Launch controller circuit animation - Eyes-on and opinions welcomed

[Curtis Enlow]

Hey all,

Getting close to pulling the trigger on building a new pad & controller for LPR/MPR operations and would appreciate opinions on the following circuit animation.

This is from EveryCircuit, so it's somewhat rudimentary, but good for proving basic concepts.

Left to right the first 'switch' (red LED) will be a keyed safety switch, next switch (yellow LED) is the arming switch which will also indicate continuity (green LED).

The power source will be a 12v 5aH sealed lead-acid battery and there will be a 10 amp circuit breaker in series between the battery and the keyed safety switch, and a warning buzzer in parallel with the yellow armed LED, as well as the launch button parallel with the green continuity LED.

Thoughts? Suggestions? Warnings? Thanks in advance


View attachment EveryCircuit Launch Controller v.1.mp4

 

[sghioto]

Just make sure all the switches are rated higher than the total current required.

 

[Voyager1]

Hey all,

Getting close to pulling the trigger on building a new pad & controller for LPR/MPR operations and would appreciate opinions on the following circuit animation.

This is from EveryCircuit, so it's somewhat rudimentary, but good for proving basic concepts.

Left to right the first 'switch' (red LED) will be a keyed safety switch, next switch (yellow LED) is the arming switch which will also indicate continuity (green LED).

The power source will be a 12v 5aH sealed lead-acid battery and there will be a 10 amp circuit breaker in series between the battery and the keyed safety switch, and a warning buzzer in parallel with the yellow armed LED, as well as the launch button parallel with the green continuity LED.

Thoughts? Suggestions? Warnings? Thanks in advance


View attachment 576853

Probably best to simulate this circuit with a 1-2 Ohm resistor in place of the LED you have where the igniter would be. LEDs have a dynamic resistance of about 20 Ohms when conducting, so it's not truly representative of the real circuit conditions. I realise that this is just a conceptual demo. Nice demo, though.

 

[Curtis Enlow]

Probably best to simulate this circuit with a 1-2 Ohm resistor in place of the LED you have where the igniter would be. LEDs have a dynamic resistance of about 20 Ohms when conducting, so it's not truly representative of the real circuit conditions. I realise that this is just a conceptual demo. Nice demo, though.

1-2 Ohms don't overheat in the sim, but then I can't indicate the wattage of the resister, either. I also cannot indicate the amperage capacity of the power supply.
Another question I have is do I need to accommodate the difference between, say, an Estes-type ignitor for LPR vs. a 12v MPR ignitor for composite motors, or will an Estes igniter reliably ignite with a 12v system?

 

[sghioto]

or will an Estes igniter reliably ignite with a 12v system?

Yes they will.

 

[Curtis Enlow]

Just make sure all the switches are rated higher than the total current required.

If I understand Ohm's Law correctly, with a 12v system and a 5aH SLA battery that can, theoretically, deliver as much as 25 amps, peak, going into a single 2-Ohm ignitor I should expect a peak amperage in the system of about 6 amps. Does this sound correct?

All of the switches are at least 10 Amps, so i should be okay with those and a 10 Amp breaker?

 

[OverTheTop]

Curtis,
The thing to be aware of is that the igniters can go short-circuit when the fire. That can stress launch systems. Your calculations above for the igniter current of 6A are good. Regarding the 5Ah battery, I would expect it to deliver more than 25A typically in a short-circuit load, so make sure your wires are big enough for that.

As to the 12V 5Ah battery, the Ah rating is how much current it can provide over how long. It can provide 12V at 1A for 5 hours, or 1/2A for 10 hours, or 1/10A for 50 hours, on a full charge. current x hours = Ah. That is what the Ah (ampere.hour) rating represents. There is another effect which kicks in which typically sees higher capacity (Ah) batteries able to drive higher currents (more plate area typically). The actual current capability depends mostly on the battery chemistry and construction and can be obtained typically in the data sheets for the battery if you really need to know.

 

[OzHybrid]

Probably best to simulate this circuit with a 1-2 Ohm resistor in place of the LED you have where the igniter would be. LEDs have a dynamic resistance of about 20 Ohms when conducting, so it's not truly representative of the real circuit conditions. I realise that this is just a conceptual demo. Nice demo, though.

AND add in a resistance for the cable to the e-match.




