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Crowbar Circuit; testing and related findings inquiry

Freemasm

Freemasm

Well Known Member
Was going to start a poll but guess I can't without a badge. I prefer to support below the RADAR.

This is not an electrical architecture philosophy debate.

Your related responses to the following would be appreciated:

- I test my crowbar regularly (at least annually). Never detected an issue
- I test my crowbar regularly. Have discovered issues
- I don't test/didn't know

The basis for this comes down to the Garmin LRUs with redundant power inputs for second power sources; not those with additional inputs because of pin current limitations. Some here do not advocate because of voltage spike fears. I'm trying to gage that potential risk. Much thx.
 
Interesting. So regarding crowbar circuits, no one tests/verifies operation/detected a malfunctioning one?

Edit = B&C technical manual is a bit wishy-washy IMO. Instructs how to verify function after install. Then it states the OV protection “can be” tested at the CI. While the approach and components make for a simple and predictably reliable system, it protects a pretty big investment. I don’t know why they didn’t state testing as “mandatory” at CI and “highly suggested” on an opportunity basis
 
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Freemasm said:
Interesting. So regarding crowbar circuits, no one tests/verifies operation/detected a malfunctioning one?
Click to expand...
I'll admit to testing mine one time, kind of accidentally. The crowbar circuit (inside the B&C voltage regulator) caused the field breaker to pop open after a small amount of water shorted out contacts in the ignition switch. (Way too windy to install canopy cover....rained that night...small amount of water got in.....longer story.)

But as for regularly testing it intentionally, no, never did that. I did test it per the B&C instructions when first built.
 
Freemasm said:
Was going to start a poll but guess I can't without a badge. I prefer to support below the RADAR.

This is not an electrical architecture philosophy debate.

Your related responses to the following would be appreciated:

- I test my crowbar regularly (at least annually). Never detected an issue
- I test my crowbar regularly. Have discovered issues
- I don't test/didn't know

The basis for this comes down to the Garmin LRUs with redundant power inputs for second power sources; not those with additional inputs because of pin current limitations. Some here do not advocate because of voltage spike fears. I'm trying to gage that potential risk. Much thx.
Click to expand...
The B&C regulator with built in crowbar is easy to test, you are not actually applying an OV to the system so obviously no danger.
Not sure how you test with a PP alt with 'built in' OV protection (supposedly) or an external B lead mounted unit.
 
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i am interested to hear where others mounted their crowbar in the circuitry. mine is in the lead from the alternator that powers the main bus.
 
Freemasm said:
Interesting. So regarding crowbar circuits, no one tests/verifies operation/detected a malfunctioning one?

Edit = B&C technical manual is a bit wishy-washy IMO. Instructs how to verify function after install. Then it states the OV protection “can be” tested at the CI. While the approach and components make for a simple and predictably reliable system, it protects a pretty big investment. I don’t know why they didn’t state testing as “mandatory” at CI and “highly suggested” on an opportunity basis
Click to expand...
To what point? This is experimental aviation; even if they made it “mandatory” people still may or may not do it.

One thing we don’t need, though, is more “mandatory” rules in the experimental community; if you want that, there is always the certified world.
 
rocketman1988 said:
To what point? This is experimental aviation; even if they made it “mandatory” people still may or may not do it.

One thing we don’t need, though, is more “mandatory” rules in the experimental community; if you want that, there is always the certified world.
Click to expand...
Yeah, not giving you that one. Stronger words should have been used in the tech manual, IMO. One isn't obligated to do it but if the B&C VR is accessible, the check takes less than a minute and consumes no materials if CBs are employed.

From the lack of responses, I'm going to guess that most aren't doing it. When something gets fried, in an OV event, someone is going to be looking to blame the OEM.

All that said, I really wanted to get an idea of how dependable these were. Looking like there isn't enough experience and any data points available going forward may have big price tags associated with them.
 
Here is a way to test an overvoltage module using two "D" cell batteries. Note polarity of D cells. OV Test.png
 
Freemasm said:
Yeah, not giving you that one. Stronger words should have been used in the tech manual, IMO. One isn't obligated to do it but if the B&C VR is accessible, the check takes less than a minute and consumes no materials if CBs are employed.

From the lack of responses, I'm going to guess that most aren't doing it. When something gets fried, in an OV event, someone is going to be looking to blame the OEM.

All that said, I really wanted to get an idea of how dependable these were. Looking like there isn't enough experience and any data points available going forward may have big price tags associated with them.
Click to expand...
Sorry, not a fan of more mandates and regulation our our community. That is one of the reasons I left the certified community.

