Runaway Generator ROTAX 912ULS

Eric

Very interesting indeed. I navigated "forum.matronics.com" in order to understand how this circuits works. Found many posts about OVP OVM14 and its development. Is there one post explaining how this protection works ?

Thanks,
Maximo.

Hi Maximo.  It's possible that Bob never posted a complete explanation of the circuit.  Hopefully the following will make sense.  Looking at Bob's schematic:

Note that U11A and U11B are the two comparators of an LM393B, contained in a single 8-pin package.  The pin numbers shown on their two triangular symbols are shared across that 8-pin device.  The power supply (pin 8) and ground (pin 4) shown on the U11A symbol are shared with U11B.

- Bus voltage enters the circuit at the upper right corner, via a fuse or circuit breaker.  Resistor R1 limits current through U3, an LM431 precision voltage regulator.

- The two-output voltage divider composed of resistors R2, R4 and R5 serves two functions. First, it provides 2.5V on the reference pin of U3 and the inverting input of comparator U11A (pin 2) when the voltage at the top of R2 is 10V.  This drives U3 to produce a stable 10V supply for the circuit.  Second, it provides an 8V reference to the inverting input of comparator U11B (pin 6).

- Under normal operation (no overvoltage condition), the voltage divider composed of resistors R7 and R8 produces a voltage lower than 2.5V, which holds the output of U11A low (output transistor on).  This pulls capacitor C12 to ground and ensures that the non-inverting input of comparator U11B is lower than 8V.  Thus, the output of U11B is low (output transistor on), pulling the gate of SCR Q15 (a TN4015H-6G) to ground, so no current can flow through it.

- When an overvoltage condition occurs (bus voltage >16.2V), the R7/R8 voltage divider provides a voltage greater than 2.5V to the non-inverting input of U11A (pin 3).  The output of U11A immediately goes high (output transistor off).  This allows capacitor C12 to begin charging through resistor R13.  It will take approximately 160mS for C12 to exceed 8V.

- If the overvoltage condition resolves in less than 160mS (i.e. a transient event), then U11A's output will switch back to low and C12 will again be discharged.  The output of U11B will stay low and Q15 will remain off.

- If the overvoltage condition persists, then C12 will continue to charge until the non-inverting input of U11B (pin 5) sees >8V.  U11B will then switch its output high (output transistor off).  This will allow the gate of Q15 to charge through resistor R14, turning the SCR on and shorting the supply to ground.  This will "crowbar" the supply, blowing the fuse or opening the circuit breaker.

- For periodic testing (as during an annual condition inspection), switch S9 is closed, which puts resistor R10 in parallel with R7.  This biases the output of the R7/R8 voltage divider upward slightly, such that U11A will "trip" at a lower bus voltage of about 13.5V.  With the battery master switch on, battery voltage is insufficient to cause a trip, but once the engine is started and the "B" charging system comes online, the circuit will immediately trip, demonstrating proper operation and gross calibration.  After this test is complete, S9 is reopened and the fuse is replaced or the circuit breaker closed, returning the system to normal operation.

It should be obvious that a pilot-accessible circuit breaker is preferable to a fuse in this system due to the hassle of replacing the fuse after every functional test.

Eric

Excelent explanation !.

I would add an SPST switch en series with the + feed of the OVM.

This could be used in the (rare) case that (with no overvoltage) Q15 or U11 fail and
the OVM shorts + with - all the time, in order to restore Gen supply to the Rectifier-Regulator.

Yes, that could work, but...

Say you're flying along and the system trips, cutting the connection from the stator to the regulator.  That event will take less than 1/4 of a second, so there's zero chance that you'll see the high voltage on your EFIS display or voltmeter.  The first indication you'll have is the EFIS alarm or alert light telling you about low voltage when the charging system stops working.

How will you know that the system tripped as a result of a Q15 or U11 failure and not a genuine overvoltage event?  Are you really willing to flip a switch that might reapply a runaway voltage condition to your avionics suite with all protection bypassed?  If it were my airplane, I would execute my electrical load-shedding plan to preserve remaining battery capacity, land when it's safe and convenient to do so and diagnose the problem at zero knots.

If you want to bypass the system after diagnosing a circuit component failure on the ground, there are a couple of options: 1. install the board using a 2-pin waterproof connector (Deutsch DTM or similar), then if you determine that the board has failed, just leave it unplugged until you have a replacement; or, 2. hardwire the board and if it fails, just cut the wires and remove it.  Either way, you can reset the circuit breaker and the charging system will work normally, but unprotected.

Personal opinion follows; take it or leave it... 😉

If you're going to have a pilot-accessible switch that can bypass an automatic safety system, you might as well not install the system.  The urge to flip that switch -- with no knowledge of what caused the system to trip -- will be very great.  I'm installing overvoltage protection because I believe it's important and I trust it to work properly when needed.  Otherwise I would skip it and just install what Rotax provided.  Carrying an automatic safety system around with the capability and intent to bypass it makes very little sense to me.

My apologies to the OP, Bob Kibby; we've thoroughly hijacked your thread!