Taking into account the safe distances and let's say you are going to just have a single lot of cable rather than a separate cable for each pad distance and say you chose to be able to launch up to a J and say you used 26G cable and remembering that the cable has an outward and return path so the resistance calculate is doubled..... Then.... we get the resistance of your cable to be......
100/1000 x 40.8 x 2 = 8.16 ohms plus a little bit for non perfect contact with dodgy clips say 0.5 ohms giving a cable value of 8.56. Round that to 9 ohms.
Your simulation is based on 12 volts.......

Now, and DO NOT FORGET THIS..... A 12 volt battery is NEVER 12 volts........ except at one point during it's discharge..... but not normally.
If it's just off charge and is lead acid gel cell it will be 13.8 and maybe up to 15v ( more likely 14.2)

Looking at a spec sheet for a match.


The Maximum Test Current is 40mA
Your test current simulation is wrong. it's using a calculation of 12v-1.8v for test LED - another 1.8 for the LED you have put in as a substitute e-match/resistance.

(12-1.8-1.8)/390=21.5mA
calculation should be for worst case.....
(15-1.8)/(390+9+1).... resistor +cable+e-match
gives 33mA. This is getting too close to the max test current for the J-Tek e-match but is technically within limits. You would also need to look at what the max current through your LED was. If it was say 30mA you would be over.
I'd use a 560 or 680 ohm resistor in the test circuit.

Estes igniters are different again. However.... it's unlikely you'll just use a single type of igniter.
Some info here.
https://estesrockets.com/wp-content/uploads/Educator/2811_Estes_Model_Rocket_Launch_Systems.pdf
The short version is..... A 12V nominal voltage Lead Acid battery-based launch controller will fire anything connected to it usually.

 

[Handeman]

Your circuit should work just fine.

I used 1/4 watt 470 Ohm resisters for the LEDs because the 470 is a very common and easy to get resister used with LEDs in 12V circuits. Don't worry too much about current levels, etc. The components and wire will survive short bursts of large current. Most igniters are about 1 Ohm so 12 Amps would be the most you would usually get through your circuit. Just don't hold the launch button down after the igniter burns and it will last a long time.

Your circuit is very similar to what I built for LPR/MPR. I used a separate momentary switch and LED for continuity and a momentary toggle and push button for the launch circuit so it was a two finger operation to actually launch the rocket. I don't remember the ratings on the switches, probably 5A AC, but they have worked for almost 20 years now. It's powered by a 250 Amp Car Starter. I'm using 35 ft. of 16 Ga. lamp cord between the controller and the clips and about 5 ft of 14 Ga lamp cord to the car starter. It's lit Estes clusters up to 4 motors without issues.

 

[Curtis Enlow]

Thanks, everyone, for taking the time to respond. There's a lot of great information and things to consider.

This system will only be used for Class 1 LPR/MPR launches. I plan on using nothing less than 16 AWG wire throughout, including the run to the pad. For the pad lead I will likely use 16 AWG two-conductor extension cable - that seems to work fine for folks; the cable is easily managed and can handle higher amps with less resistance than zip cable.

The LED indicators are 1/2" panel-mount indicator LEDs 12-24v rated with built-in limiting resistors. Tech data is limited, but they should never experience more amperage than the mA range, but I will test them out when I get them, though they seem to work flawlessly in other controller applications.

I'm having a difficult time finding mushroom-head (best for child hands) momentary contact push button switches with higher than 10A ratings for the 'fire' function. Even industrial application switches seem to be designed mostly for solenoid or relay circuit applications. I've read, literally, dozens of controller build threads here (in addition to external sources) and I've yet to read a single incident of failure, so - while certainly possible - I've yet to encounter a story of a controller being damaged by an extended direct short at the pad. The key safety and arming switches are rated to 25A and I will have a 10A circuit breaker at the power supply, so I feel my precautions are reasonable.

Thanks, again! Any further input or thoughts are welcomed

 

[OzHybrid]

Make sure your circuit breaker will survive a short circuit. Specification parameter is max inrush current. It should be a big number. .

 

[OzHybrid]

Thanks, everyone, for taking the time to respond. There's a lot of great information and things to consider.

