Shunts... For the last time, I swear!

[marcusSRG]

Ok, I made a post a while back regarding various electronics switches and shunts. That was a good thread, but the discussion went more towards switches and phono jacks. I'm still hung on shunts a little but I think I got it...I just want to get a couple of things cleared up.

I know a shunt is basically a safety switch along the connection between a timer/accelerometer channel output and the rocket's airstart/staging igniter(s). This switch can be opened or closed, depending on setup, to safe the igniter(s) from an accidental output from the electronics...

BUT, doesn't a shunt still allow continuity of the outboards to be verified by the electronics WHILE it's engaged?? I attached a crude schematic of two possible "igniter safing" setups...you'll see I'm no electrical engineer!



So...isn't picture "A" a true shunt? Here the shunt would be "engaged" when the switch is closed....and the electronics should be able to verify continuity, right?? In picture "B", the switch would inhibit a continuity check for sure...but would a good TRA RSO consider "B" to be a proper shunt??

I plan to apply all of this stuff to my LOC Ultimate and 429-SS rebuilds for airstart flights. I will be using a PET2 timer. To conform with TRA code I'll be safing my outboard igniters with shunts (as advised here) as well as utilizing the PET2's pull-pin AND g-switch arming feature to lock out the timer until the rocket is prepped, as demonstrated by dixontj93060 with his two stager here: https://www.rocketryforum.com/showthread.php?21235-Two-Stager

Thanks.

 

[CarVac]

Picture B is not just "not a true shunt", but rather not a shunt at all.

You can avoid all of this trouble by just inserting the upper stage igniters on the pad.

 

[cerving]

BTW, has anyone actually tried testing a shunt on a Q2G2 and a 9v battery to see if it will still fire? I calculated that a 16 ga shunt should prevent it from firing, assuming that the Q2G2 wire is 8" and the 24 guage stranded wire to the altimeter is another 8". If I have a chance this weekend, I want to give it a try... one I get some extra Q2's to burn.

 

[marcusSRG]

Picture B is not just "not a true shunt", but rather not a shunt at all.

You can avoid all of this trouble by just inserting the upper stage igniters on the pad.

Cool. That's what I thought. As for inserting the igniters at the pad, don't you have to do this anyway?? Or 'in a designated area'?

 

[CarVac]

If you insert the igniters before the rocket is on the rail, then they are required to be shunted.

 

[mikec]

If you insert the igniters before the rocket is on the rail, then they are required to be shunted.

For TRA, perhaps. There is no such requirement for NAR and we went through a rather extensive discussion of this whole topic earlier; see https://www.rocketryforum.com/showthread.php?47156-Request-Photos-of-shunts-and-grounding-charges

Installing igniters in an upper stage on the rail is not practical in some cases and has its own risks; hence the NAR designated area rule.

 

[FredA]

Disconnect and shunt using a double-pole switch.
Disconnect to altimeter so that it can't fire the ematch regardless of the resistive divider.
Shunt so that stray RF is killed by the short.

Shunt only MAY STILL FIRE since some current will still flow in the ematch.

 

[CarVac]

For TRA, perhaps. There is no such requirement for NAR and we went through a rather extensive discussion of this whole topic earlier; see https://www.rocketryforum.com/showthread.php?47156-Request-Photos-of-shunts-and-grounding-charges

Installing igniters in an upper stage on the rail is not practical in some cases and has its own risks; hence the NAR designated area rule.

Not relevant for this thread; the OP is following Tripoli rules.

 

[cerving]

It might be a good idea to forego the very-low current ematches in favor of something that takes a bit more current to light, this would probably be safer with a shunt provided the shunt's resistance was very low (16 ga wire or heavier should do it). This is all about accepted practice, what commercial igniters are typically being used for airstarts with AP motors? From what I have read, it seems like there are a lot of people out there (maybe the majority...) are doing this with homemade igniters; there appears to be a lot of variation in the amount of current that's required to fire them.

Disconnect and shunt using a double-pole switch.
Disconnect to altimeter so that it can't fire the ematch regardless of the resistive divider.
Shunt so that stray RF is killed by the short.

Shunt only MAY STILL FIRE since some current will still flow in the ematch.

 

[bobkrech]

A is a shunt, B is a connect/disconnect.

The igniters should not fire if the shunt is installed. A ground test will verify if this is so.

If you power up the altimeter with the shunt in place, the altimeter will think there is a good e-match/igniter installed even if there is none, or the e-match/igniter is bad. Many altimeter only check for continuity during the powerup check, so if you powerup with the shunts connected and then disconnect them and you e-match/igniter is bad, you will have no indication of it. It is best to raise your rocket to the launch position, remove the shunts and power up the altimeters. This way the altimeter will detect a bad e-match/igniter and signal you that there is a problem.

Bob

 

[jderimig]

A is a shunt, B is a connect/disconnect.

The igniters should not fire if the shunt is installed.

Bob

If a "perfect" shunt is installed an e-match should not fire. However there is no perfect shunt. Assume a 0.01ohm shunt in parallel with a 1 ohm ematch. Perfectly possible with switch or dirty contacts. Discharge a 10 amp source (easily possible from a charged capacitor bank) and 100ma will flow through the ematch. Possibly Poof!

Source the power with stiff power source like a LiPo and the igniter AND shunt will burn.

Shunts only with low resistance igniters are NOT safe.

 

[marcusSRG]

Thanks, guys. That's exactly what I wanted to know!

 

[FredA]

Shunts only with low resistance igniters are NOT safe

Like I said...disconnect and shunt with double-pole switches.
It's EASY.
It's reliable -- pyro outputs fire for 100's of milliseconds....contact opening due to vibration [if any] may subtract from this time, but the match will still have plenty of time to fire....

 

[Handeman]

As others have stated, a shunt is a cross connection between the ematch/igniter leads so RF can't cause them to fire. That is the way some ematches are shipped, the ends twisted together. A DPDT switch should be used so the source is isolated from the ematch/igniter and the leads of the ematch/igniter are shorted together. The attached drawing shows the switch and how it should be wired. The key is the connection indicated by the red line. That is the connection that is made when the switch is in shunt position.

You will not be able to do continuity testing while in shunt position, but the timer is also isolated so there is no chance of current firing a match during continuity tests either.

 

[FredA]

I'm always amazed at how many people can't wire a switch!
Not to poke at specific people here....it's just something I find over and over....

This is why I say we should not make our electronics any more complicated than necessary!

Here is how to wire a double-pole switch to disconnect and shunt a pyro output.

 

[bobkrech]

I'm always amazed at how many people can't wire a switch!
Not to poke at specific people here....it's just something I find over and over....

This is why I say we should not make our electronics any more complicated than necessary!

Here is how to wire a double-pole switch to disconnect and shunt a pyro output.

View attachment 116199

Not quite right. This is.

Bob

 

[troj]

Not quite right. This is.

View attachment 116215

Not everyone reads electric diagrams; the difference being to be sure to short it to ground correct?

-Kevin

 

[FredA]

Duh......Thanks Bob....see even I get the diagram wrong....that's what I get for trying to answer quick before going out the door....

Kevin -- ground is a relative term.....most altimeters don't identify which output, if any, is ground.
So don't assume one is ground...
Just disconnect one leg and short the e-match.

 

[UhClem]

Shorting the igniter may or may not make it safer.

While it protects against stray voltages it makes the igniter more susceptible to magnetic induction. So if you do short the igniter, you should use twisted pair wire between the short and the igniter to minimize the loop area.