Stratologger & 4 ematches

I need to set off 4 ematches at the same time. I use a lipo battery 2s 850mah. Will it work or am I likely to blow the alt up.


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Stratologger can pass 10Amp so it will be okay, but protection circuit in lipo battery may shut the battery off. How much current are you trying to source and do you have a datasheet for the battery?


-Alex

Wire the ematches with 2 ematches in series in parallel with another 2 ematches in series. The load will appear to your altimeter and battery exactly the same as 1 ematch.
And it will work perfectly as designed (as long as you are using commercial ematches).

Pyropetepete said:

I need to set off 4 ematches at the same time. I use a lipo battery 2s 850mah. Will it work or am I likely to blow the alt up.


Sent from my iPhone using Rocketry Forum

Click to expand...

what are the e-match specs?

Bob

Not all Lipo batteries have protection circuits. The ones typically used for RC do not have the circuit.

All Fire Current: 0.5 Amps.
No Fire Current: 50mA

Resistance of the e-match?

I don't like the in series method for this if redundancy is considered, because if one opens up, the entire chain is dead. For this application, I prefer 4 in parallel, but better yet, 2 in parallel on each of 2 independent altimeters.

For a series parallel arrangement, if the match failure rate is 1%, the failure rate of a chain is 2%, and 2 chains in parallel are (2%)^2 = 0.04%.

For the 2 or 4 parallel arrangement, if the match failure rate is 1%, the failure rate of 2 in parallel is (1%)^2 = 0.01%, and 4 in parallel is (1%)^4 = 0.00001%

You also need to consider the failure rate of the altimeter. I

1f it's 1% for a single altimeter, the system failure rate is 2% with 1 e-match and 1% with 2 parallel e-matches.

1f it's 1% for a per altimeter in a dual altimeter system, the system failure rate is 0.04% with 1 e-match per altimeter and 0.01% with 2 parallel e-matches per altimeter.

To answer your current question. for 2S Lipo ~ 8 volts. Current thru 1 e-match is I = V/R.

If R = 1 ohm, I = 8 amps, if R = 2 ohm, I = 4 amps, if R is 3 ohms, I = 2.7 amps, if R= 4 ohms, I = 2.5 amps.

With 4 in parallel, if R = 1 ohm, I = 4 x 8 = 32 amps, if R = 2 ohms, I= 4 x 4 = 16 amps, if R = 3 ohms, I = 4 x 2.7 = 10.8 amps, if R = 4, I = 4 x 2.5 = 10 amps.

1S Lipo ~ 4 volts. Current thru 1 e-match is I = V/R.

If R = 1 ohm, I = 4 amps, if R = 2 ohm, I = 2 amps, if R is 3 ohms, I = 1.3 amps, if R= 4 ohms, I =1.25 amps.

With 4 in parallel, if R = 1 ohm, I = 4 x 4 = 16 amps, if R = 2 ohms, I= 4 x 2 = 8 amps, if R = 3 ohms, I = 4 x 1.3 = 5.4 amps, if R = 4, I = 4 x 1.25 = 5 amps.

Bob

bobkrech said:

Resistance of the e-match?

I don't like the in series method for this if redundancy is considered, because if one opens up, the entire chain is dead. For this application, I prefer 4 in parallel, but better yet, 2 in parallel on each of 2 independent altimeters.

Click to expand...

Another point to consider is with a series connection the continuity check function of the altimeter is preserved. That is why it is standard practice for the pyro guys to wire the ematches in series, that is how they are designed to be used. In parallel wiring the altimeter will report good continuity even if up to 3 matches are open. Usually ematches don't open up in-flight, they are either open to begin with (which the continuity check will detect) or they have continuity but the pyrogen doesn't ignite. In the latter case the other ematches in the chain will still see current and get a chance to fire. You don't want to throw away the continuity check function that the altimeter designers have provided for you.

Also an open is not the only hazard. If you have 2 ematches in parallel and one is shorted the other won't fire.

I would have recommended 4 in series for this reason but I would like 2X the required voltage across the change. A 2S Lipo would not satisfy this requirement if the ematch resistance is above 1 ohm each.

If two channels are available I would recommend 2 ematches in series per channel. The continuity check is preserved and the lower current draw will be easier on the fets.

If only one channel is available then 4 in parallel is but you are totally blind continuity check wise. Also if your lipo has a protection circuit you are much more likely tripping that with 4 in parallel than with a series or series-parallel arrangement.

Can't argue against your approach if the probability of an e-match continuity failure after launch is very small. If an after lift-off open is really significantly lower probability than a no-fire, then is would be preferred, but I haven't seen any data on that.

As you stated, the great advantage of the series configuration is a lower current draw than the parallel configuration however as you also stated you really want at least 2x times the all-fire current (voltage) to insure fast activation of the e-match so you really need to know the e-match resistance.

Also as we both pointed out, two channels or 2 independent altimeters offer redundancy which reduces the probability of a non-event.

Bob

If i understand correctly what I read on pyro forums, after firing, e-matches are likely to break the circuit or increase resistance from 1-2ohm to 100+ohm.

I'm curious, If e-matches are wired in series, is it possible for one e-match to fire a tad earlier and prevent other e-matches from reaching ignition temperature?


-Alex

azzie said:

If e-matches are wired in series, is it possible for one e-match to fire a tad earlier and prevent other e-matches from reaching ignition temperature?
-Alex

Click to expand...

Anything is possible but they (commercial matches) are designed not to do that. Standard practice in the pyrotechnics industry is series wiring, sometimes hundreds in the chain, very high reliability in that mode.

Edit: All these recommendations are assuming commercial high quality ematches like MJG where there is miminal parameter variation match to match. If you are using homemade or matches of questionable origin all bets are off.

To add to what John said, an e-match will have activated long before the bridgewire opens. Once current is applied to a string of e-matches, commercial e-matches will all experience the same current, which generates the same power in each match head so the rate of temperature rise is about the same in each match. Match compound ignites between 300-400C and once it ignites it doesn't need any more power to continue to burn. The bridge wire won't melt till about 1400 C, so there's a large margin in temperature.

The reason why you want your e-matches in series for blasting is that you don't want unexploded charges in your debris. If you have series continuity in your string, once powered, all matches fire. Also the e-match run is long, possibly over several thousand feet with over 100 matches. Wire has resistance and I = V/R. Blasting typically uses a capacitive discharge firing system to send several hundred volt pulse of several amps down the wire to fire the string. E-match activation is typically in a millisecond or so.

Bob