This system will only be used for Class 1 LPR/MPR launches. I plan on using nothing less than 16 AWG wire throughout, including the run to the pad. For the pad lead I will likely use 16 AWG two-conductor extension cable - that seems to work fine for folks; the cable is easily managed and can handle higher amps with less resistance than zip cable.

The LED indicators are 1/2" panel-mount indicator LEDs 12-24v rated with built-in limiting resistors. Tech data is limited, but they should never experience more amperage than the mA range, but I will test them out when I get them, though they seem to work flawlessly in other controller applications.

I'm having a difficult time finding mushroom-head (best for child hands) momentary contact push button switches with higher than 10A ratings for the 'fire' function. Even industrial application switches seem to be designed mostly for solenoid or relay circuit applications. I've read, literally, dozens of controller build threads here (in addition to external sources) and I've yet to read a single incident of failure, so - while certainly possible - I've yet to encounter a story of a controller being damaged by an extended direct short at the pad. The key safety and arming switches are rated to 25A and I will have a 10A circuit breaker at the power supply, so I feel my precautions are reasonable.

Thanks, again! Any further input or thoughts are welcomed

Just remember that electronics should be treated with max values as that. Max and a tiny bit and POOF, out comes the magic smoke.

 

[sghioto]

I'm having a difficult time finding mushroom-head (best for child hands) momentary contact push button switches with higher than 10A ratings

You can use the lower amp switch to control a 12 volt relay.

 

[OzHybrid]

You do not need a switch rated higher than 10 amps. There is no MPR match that would ever need that and 10 amps is sufficient to fire 10 matches in parallel. That's a big cluster.......
What you need is a method to current limit to 10 Amps. So a resistance of 1.2 Ohms. 12/1.2=10 ( I'm breaking my own rules here and using the voltage of a nominally12V battery as 12.....) The easiest way to do that is to choose a firing cable that has that resistance. cable resistance is usually quoted in tables as resistance per 1000 ft so bearing in mind the resistance will be doubled by outward and inward you want a value of half 0.6 ohms/50ft (as you've said this launcher is MPR only) so the cable you want is (1000 x 0.6)/50=12 ohms per 1000 ft
From the table below.


20G has a resistance of 10.1/1000ft and 22G has 16.5. Although the 22G will allow slightly too much current in the event of a short circuit, it's not an electronic component and it's unlikely that it will in reality exceed the current limit of the switch due to other inherent resistances in your circuit.
In this way the firing cable length becomes a functional current limiting resistance. Designed. Not just an arbitrary choice.

If you decide to use the 16G cable because it's less liable to be damaged, you can use a limiting resistor in series with it. A 50 ft 16G cable has a full return path resistance of 0.818, you need 1.2 so 1.2-0.818=0.382 nearest standard value would be 0.39 the wattage rating would be Amps squared x resistance.10x10x0.39=39 watts 50W would be your nearest wattage rating. This will get hot quickly if you have a short for too long.
Also note that during a short, your cable will be dissipating 82 Watts or 1.64W per foot which is safe for the cable.

With the 20G cable. The cable will dissipate 10x10x1.2=120 Watts or 2.4 Watts per foot. This should also be fine for the cable.

You can also take the route that some use and throw a bunch of switches and a battery in a box and wire it up and never have a problem. But that's not designed. That's luck.

Now you're thinkin, did he fire 5 or 6.
Do ya feel lucky???

 

[Curtis Enlow]

Took a long time to find this part of the puzzle. Leaving it here for future seekers:

 

[Curtis Enlow]

You do not need a switch rated higher than 10 amps. There is no MPR match that would ever need that and 10 amps is sufficient to fire 10 matches in parallel. That's a big cluster.......
What you need is a method to current limit to 10 Amps. So a resistance of 1.2 Ohms. 12/1.2=10 ( I'm breaking my own rules here and using the voltage of a nominally12V battery as 12.....) The easiest way to do that is to choose a firing cable that has that resistance. cable resistance is usually quoted in tables as resistance per 1000 ft so bearing in mind the resistance will be doubled by outward and inward you want a value of half 0.6 ohms/50ft (as you've said this launcher is MPR only) so the cable you want is (1000 x 0.6)/50=12 ohms per 1000 ft
From the table below.
View attachment 577014

20G has a resistance of 10.1/1000ft and 22G has 16.5. Although the 22G will allow slightly too much current in the event of a short circuit, it's not an electronic component and it's unlikely that it will in reality exceed the current limit of the switch due to other inherent resistances in your circuit.
In this way the firing cable length becomes a functional current limiting resistance. Designed. Not just an arbitrary choice.

