Van's Air Force
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Crowbars and circuit breakers
- Thread starter Bavafa
- Start date
AlexPeterson
Well Known Member
Not at all. It puts a dead short on the circuit, tripping the breaker.
https://en.wikipedia.org/wiki/Crowbar_(circuit)
https://en.wikipedia.org/wiki/Crowbar_(circuit)
Alex,
Can one use a circuit breaker in place of a crowbar? As I understand it, a crowbar is to protect a circuit and isn?t what a CB does?
You were closer on hour first post. Crowbars protect against over voltage, and also trip the CB. CBs are just sensitive to current.
Sam Buchanan
been here awhile
No, a breaker trips when current exceeds the rating of the breaker, the OV module reacts to voltage.
Toobuilder
Well Known Member
Just thinking out loud here....
So if one was to use a crowbar short of the field control wire, the CB pops or the fusible link burns through preventing the alternator from making power and thus, its a "hard" fault - it would take a separate and distinct action to reset. Works great as long as the field control circuit remains functional through all alternator failure modes.
However, if the OV device controls a relay on the output side of the alternator, then it seems like opening the output lead would allow the system voltage to return to normal, triggering a relay reset and output connection and subsequent reoccurrence of the OV condition. I can see a scenario where the "trip, reset, trip, reset" cycle would cause plenty of problems. Is a latching relay the solution here?
So if one was to use a crowbar short of the field control wire, the CB pops or the fusible link burns through preventing the alternator from making power and thus, its a "hard" fault - it would take a separate and distinct action to reset. Works great as long as the field control circuit remains functional through all alternator failure modes.
However, if the OV device controls a relay on the output side of the alternator, then it seems like opening the output lead would allow the system voltage to return to normal, triggering a relay reset and output connection and subsequent reoccurrence of the OV condition. I can see a scenario where the "trip, reset, trip, reset" cycle would cause plenty of problems. Is a latching relay the solution here?
Carl Froehlich
Well Known Member
No one would wire an alternator output solenoid as you discribe.
The output solenoid is powered from a 5amp or so breaker. The OV Crowbar module, when tripped, shorts across this breaker so it opens. When it opens, the alternator output solenoid has power removed so it opens as well, isolating the alternator from aircraft power. You must take action to reset the breaker, thus you will not get a ?cycle?.
Carl
Toobuilder
Well Known Member
Fair enough, but is such a scheme "acceptable" aviation practice? This is a question out of ignorance, not argument.
It seems that the desired result is many components removed from the initial fault. The OV event triggers the OV relay to crowbar the output relay power to ground which causes the CB to open, and theoretically, the output relay opens, disconnecting power from the ship. Thats a lot of electromechanical hardware in the loop, all in series. Since every single component must funtion to get the desired result, it seems like such a scheme drives the probability of failure up considerably. Not to mention the time delay required for all these dominos to fall.
If this is common practice in the world of electronics, I'll shut up and color. It just offends my sense of simplicity.
It seems that the desired result is many components removed from the initial fault. The OV event triggers the OV relay to crowbar the output relay power to ground which causes the CB to open, and theoretically, the output relay opens, disconnecting power from the ship. Thats a lot of electromechanical hardware in the loop, all in series. Since every single component must funtion to get the desired result, it seems like such a scheme drives the probability of failure up considerably. Not to mention the time delay required for all these dominos to fall.
If this is common practice in the world of electronics, I'll shut up and color. It just offends my sense of simplicity.
If you really want simple, the old Grumman singles (derived from the BD-1) used a single NED (Noise Emitting Diode - makes a very loud noise when connected backwards across a sufficiently large potential) for the task. Of course, this took out your reset capability inflight.....
Carl Froehlich
Well Known Member
Few people use alternator output solenoids at this point. Typically the only reason why you would is because you are using an auto alternator that has an internal regulator but does not have OV protection - so you have to add it. This was fairly standard for those not using something like the a B&C alternator and external regulator, perhaps 17-20 years ago.
So - a simple breaker switch for your alternator field/source voltage (B&C or Plane Power unit) and you are done.
Carl
rv7charlie
Well Known Member
Yep, that and an extra $500 is all you need to be 'simple'.
All-in-one regulators like the B&C are excellent products, but they are anything but 'simple'.
edit: And 'few' depends on who you're counting.
All-in-one regulators like the B&C are excellent products, but they are anything but 'simple'.
edit: And 'few' depends on who you're counting.
