Using an Eggtimer Proton with switched deployment power

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Charles_McG

Charles_McG

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Last night I converted both my Eggtimer Protons to switched deployment power. Well, not switched, exactly, one will be twist/tape, one has a shorting plug on a pigtail. A photo is in the 'What rocket stuff did you do today' thread.
https://www.rocketryforum.com/threads/what-did-you-do-rocket-wise-today.48649/page-813#post-1962937

I discovered something I didn't expect. With deployment power off, all the enabled channels read blue/on, even when they are open circuit. They only go to red/off when deployment power is connected. Cris tells me that this is functioning as designed.

Now, that flips what I think good Visual Management should be. I've come up with a Mistake Proofing plan - leave one channel enabled but open circuit. (I don't need all six for my flights.) That way I'll get a visual indication that deployment power is indeed back on (at/near the pad) AND it may even prevent me from arming the Proton until the red channel is reconfigured.

Has anyone thought of/found other procedures?
 
I'm trying to get a dispensation from the Pope to add a 10K resistor across the switch... that would allow enough trickle current for the continuity status to work properly. This behavior is due to the automotive driver chips that are used on the outputs... they need to have a tiny amount of current to report continuity status on the load (which is normally a fuel injector or spark plug coil). I hope they grant it... those parts are a key feature of the Proton, they're rated to 10A and up to 28V, they have built-in overload and thermal cutout, open-circuit and short detection, and they're rated for use in higher than average temperatures (since they're designed to be encased in a metal ECU). The datasheet doesn't have a MTBF figure, but since they're designed to be fired many millions of times over the life of an automobile, the 100 or so times that they would be likely to be fired in this application is no sweat.
 
cerving said:
I'm trying to get a dispensation from the Pope to add a 10K resistor across the switch... that would allow enough trickle current for the continuity status to work properly.
Click to expand...

If you plan to introduce that resistor into a later revision of your HW (vs. adding a big wired one to the wiring harness), consider using two series redundant resistors, spaced a bit apart to reduce chances of one shorting.
That's the approach that we used in a secondary arming system for the upper stage in a club project. In our case, the primary arming is still a mechanical switch and both are in series, so no dispensation is required. I hope you get yours.

Reinhard
 
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Resistors don't fail shorted, especially 1/8W ones with a value like 10K, but two 4.7K resistors would work too.
 
cerving said:
Resistors don't fail shorted, especially 1/8W ones with a value like 10K, but two 4.7K resistors would work too.
Click to expand...

True. But, at least in our case, they are SMD components with the terminals not far apart. Conformal coating would have achieved the same, but is inconvenient for test and rework. Also, we didn't spend time researching if they fail _always_ non shorted but instead just added the second one.

Edit: Apparently they can short, but it's less likely than failing open.
https://www.navsea.navy.mil/Portals/103/Documents/NSWC_Crane/SD-18/ResistorsFailure.pdf

Reinhard
 
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John Kemker said:
LOL!

Every resistor I've ever seen that failed, failed OPEN. Nature of the beast!
Click to expand...
Steve Shannon said:
Resistors == fuses
Click to expand...
As is so often the case, what seems like a reasonable assumption turns out not to always be the right one.


Tony

I use wire wound on my outputs with the Stratalogger and LiPo batteries to avoid over current, turns out according to the link above they have a 9% failure rate of failing short. The text does not match the chart exactly, saying "Resistors failing in the short circuit mode are rare and only accounting for 3 to 9% of all resistor failures."
resitor-failures.png
 
manixFan said:
As is so often the case, what seems like a reasonable assumption turns out not to always be the right one.


Tony

I use wire wound on my outputs with the Stratalogger and LiPo batteries to avoid over current, turns out according to the link above they have a 9% failure rate of failing short. The text does not match the chart exactly, saying "Resistors failing in the short circuit mode are rare and only accounting for 3 to 9% of all resistor failures."
View attachment 406692
Click to expand...

Thanks, Tony. Good to know.
 
Those stats are for wire wound resistors.. I’d be willing to bet the failed-shorted rate is much lower for carbon resistors.
 
That table is from a navy document. The next page down lists the overall failure rate for fixed chip film resistors (not specifying carbon v metal film) as 10e-7 to 10e-8.

I would also note that in the case of the Proton, there’s already a current limiting resistor in the continuity path. The idea is to double up on the battery side, I think. Really bare minimum current.
 
Thanks for the info on this thread. I built mine over the weekend and was confused at the result in the dual battery configuration. I briefly considered adding a resistor between the two B+ pads, but decided to research first.

Another option might be to test for voltage on the pyro circuit and alarm if not found. Could be a UI warning like the accelerometer calibration or audible.

I fell into the thread on switches. Ugh.
 
Guidance regarding the use of the Proton with external switches will be coming forthwith after the TRA/vendors teleconference on Saturday.
 

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