If you decide to use the 16G cable because it's less liable to be damaged, you can use a limiting resistor in series with it. A 50 ft 16G cable has a full return path resistance of 0.818, you need 1.2 so 1.2-0.818=0.382 nearest standard value would be 0.39 the wattage rating would be Amps squared x resistance.10x10x0.39=39 watts 50W would be your nearest wattage rating. This will get hot quickly if you have a short for too long.
Also note that during a short, your cable will be dissipating 82 Watts or 1.64W per foot which is safe for the cable.

With the 20G cable. The cable will dissipate 10x10x1.2=120 Watts or 2.4 Watts per foot. This should also be fine for the cable.

You can also take the route that some use and throw a bunch of switches and a battery in a box and wire it up and never have a problem. But that's not designed. That's luck.

Now you're thinkin, did he fire 5 or 6.
Do ya feel lucky???

Norman, I do really appreciate not only your engineering approach, but the time and effort you have taken in your responses. I do take it to heart, and I have spent a LOT of time actively researching this little project, secretly wishing I could throw a bunch of switches and a battery in a box and have a lighthearted go of it, and even if it did all melt down into a sparky, gooey mess, just have a good laugh, get out the Amazon card and hit the drawing board again. But I can't. I have to let it all stew in its own juices in the back burner in my head for three or four weeks while I fall asleep considering the options and possibilities. Trust me, it's a very good thing I don't know Algebra, Trig or Calculus or I'd never fly a rocket again in mortal life.

To that end I am now committed to 14 AWG silicone cable for all the hook ups in the box so that is all good up to 20A. I understand your concern about a spike at the pad, but wouldn't a breaker between the battery and the safety switch take care of that? I have considered an inline fast-blow fuse at the business end, just in case the breaker didn't react quick enough on the off-chance that an igniter should actually decide to weld its leads together in a dead short on the pad and some little person froze holding down the 'Fire' button in the excitement and heat of the moment. I'm just trying to sense the transition of reasonably prudent to needlessly cautious; I do love Ed Heinemann's famous quote, "Simplicate and add lightness."

I actually thought about a fully automated relay system with not much more than a Proceed key and big red 'ABORT' button with a programmable Arduino launch control and an 'Arm' relay that would unlatch just so many milliseconds after the 'Fire' command, or after it senses a loss of continuity or a spike in amps, but....yikes, I do want to fly this Spring/Summer some time! Maybe later when I have a working alternative and time to devote to it - I think that could be a fun project.

 

[OzHybrid]

Norman, I do really appreciate not only your engineering approach, but the time and effort you have taken in your responses. I do take it to heart, and I have spent a LOT of time actively researching this little project, secretly wishing I could throw a bunch of switches and a battery in a box and have a lighthearted go of it, and even if it did all melt down into a sparky, gooey mess, just have a good laugh, get out the Amazon card and hit the drawing board again. But I can't. I have to let it all stew in its own juices in the back burner in my head for three or four weeks while I fall asleep considering the options and possibilities. Trust me, it's a very good thing I don't know Algebra, Trig or Calculus or I'd never fly a rocket again in mortal life.

To that end I am now committed to 14 AWG silicone cable for all the hook ups in the box so that is all good up to 20A. I understand your concern about a spike at the pad, but wouldn't a breaker between the battery and the safety switch take care of that? I have considered an inline fast-blow fuse at the business end, just in case the breaker didn't react quick enough on the off-chance that an igniter should actually decide to weld its leads together in a dead short on the pad and some little person froze holding down the 'Fire' button in the excitement and heat of the moment. I'm just trying to sense the transition of reasonably prudent to needlessly cautious; I do love Ed Heinemann's famous quote, "Simplicate and add lightness."

I actually thought about a fully automated relay system with not much more than a Proceed key and big red 'ABORT' button with a programmable Arduino launch control and an 'Arm' relay that would unlatch just so many milliseconds after the 'Fire' command, or after it senses a loss of continuity or a spike in amps, but....yikes, I do want to fly this Spring/Summer some time! Maybe later when I have a working alternative and time to devote to it - I think that could be a fun project.

