Current limiting resistor inline with e-match

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wonderboy

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This weekend I had a successful flight of my LOC 7.5" Doorknob. In another thread I showed how I extended the airframe to a more scale appropriate length and added a true av-bay for dual deploy. I ran redundant altimeters, a Missile Works RRC3 and RRC2L. On recovery of the rocket, I found that the RRC3 (primary altimeter for the flight) did not fire either charge. I also noted that upon cycling power, the altimeter no longer detected the charges (no continuity beeps).

A quick e-mail to Missile Works resulted in discovering the problem (Jim explained what likely happened and what to look for). There was a short in the e-match and the current exceeded the RRC3 3 amp rated capability. The battery feed trace on the top side of the PCB that feeds the terminal block for the e-matches blew.
20220718_090822.jpg

It's an easy repair to make, and I'm going to tackle this one but it reminded me that I need to get a current limiting resistor back in-line on this to protect the circuit. When I first built the rocket, I had a large power resistor installed (it was one I had on-hand). It was a 1 ohm large aluminum bodied resistor with mounting lugs rated at 50 watts. I started thinking about this more and wanted to calculate the proper value to use with my setup, so I made a spreadsheet that you can plug numbers into.

Note that there is an alternative wiring scheme documented in the Missile Works user manual where you run a separate pyro battery directly to the e-match and only connect the other lead to the RRC3 output negative terminals. This would be another workaround for the burned trace, but I'm not running that wiring arrangement.

My parameters are:
2-cell Lipo power (max battery V = 8.4V, min 7.5V)
MJG e-match nominal 1 ohm +/- 0.2 ohms
minimum recommended all-fire current for e-match: 1 Amp (technically it is good down to 0.6 Amps, but MJG recommends 1 amp for margin)
Max current capability of RRC3 = 3 amps

So given all this, I calculated that a 3 ohm 25 watt resistor in-line will provide protection in case of a dead short in the e-match circuit (at max battery voltage) and still provide greater than the minimum all-fire current with the lowest possible battery voltage.
These guys: https://www.amazon.com/gp/product/B07H5GBF2D/ref=ppx_yo_dt_b_asin_title_o00_s00?ie=UTF8&psc=1

If anyone else wants to see what I did, or run some numbers for themselves, here is my spreadsheet:
Spreadsheet Link
 
Looks like that will work. However, a standard 1/4W resistor will not handle the 20 watts if there's a shorted ematch. But they're cheap and better than burning the traces of the altimeter board. You could get something like this for a permanent solution...
https://www.amazon.com/NTE-Electronics-25W3D0-Flameproof-Resistance/dp/B007Z7MUZ4/

The best solution is for the altimeter to be designed as a current source and not a voltage source. 2 amps for example.

You can add the circuit external to the board using an LM317 regulator and one resistor.

1658186065814.png
From: https://www.ti.com/lit/ds/symlink/lm317.pdf?ts=1658139049597

R1 = 0.4 ohm for a 3A limit, or 0.6 ohms for a 2A limit. Use a 5 watt resistor for continuous use.
 
You do not need a resistor rated for continuous service at that power level since this is a pulsed application. A 5W wire wound (for its good short term overload rating) should be more than adequate.

Although a good 1W flameproof should do the trick.

Current source outputs are nice but can have compliance voltage issues. The circuit to limit current eats up quite a bit of the available voltage.
 
While one is at redesign, it may be worth considering a high resistance in parallel with the ematch. That acts as a static bleed resistor. Value should be chosen to be sufficiently high that an ematch is not detected when one is not in place.
 
Current source outputs are nice but can have compliance voltage issues. The circuit to limit current eats up quite a bit of the available voltage.
For ematches, the current source will not cause a problem with the voltage drop if the battery is above 4 volts at the current level. This is a standard means of firing squibs and bridge wires for industrial/mil/aero applications.

Always measure the ematches for about 1 ohm.

Ground test any modifications you add to the altimeter setup.
 
This is a standard means of firing squibs and bridge wires for industrial/mil/aero applications.
I know. Mostly because they use devices with well characterized all fire current/duration. Design a circuit to provide that current (plus a little extra) for a little more than the required time.

Makes it a lot easier to answer questions if one doesn't fire. (Happened to me once.)
 
This is great discussion, I appreciate the information. I know that in practice, many components tolerate a short period beyond their rated power handling capacity, but I wasn't sure how to analyze this. For my application, on the RRC3 and RRC2L missile works altimeters, I calculated 3 ohms provides protection to always below the altimeter limit of 3 amps, and maintains the minimum recommended all fire current of 1 amp (considering battery voltage ranges and resistor/e-match resistance variation). In steady state, this dissipates 25 watts. I was able to find a 3 ohm 25 watt power resistor on amazon. I agree it is overkill, but it does have the advantage of being very robust (aluminum body with mounting lugs). Since my application is not aimed at maximum performance, I'm planning on this simple solution just to protect my electronics from any further incidents.

