Noise Problem. Both Comms Breaking 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.