A good quality circuit breaker will trip out. True. And it's a good idea to have one. But by current limiting you allow yourself to hold that button down for a bit longer. If you were using nichrome wire to get something that's being difficult to light (and I've done a coil of nichrome up a rocket slot), it would give you the time to get it lit. In normal situations, it's not important. But in difficult situations it's useful.
There are electronic ways to current limit. It's not technically difficult. But that's another story.
I've given you the calculations for a system that will work. They are the same ones I would use. Do with them what you want. I used to work for the BBC Special Effects department designing and building firing systems for the SFX designers there. Happy to discuss any calculation anyone wants to put down on paper if they have any issues with what I've written.

Silicone-covered wire internally is a good idea. But do not use it externally as it has poor abrasion resistance and usually has many more strands which could result in a copper hair in an unfortunate spot. Watch out for that.. PVC-covered cable has goodish abrasion resistance. And is relatively cheap.

20 Amps @12V nominal is enough energy to get a suntan from. I've TIG welded stainless steel with less energy input. To be honest, just going for more and more available current demonstrates a lack of understanding of the process of firing an igniter. This is a LPR/MPR firing system. In normal operation, you just need to get 1V and 1A =1Watt of energy to any commercially made igniter to get it lit. (Okay some will be 3v and 0.33 Amps)

Norm

 

[kc9qzf]

Been doodling around with launch systems for a few years (25+) and over that time I have developed a few rules I like to follow when I am doing one:
1. Battery, remember the battery voltage (SLA type) will vary from 13+V when fully charged and will drain down to 12V when fully discharged.
2. Durring very high current drain, the series resistance of the battery will become very evident and the voltage you will see during these events will often suprise you, I have seen 10.5V during a heavy load. The battery will recover very quickly so just because it says 12V doesn't mean it will always stay that way.
3. Assume you can drive a 1.0 Ohm load. If you design for this then you will have a launch system that can stand the vagarities of the launch event. I have seen some very strange things happen, including a full short of the leads at the clips which can cause a weaker system do die at a time when you really dont want to be spending hours debugging the launch equipment.
4. Limit your continuity current as much as possible. We used to say "flashbulb" safe, but running currents as high as the lowest no-fire current is just too uncertain, especially when you might be standing right next to the motor. I limit the continutiy current on all of our systems to around 1.0mA, yes, this means you wont be driving the extra bright led directly but a few cents spent making a safe continuity circuit will pay off in many great launches.
5. Never assume less is more, be able to provide both voltage and current to the ignition point, let the motor starter sort out the specifics. I normally want to be able to provide at least 10A continuous to the clips. This means minimizing resistance by using higher rated devices everywhere. 20A relays etc.
6. Larger wire might be a bit heavier, but use wire that is capable not only of the voltage and current you need, but is capable of being stepped on, tripped over, exposed to UV without issues.
7. Always provide a removable "key" so while you are out putting your rocket on the pad, someone (small helper, to a less than safe thinking friend) cant arm the system.
8. Provide an AUDIBLE warning, either at the pad or on the controller. LEDs are ok, but a buzzer is better.

Dan Fox
KC9QZF AE
Wilson F/X (i'm the F/X)

 

[Curtis Enlow]

A good quality circuit breaker will trip out. True. And it's a good idea to have one. But by current limiting you allow yourself to hold that button down for a bit longer.

That's a great point. I will look at current limiting at the business end. I need a pigtail at the pad end of the extension cable so an appropriate current-limiting resistor would be easy to rig.

I am a little confused about something - I would need a 50 watt resister, but what Ohms value? All the 50 watt resisters I can find are 1Ω or greater, quite a bit more than the 0.39Ω I need...? Nevermind, found a 0.4 Ohm 50-watt. Is this what I need in series for the limiting resister?

I've given you the calculations for a system that will work. They are the same ones I would use. Do with them what you want. I used to work for the BBC Special Effects department designing and building firing systems for the SFX designers there.

Gotta love BBC! TV in the States is @#%! Picked up BritBox for a re-run of 'Life on Mars' but was disappointed not to find 'Danger UXB' listed I'm a location sound guy in the video industry, myself.