Once installed, I do plan a couple rounds of ground tests just to confirm the operating window (using fully charged and also most depleted batteries).
 
I really like the RRC3 and I flew one regularly for a few years, but this report does seem to highlight a design flaw. e-match shorts for pyros are not that uncommon in this sport. After all, we are purposely blowing up the two wires we don't want to short out. Modern altimeters should have better current limiting capabilities. Either current limiting drivers like the Eggtimer uses or at least current limiting resistors in the circuit. At a bare minimum, I think the pcb traces should allow for the drogue to short and burn out, while not frying the traces for the main.
 
I really like the RRC3 and I flew one regularly for a few years, but this report does seem to highlight a design flaw. e-match shorts for pyros are not that uncommon in this sport. After all, we are purposely blowing up the two wires we don't want to short out. Modern altimeters should have better current limiting capabilities. Either current limiting drivers like the Eggtimer uses or at least current limiting resistors in the circuit. At a bare minimum, I think the pcb traces should allow for the drogue to short and burn out, while not frying the traces for the main.
I thought most had fusible resistors for just this purpose. The resistor blows as a fuse and saves the tracks and components. A resistor is easy to replace.
 
I thought most had fusible resistors for just this purpose. The resistor blows as a fuse and saves the tracks and components. A resistor is easy to replace.
Auto-reset thermal breakers can be sized to avoid burning out traces or components. Fusible links disable the board until they're replaced. A thermal breaker cools and reset in a few seconds, repeating the cycle until the short goes away. I've been using these in all my launch controllers since 1995. And in some of the circuits of designed for other applications.
 
Auto-reset thermal breakers can be sized to avoid burning out traces or components. Fusible links disable the board until they're replaced. A thermal breaker cools and reset in a few seconds, repeating the cycle until the short goes away. I've been using these in all my launch controllers since 1995. And in some of the circuits of designed for other applications.
These things can be tiny (the size of a typical SMD passive - say 1210ish) and relatively cheap. They should be more commonly found in flight electronics - unless I'm missing something?

eg. https://au.mouser.com/datasheet/2/54/cb-2907526.pdf

TP
 
Auto-reset thermal breakers can be sized to avoid burning out traces or components. Fusible links disable the board until they're replaced. A thermal breaker cools and reset in a few seconds, repeating the cycle until the short goes away. I've been using these in all my launch controllers since 1995. And in some of the circuits of designed for other applications.
Like a PTC?
 
You can also look for a lipo that won’t source more than 3A. That’s the technique that Adrian uses for the Ravens with the lipos that he sells.
 
Smart FET's. They shut off if there's an overcurrent/overvoltage/overtemp event, vs. "standard" FET's which typically fuse closed leading to all sorts of bad things happening.
 
TBH the problem most likely comes from the use of oversized LiPos or incorrectly sized ones, the RRC3 is recommended to use a 9v battery not a lipo.
Yep, the manual clearly states the preference for 9V batteries and what will happen with higher amperage batteries, but it also says you can still use lipos as long as you ensure your eMatches will never short out — that isn’t a real world consideration. I think it should just say to stick with 9V batteries (or low amp batteries — but not too low) otherwise the risk of a drogue match short could render the altimeter dead. Reliable deployment is a substantial safety concern and most new fliers won’t understand how to calculate amps and load.

9D30FA38-01A7-4C7B-B4F5-BD7F2D8B4462.jpeg
 
The 2-cell Lipo stays in the required voltage range, but as noted it is capable of providing more than the altimeters rated 3 amps. I don't like the idea of going through so many 9V batteries (hearing how people use a new one for every flight). To me that is too wasteful. So to use a Lipo, all that's needed is a current limiting device, and the simplest in my opinion is a current limiting resistor external to the altimeter in-line with the e-match. I didn't mean to imply that the missile works altimeter was deficient in some way, but rather to just provide an option to protect this or any other altimeter due to a short condition. I was trying to show examples (with the spreadsheet calcs) that it is possible to choose a resistor that will limit the current to less than the maximum allowed by an altimeter and still provide the minimum all-fire current (both at worst case battery voltage and resistance conditions for each situation).
 
You could deliberately use a resistor that is underspecced for the purpose and consider it a disposable (fuse) asset that you wire in series with your igniter under all circumstances?
 
You could deliberately use a resistor that is underspecced for the purpose and consider it a disposable (fuse) asset that you wire in series with your igniter under all circumstances?
Or better yet put another ematch in series and put in the charge well. You get your current cut in half and ematch redundancy for free.
 
I'm seeing all kinds of wonderful solutions. If this is really a problem, why not just use something simple like a properly sized fuse in series?

Though I think I would probably just use a small relay.
 
I'm seeing all kinds of wonderful solutions. If this is really a problem, why not just use something simple like a properly sized fuse in series?

Though I think I would probably just use a small relay.
Yes use a 1/8W 1ohm carbon composition resistor as a fuse and stick in the charge well with the ematch. ;)
 
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