The theory of shunting

I'll admit it. I am not the brightest bulb when it comes to electrical matters. I have been trying to wrap my head around the shunting thing. On larger projects, I sometimes, not always, will have two keyed switches. One between the battery and altimeter and the other between the altimeter and ematch or igniter. From what I have been reading shunting is different, or is it? I consider myself to be pretty safety minded. I will test the altimeter and outputs prior to installation. Once installed I'll test again before RSO. Both switches remain off until loaded on the rail. So is there more that needs to be done? Currently working on a Nike-Apache scratch build two stage. I want to make sure that it will pass inspection when it comes to launch. I will be using a dedicated Raven for sustainer ignition and another Raven to control sustainer dual deploy. I plan on using individual keyed switches for both but not the secondary switch. Hopefully that won't be a RSO problem.

So if anyone can help me understand this shunting thing I would appreciate. You might have to draw me a picture a picture or two, maybe three.

I would love to know about this concept. From what I understand, shunting is shorting out the ematch/igniter. I have no idea what this accomplishes and I have never done it. I will say however, when setting up an electrically fired fireworks show, all the squibs come with the ends of the wire stripped and twisted. Same idea????
-Ken

I have read many threads on TRF looking for a consensus on this topic and so far there seem to be two camps, shorting a circuit (specifically in reference to an e-match or igniter) or creating an open circuit (specifically in reference to the power source). I may not be using the correct terminology but you get the idea.

It would be nice to understand these concepts better, so I look forward to reading the responses.

The safest way to safe the igniter firing circuit is a combination of opening the circuit and shorting the igniter.

Step one is to open the circuit between the energy source and the igniter. That prevents a failure in the firing circuit from causing trouble.

Next is to short the igniter. This protects you from stray electric fields that might provide sufficient voltage to the open circuit. But shorting the circuit creates a low impedance loop which couples nicely to magnetic fields. So you must also use twisted pair wire between the shorting device and the igniter in order to minimize magnetic coupling.

A double pole double throw switch will do the trick. The commons are connected to the igniter, one set of contacts connects to the firing circuit and the other is simply shorted. In one position the igniter is connected to the firing circuit and in the other it is completely isolated and shorted. If you go this route a 100K resistor from each firing wire to ground is in order to bleed off static charge.

This is the way that you have to do it keep a national range happy. But they have much nastier EMI environments than we do. Nice big megawatt radars bathing the launch site and that sort of thing. Simply opening the circuit is usually sufficient for us.

In the old days, the primary igniter used was often a flashbulb. These could be set off by static electricity, so shunting was important. Proper electric matches have a rated "no-fire current" below which they will not trigger so this is no longer as critical.

Even today if a shunt is in place and the electronics mistakenly fire the output, the charge will travel through the low-resistance shunt path instead of the electric match. For example, if a unit is somehow turned on before being set upright on the pad, it could detect launch and go into armed mode. (Again, older electronics were more sensitive to these sorts of errors.)

JohnCoker said:

Even today if a shunt is in place and the electronics mistakenly fire the output, the charge will travel through the low-resistance shunt path instead of the electric match.

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Current will flow through both the shunt and the electric match. This is especially true if you are using a very low output impedance power source like high 70C discharge LiPo's. As UhClem posted, shunts offer additional protection only when used in conjunction with an open switch.

jderimig said:

Current will flow through both the shunt and the electric match. This is especially true if you are using a very low output impedance power source like high 70C discharge LiPo's. As UhClem posted, shunts offer additional protection only when used in conjunction with an open switch.

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^^^This.^^^

An easy wat to think about shunts.

electricity always take the easiest course to ground.

having a Shunt in place allows a pathway for any current " an easier course" since even if its minute there is more resistance in the e-match than in the shunt, so if there were to be some kind of power sent to the "e-match" etc that current would take the "short" cut (hence the name) to where its shunted to

its just another method of protection!

Here is video of a shunt test using a 300ma 30C Lipo battery. Enjoy

https://youtu.be/sasoJB_P0Rg

Here's my video. https://youtu.be/Sf9JqrsIUY4

I used a 1S, 130 mah LiPo and a Brand-X e-match as these are the components, along with a Raven-3, I use for my multi-stage flights. I purposely use a match with long leads and shunt near the source and I tightly twist the shunt together with pliers to minimize its resistance.

Sorry for fumbling connecting the battery at first. One connected I held in place for 4 to 5 seconds. Didn't want to risk longer, as it was I could feel the battery heating through the gloves. The match lit immediately once the shunt was cut.

This was a worse case test as in real life, the battery connects through the Raven which would further limit current and hopefully self-destruct before the lipo catches fire should the Raven fail on power-up in such a way as to pass full battery current to the match.

Along with John's video, this shows the importance of ground testing using test a configuration that match your real-world flights. Safety features are useless unless proven by actual testing.

I have read the posts and feel that I have a little better understanding. I can see you really need to understand the concept before putting it in practice. Ground test, ground test, ground test. Just wondering if I were to bring my 2-stage Nike Apache to Airfest this year (I probably won't have it finished) with just a keyed switch or redundant switches would I be allowed to fly? I guess my main curiosity with shunting is seeing if it will prevent me from flying two stage rockets. I have never used any shunts in anything that I have flown. I have flown rockets on motors ranging from C-L. Alot of them using electronics.

Just wondering if Dave Schultz has any video, pictures or drawings of wiring incorporating shunts and switches. Professor Dave your students await.

..deleted due to error in post

FondyKevin said:

Just wondering if Dave Schultz has any video, pictures or drawings of wiring incorporating shunts and switches. Professor Dave your students await.

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Nyet.

My favorite altimeter is the AltAcc which includes an arming switch. The 9V battery is installed which powers up the altimeter and the altimeter is then mounted in the rocket. The arming switch disconnects power from the outputs until it is closed.

AltAcc2C manual


For staging I have used an GWiz LC Deluxe with two external switches. One for power and the other for the pyro power source. Both had to be closed to enable the outputs.

No shunts.

Part of my "hands-on education" as a pyrotechnician, (which is mostly OTJ, by the way) was the difference between open circuit and static-immune shunted.

...No contest. Shunted is safer.

With that said, I pre-make my charges at home, and transport them to the launch site shunted; they fly open circuit.

All the best, James