Screw switch fail

[amiliv]

Walk all the way down to pads in crazy heat... Just for Eggtimer TRS to report no continuity on ematch when armed. Walk back with the rocket in hand. In comfort of air conditioned room this is what I found under the switch:



Little goblins stole some copper. Or whatever else happened on the previous (picture perfect) flight to that PCB trace from terminal block to the screw switch part of uh screw switch. Nothing looks burnt or melted. Just peeled off. Odd.

At least TRS forces you to do continuity check as precondition for arming it for flight.

 

[rocket_troy]

I have seen that before with PCBs - it's normally the result of a short that virtually "explodes" the copper away at the most vulnerable point. You don't necessarily see obvious evidence of excessive heat around the failure although very close inspection of the very ends of the peeled trace(s) can often shed some light of whether it's a mechanical tear or whether a stampede of rampant electrons were the culprit. Are you running the timer off a Lipo? Is it possible the squib from the previous flight might have shorted somehow after firing? You don't by any chance still have that pyro from the previous flight?

TP

 

[amiliv]

Nope, that ematch went into trash bag two months ago. Though I find it plausible that it may have shorted after firing. Yup, it was LiPo, 300 mAh, 2-cell, 35-70C. So if it shorted, that would have put a lot of current through that switch. I guess, if something had to give way, better a cheap switch than expensive altimeter.

 

[Arpak]

This may be another argument for an inline resistor on pyro channels when using a lipo?

 

[manixFan]

I think rocket_troy is correct in his assessment - I’ve seen exactly the same thing when too much current went through a trace and the heat destroyed the bond and breaks the copper. So more of a symptom of too much current than a failure. I use current limiting resistors now on bigger lipo batteries. If you haven’t flown that altimeter yet you should test the outputs of the firing channels to make sure they’re still ok.

Tony

 

[amiliv]

Yup. I do plan to test it. The altimeter is probably fine. It's Eggtimer TRS, and the firing channels are completely electrically isolated (via optoisolators) from the rest of the altimeter. They don't even share common ground, and they are on a separate battery.

 

[manixFan]

It’s the firing output I’m concerned about. I looked through the TRS doc and could not find a max rating for the outputs. We’ve managed to fry opto’s in a launch firing circuit when the leads were shorted together and the LCO held the firing button down. So the altimeter may seem to be ok but the outputs may not fire.
Was that a switch to the altimeter power or the pyro power? I assume pyro based on your comments.


Tony

 

[amiliv]

Yup. The switch was on the pyro power, to comply with the new NAR/Tripoli safety regulations.

 

[Arpak]

Yup. The switch was on the pyro power, to comply with the new NAR/Tripoli safety regulations.

Which ones, did I miss something?

 

[Voyager1]

Which ones, did I miss something?

These ones!
https://www.rocketryforum.com/threa...le-regarding-wireless-remote-switches.157373/

 

[Voyager1]

I have seen that before with PCBs - it's normally the result of a short that virtually "explodes" the copper away at the most vulnerable point. You don't necessarily see obvious evidence of excessive heat around the failure although very close inspection of the very ends of the peeled trace(s) can often shed some light of whether it's a mechanical tear or whether a stampede of rampant electrons were the culprit. Are you running the timer off a Lipo? Is it possible the squib from the previous flight might have shorted somehow after firing? You don't by any chance still have that pyro from the previous flight?

TP

Definitely a short. You'll notice that it's on the narrower PCB track because it couldn't handle the short circuit current, so it went first. When I use those screw switches, I cut the end with the two rectangular solder pads off. I then mount the PCB with two screws that also act as the terminal posts either side of the switch screw. That gives you a little more current capacity without relying on asymmetric PCB tracks. I agree that this is a good argument for installing current-limiting resistors.

 

[amiliv]

Which ones, did I miss something?

Wireless switches are not sufficient anymore. There needs to be either physical switch, or the battery needs to be disconnected until you get the rocket next to the pad (doesn't need to be upright on the pad). That's why I have physical switch on the TRS (which has built-in wireless switch). Having a switch on the battery for pyro satisfies this.

 

[OverTheTop]

I am a bit late to the party here, but yes, it looks like an overcurrent event. You can bolster the current capacity of the tracks by soldering on a chunky piece of tinned-copper wire between the pads. If you are feeling energetic you could remove the solder mask and solder it to the track along the way too. Probably overkill IMHO, although it does depend on what gauge of TCW you use.

 

[jimzcatz]

Another reason why I NEVER use switches!!

 

[SecondRow]

Wireless switches are not sufficient anymore. There needs to be either physical switch, or the battery needs to be disconnected until you get the rocket next to the pad (doesn't need to be upright on the pad). That's why I have physical switch on the TRS (which has built-in wireless switch). Having a switch on the battery for pyro satisfies this.

1. That was a TRA only rule. NAR never adopted it.
2. That rule was overturned.

https://m.facebook.com/groups/cjscjn/permalink/10160313934639046/

 

[Worsaer]

From Tripoli’s official site:

http://www.tripoli.org/WirelessRemoteSwitches

 

[FredA]

Another reason why I NEVER use switches!!

Um....the switch wasn't the primary failure - it just showed that there is a major problem.
Don't throw the baby out with the bath water.

 

[djs]

Maybe related (or not), but I've seen some corrosive activity on screw switches if they're exposed to ejection charges in some way. Usually in my case it's gunk that needs to be cleaned off, but I could also see it eating at the traces to the point where it's mechanically easier to have them go kafloopy like in this picture.

 

[Pete.D]

Another reason why I NEVER use switches!!

The switch didn’t fail: the copper trace acted as a fuse in that it protected associated components (like battery, connectors and active components) from damage due to excessive current, which was due to a short circuit elsewhere in the system. The current would have been in excess of 20 Amps, limited mainly by the internal resistance of the battery. If that copper trace hadn’t melted another one would have, or the battery would have caught fire.

 

[Arpak]

These ones!
https://www.rocketryforum.com/threa...le-regarding-wireless-remote-switches.157373/

Wireless switches are not sufficient anymore. There needs to be either physical switch, or the battery needs to be disconnected until you get the rocket next to the pad (doesn't need to be upright on the pad). That's why I have physical switch on the TRS (which has built-in wireless switch). Having a switch on the battery for pyro satisfies this.

Ah gotcha, good thing I've been using switches alongside wireless devices regardless, personally I like being able to decide when the thing is sucking down battery power. Thanks for the info.

 

[Bill Hanson]

From Tripoli’s official site:

http://www.tripoli.org/WirelessRemoteSwitches

As the TRA site states, those rules have been rescinded.

In February 2020, the Tripoli BoD adopted a rule requiring a physical disconnection from power for any electronics controlling energetics. At the time they felt this was required by NFPA 1127.

After further discussion during its September meeting, the board of directors has concluded that additional physical disconnects are not required and the February 2020 rule has been canceled.

The rule requiring that energetics and firing circuits be inhibited until the rocket is in firing position remains in place.

 

[amiliv]

As the TRA site states, those rules have been rescinded.

Ah. I missed that one. Thanks for the update. It means I could just bypass the physical switch since it's redundant (TRS has wireless switch built in).