Silicone-covered wire internally is a good idea. But do not use it externally

Agreed. Love the ease of routing but I have found it's a pain to solder. I'll probably go with mechanical crimping and heat shrink on the terminals.

I have read elsewhere on this forum that Aerotech recommended 12v @ 3 amps for their First Fire igniters, so that is what I went with.

Thanks for your advice, Norm!

 

[OzHybrid]

Been doodling around with launch systems for a few years (25+) and over that time I have developed a few rules I like to follow when I am doing one:
1. Battery, remember the battery voltage (SLA type) will vary from 13+V when fully charged and will drain down to 12V when fully discharged.
2. Durring very high current drain, the series resistance of the battery will become very evident and the voltage you will see during these events will often suprise you, I have seen 10.5V during a heavy load. The battery will recover very quickly so just because it says 12V doesn't mean it will always stay that way.
3. Assume you can drive a 1.0 Ohm load. If you design for this then you will have a launch system that can stand the vagarities of the launch event. I have seen some very strange things happen, including a full short of the leads at the clips which can cause a weaker system do die at a time when you really dont want to be spending hours debugging the launch equipment.
4. Limit your continuity current as much as possible. We used to say "flashbulb" safe, but running currents as high as the lowest no-fire current is just too uncertain, especially when you might be standing right next to the motor. I limit the continutiy current on all of our systems to around 1.0mA, yes, this means you wont be driving the extra bright led directly but a few cents spent making a safe continuity circuit will pay off in many great launches.
5. Never assume less is more, be able to provide both voltage and current to the ignition point, let the motor starter sort out the specifics. I normally want to be able to provide at least 10A continuous to the clips. This means minimizing resistance by using higher rated devices everywhere. 20A relays etc.
6. Larger wire might be a bit heavier, but use wire that is capable not only of the voltage and current you need, but is capable of being stepped on, tripped over, exposed to UV without issues.
7. Always provide a removable "key" so while you are out putting your rocket on the pad, someone (small helper, to a less than safe thinking friend) cant arm the system.
8. Provide an AUDIBLE warning, either at the pad or on the controller. LEDs are ok, but a buzzer is better.

Dan Fox
KC9QZF AE
Wilson F/X (i'm the F/X)

A fully discharged 12 SLA is 10.5 volts. But 12 volts is a good cutoff point if you want it to survive. They do not like being fully discharged. Still quite useable at 12V. it still has 50% capacity....... Table below for a fully charged battery that has been taken off charge and allowed to settle its voltage.
This brings up the other PITA thing about SLA technology. You really only get 50% of the capacity and out in the real world probably less than you think as the capacity decreases with increased current drain. All SLA and wet lead acid capacities are quoted at a discharge of 1/10 of their rating. So a 5Ah battery will only give you that at a discharge of 0.5A for 10 hours. ( if you are lucky....I've seen some of the generic chinese SLA completely fail to even meet the specification)

I agree that you need to look at the internal resistance of the SLA. But any quality SLA will only be a couple of ohms worst case. The 12v 7Ah is the most common as they are used for nearly everything in the small backup battery area. Panasonic still make the best ones, which is why you'll pay a premium for them.

This is a design for a single launch controller for LPR only. Your design specification is overkill if it's just for that, but is what I would use for a multi-fire launch controller. Or a single HPR launch controller.

And if I were building those, I'd be tempted to use the same parts for a single LPR launch controller just because they were in stock. Not because it was required for this specific purpose.


Norm

 

[kc9qzf]

A fully discharged 12 SLA is 10.5 volts. But 12 volts is a good cutoff point if you want it to survive. They do not like being fully discharged. Still quite useable at 12V. it still has 50% capacity....... Table below for a fully charged battery that has been taken off charge and allowed to settle its voltage.
This brings up the other PITA thing about SLA technology. You really only get 50% of the capacity and out in the real world probably less than you think as the capacity decreases with increased current drain. All SLA and wet lead acid capacities are quoted at a discharge of 1/10 of their rating. So a 5Ah battery will only give you that at a discharge of 0.5A for 10 hours. ( if you are lucky....I've seen some of the generic chinese SLA completely fail to even meet the specification)
View attachment 577172
I agree that you need to look at the internal resistance of the SLA. But any quality SLA will only be a couple of ohms worst case. The 12v 7Ah is the most common as they are used for nearly everything in the small backup battery area. Panasonic still make the best ones, which is why you'll pay a premium for them.

