voltage during the transient.

The other reason to do so is because the regulator is there to protect &
serve the battery. You don't care all that much about the bus being a
half volt too high or low; you DO care about the battery being that, and
the bus does have drop. So you want to sense near/at the battery.

But that brings a dilemma. You must have the sense line draw some
current. Why? So the regulator is sure its sense line has not become
disconnected from the battery. {If it does come loose, the regulator
says "0 volts! More power to the field, Scotty" to compensate, and
the smell of burnt silicon soon fills the area. There were some early
transistorized auto regulators that did just that; long gone I hope!} But
then, even with but a few mils of draw, if it does that for 2 months in
the hangar....your battery is dead. So it must be switched, and that....


The other possibility is to open the box, and put an appropriate filter
between pin 3 and the OVP circuit such that it doesn't effect what the
VR sees.

I wonder if an appropriate ferrite toroid on the sense lead could stop the
noise spikes from tripping the OVP without degrading regulation. I can't
recall what such toroids look like [i.e SPICE] when saturated with DC as
it would be...

David Lesher wrote:
MikeMl writes:
...There
is almost certainly a catch (snubber) diode across the field winding
oriented such that the field current decays with a time constant of RL,
where R is the coil resistance, and L is the coil inductance.

I'd hope.. but is there one??

Every transistorized regulator that I have ever looked inside of has had
a snubber diode (inside the regulator) connected across the field
winding. Both type B high-side and type A low-side regulators.

Two is that the crowbar is downstream of the regulator (directly across
the field winding).

Doesn't this assume the regulator is high-side; with the bottom of the
field grounded? I thought the consensus was they were low-side, al-la GM;
i.e. the top end of the field gets fed with +13V from the aux diodes or
battery; and the regulator is in the leg to ground.

According to B&C's website, the LR3C is a high-side, grounded-field type
of regulator.


Neither method (unless the LR3C is a lot more complicated than I think
it is) prevents a "load dump" as David Lesher describes it! This is why
I think it is a poor design. It is no better than a more conventional
OVP module such as used in Cessnas and Pipers which simply breaks the
connection between the Field Breaker and the input to the regulator.

The issue there is opening that will need a fast, high voltage device
that you can turn off. The SCR crowbar gets turned on to protect,
an easier task.

Without a snubber diode, the open-circuited voltage across the field
winding can spike to thousands of volts. You might get the field current
to cease faster, but you will be replacing lots of regulator transistors.


The issue with load dumps is not just the spike peak voltage; it's
how long they last...i.e. total energy. The no-battery stunt was so
harmful because the Xl of the field ensured it could not halt alternator
quickly. So it wasn't a few ms of spike, it was many hundreds of ms
worth...

My measurements indicate that it takes about 250msec for the field
current to decay during a load dump event.


Actually, I have been thinking about Steve's problem some more, and the
transient which is tripping the crowbar could well be coming from a
"load dump lite".

I don't recall the OP's remarks; did he indicate the pump cycling
was tripping the breaker? Or was it an unknown?

I agree the pump cycling could be an issue; your savior is often the
low impedance of the battery eating those spikes. If it's not, the
spikes may have gotten bigger, or the Xc of the battery lower...

One spike solution is large MOV's across the spike creator. The gotcha
there is: MOV's actually are sacrificial, as small spikes erode them
over time; large ones will cause them to explode... [If you ever shake
a "surge protected outlet strip" and it rattles; that's the ex-MOV
inside...]


I called B&C today to ask about this, and the only suggestion was to
connect pin 3 (sense input) of the LR3C as close to the battery as
possible. In other words, the battery is the spike filter of last
resort, so if there is a lot of wire (resistance) between pin 3 and the
battery, then the impedance along the wire (and in the master relay)
could allow pin 3 to see a higher voltage during the transient.

The other reason to do so is because the regulator is there to protect &
serve the battery. You don't care all that much about the bus being a
half volt too high or low; you DO care about the battery being that, and
the bus does have drop. So you want to sense near/at the battery.

But that brings a dilemma. You must have the sense line draw some
current. Why? So the regulator is sure its sense line has not become
disconnected from the battery. {If it does come loose, the regulator
says "0 volts! More power to the field, Scotty" to compensate, and
the smell of burnt silicon soon fills the area. There were some early
transistorized auto regulators that did just that; long gone I hope!} But
then, even with but a few mils of draw, if it does that for 2 months in
the hangar....your battery is dead. So it must be switched, and that....


The other possibility is to open the box, and put an appropriate filter
between pin 3 and the OVP circuit such that it doesn't effect what the
VR sees.

I wonder if an appropriate ferrite toroid on the sense lead could stop the
noise spikes from tripping the OVP without degrading regulation. I can't
recall what such toroids look like [i.e SPICE] when saturated with DC as
it would be...

I thought about an external L-C filter, but that could screw-up the
voltage regulator's control loop dynamics. The RC filter seems safer.

Lancair IV-P Flyer wrote:
On Apr 2, 9:26 am, MikeMl wrote:
....
Mike, given this information, what do you suggest I do next to try and
isolate the cause?

Do the static test on the LR3C using the lab power supply I outlined in
the earlier post. You can do it with the LR3C either in or out of the
aircraft! If doing it on the bench, use a 28V 1A lamp as a substitute
for the alternator field.