If you want to test at the CI, there is nothing stopping you. There is also nothing to prevent the OV protection to fail on the next flight after the test…

Certainly not saying don’t test; just that it not be mandated.
 
Basic crowbar circuit (TR1 is not part of the crowbar) https://www.electronics-notes.com/a...ac/overvoltage-protection-crowbar-circuit.php
So ... 4 components.
While I do have an EE degree I don't recall how to calculate MTBF for circuits, but there's little to fail here. Especially since the SCR never conducts unless the crowbar fires. More likely to have a wire break / fall off than a component failure.

In fact when the crowbar fires it will dump what ever power is available thru the SCR. Hopefully the fuse (or breaker) will blow before the SCR does. See where I'm going with that?

I have a crowbar, and frankly I've never thought to test it subsequent to my initial test.

Oh, and if you put a crowbar on the alternator output (instead of the F lead), be prepared for it to get abused with over 60 amps for a while, until the main alternator fuse blows. A standard fuse (i.e. not a 'fast blow') can require *double* its rated current to open in 1 second. Can my 60 amp alternator even put out 120 amps to a dead short? Will the crowbar tolerate 90 amps for multiple seconds? A typical SCR will have a 1.0 volt drop (i.e. not a perfect switch), so 90 amps will be 90 watts of power to dissipate in heat.

Now i want to go grab an old SCR and run 90 amps through it... and film the results! I suspect there will be a loud noise. :unsure:
 
blaplante said:
Basic crowbar circuit (TR1 is not part of the crowbar) https://www.electronics-notes.com/a...ac/overvoltage-protection-crowbar-circuit.php
So ... 4 components.
While I do have an EE degree I don't recall how to calculate MTBF for circuits, but there's little to fail here. Especially since the SCR never conducts unless the crowbar fires. More likely to have a wire break / fall off than a component failure.

In fact when the crowbar fires it will dump what ever power is available thru the SCR. Hopefully the fuse (or breaker) will blow before the SCR does. See where I'm going with that?

I have a crowbar, and frankly I've never thought to test it subsequent to my initial test.

Oh, and if you put a crowbar on the alternator output (instead of the F lead), be prepared for it to get abused with over 60 amps for a while, until the main alternator fuse blows. A standard fuse (i.e. not a 'fast blow') can require *double* its rated current to open in 1 second. Can my 60 amp alternator even put out 120 amps to a dead short? Will the crowbar tolerate 90 amps for multiple seconds? A typical SCR will have a 1.0 volt drop (i.e. not a perfect switch), so 90 amps will be 90 watts of power to dissipate in heat.

Now i want to go grab an old SCR and run 90 amps through it... and film the results! I suspect there will be a loud noise. :unsure:
Click to expand...
i have read that a crowbar on the f lead may not stop current output in certain alternator failure modes, like an internal short. my set up feeds power from the alternator thru a 100 amp relay. a detected over voltage will only stop current holding the relay closed. whether or not there is wear on the contactors of the relay i do not know. i do turn off the alternator [f lead] before shutdown to prevent the relay possibly doing any arcing from a current thru it when opened.likewise i turn on the alternator before engine start.
when i did my crowbar i really found a lot of conflicting info [b and c too] . this was more than 5 yrs ago.
 
bobnoffs said:
i have read that a crowbar on the f lead may not stop current output in certain alternator failure modes, like an internal short. my set up feeds power from the alternator thru a 100 amp relay. a detected over voltage will only stop current holding the relay closed. whether or not there is wear on the contactors of the relay i do not know. i do turn off the alternator [f lead] before shutdown to prevent the relay possibly doing any arcing from a current thru it when opened.likewise i turn on the alternator before engine start.
when i did my crowbar i really found a lot of conflicting info [b and c too] . this was more than 5 yrs ago.
Click to expand...
True, a crowbar on the F lead in some situations will not shut down the alternator.
Glad to hear you are actually running an OV protection relay, not directly using a crowbar on the B+ lead.
Hoping that no one thinks they should put the crowbar circuit directly on the B+ lead because that would be 'better'.

Interestingly the only OV failure I've heard about locally was a certified plane that was grounded as the mechanic was working on the charging system. CFI and student somehow missed the fact that it was grounded. Started up, did run up, took off, got smoke in the cockpit, made emergency landing. All the radios and GPS were fried. I'm sure OV events happen, but this is the only one I know about first hand.
 
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