This is a design for a single launch controller for LPR only. Your design specification is overkill if it's just for that, but is what I would use for a multi-fire launch controller. Or a single HPR launch controller.

And if I were building those, I'd be tempted to use the same parts for a single LPR launch controller just because they were in stock. Not because it was required for this specific purpose.


Norm

Norm,
I agree, these were just some rules I use, not requirements.
Dan

 

[Grog6]

You can actually have it be too low of an impedance; if you make the igniter explode in a black powder motor, it can make the motor explode. A car battery and one launcher I made needed 50 feet of lamp cord to not blow up motors. It worked fine with the long cable.

 

[OzHybrid]

There has been a trend toward using FET devices as the power controller. Probably because of the incredibly low ON resistance. FET devices have a couple of characteristics that make them difficult to use for firing circuits. The main disadvantage is that they are a voltage controlled device. They don't need any REAL current to fire them. If the gate controlling it is allowed to float it will pick up enough voltage from any radio signal to fire it....
It's also more difficult to current limit them than a transistor.
As GROG6 has pointed out, too much current can cause an igniter to act more like an explosive than an igniter. This is because enough energy is available at the fusible link to instantaneously vaporise the link. I've actually seen a hole get blown through the e match where you go one step further and the fusible link does not transfer sufficient energy to the igniter compound because of the speed the link vaporises.
A current limit of around 4 amps set in the firing circuit would avoid that. Easily achieved by a suitable transistor and a feedback resistor.
Norm

 

[rcktnut]

I guess nothing wrong with putting a bunch of bells and whistles into a launch controller. For a single pad I kept mine simple. I had it for 20+ years still works fine. No lights just a piezo buzzer to check continuity, based basically like an Estes or Aerotech controller. I use a 7Ah SLA battery. Good for LPR to HPR. No circuit breakers, resistors, etc. needed. Never had a single problem with it.

 

[asm109]

And the day you want to launch a hydraVII with 7 igniters wired in parallel? Resistance is much reduced and therefore the current flow increased.

 

[Grog6]

Mosfets are NOT a good idea for the switch in a launch set. I'll tell you why. You need a mosfet, a voltmeter, and a dry table. the number on the side of the mosfet is it's type; you can search google, and see which pins are which. They are source, gate and drain. An N-channel mosfet needs the drain + , the source -, and the voltage on the gate turns on the channel between them. do, the Voltmeter on ohms, clip the black meter lead to the source terminal, and the red read to the drain. It should be open circuit.
But here's why it's a dangerous thing:Tale and touch the gate lead with a finger, it will usually turn on.touch the black lead with your other hand, then touch the gate, it will turn off, touching red will turn it on, thru the resistance of your dry body. The circuitry needs to be carefully designed; I've seen an open switch contact make a mosfet a random voltage detector, where it would switch when i reached for it, from the static on my hand.
something else on switch ratings. If you see 10a125v .1a dc or somesuch, it means the traveling contact is vulnerable to wear, and it won't hit it's million operation limit in the datasheet. That switch can handle 2x current for 20 secnds or so, look for overload ratings. DC is hard on switches, BUT not dc below 28 Volts. Only gold contacts wear at 12v. Coin silver contacts will last forever at the overload rating. But only below 28v.

 

[kc9qzf]

A properly designed circuit can safely use solid state switching.

 

[OzHybrid]

A properly designed circuit can safely use solid state switching.

Which circuit shown in this thread would you call properly designed? And if there isn't one and people search for a circuit, they're going to get a badly designed one.
If you have a properly designed circuit suitable for a firing system, please publish it on this thread.
One of the problems you run into is the "but what about when I want to connect a cluster of 10 igniters in parallel for my mega project?" unrealistic expectations of what can be done. It can be done of course, but would require additional hardware to guarantee ignition as opposed to hoping for ignition when you pressed the button.

 

[kc9qzf]

When I get back to civilization, I will.

 

[sghioto]

Which circuit shown in this thread would you call properly designed

Wasn't about the circuits shown in the thread. It was about your comment concerning a trend in using FET devices which Grog6 followed up by saying it isn't a good idea. The reply from kc9qzf and I agree with is a launch controller using FETs that is properly designed for using FETs is practical.