8 replies to this topic

[Jamesgr86]

Hey guys, I've been experimenting with a few things, trying to figure out the best power supply for my rocket launch controller.  Since I've made everything from scratch, and I mean everything, I'm down to trying to find a good power source to ignite my crappy igniters.  A normal 6v isnt working.  Iv'e gone as far as to remove the fly back transformer from one on my many PSU's laying around my house and I have a circuit diagram to use that, but I don't want to, or ONLY as a last result.  Iv'e taught myself everything I know in my life by doing trial and error tests, and I'm only just into learning how circuits work.  I know a capacitor will store the energy I need, and i was thinking a 16v ignition would be enough.  My question is if anyone can help me with a simple circuit that will charge the capacitor(s) with a L.E.D. that once charged lights up, and THEN I can hit the Launch button.  To my knowledge the L.E.D will just discharge the capacitor, so how can i discharge the voltage I need on command?  That's the only thing I really need to know I guess, once I know that I could build the circuit.  Just having a hard time finding the right words to google the issue.  Hope my explanation is enough.  Thanks in advance.

[megabusa]

Hi James, Welcome first of all !

 

What are the igniters you have ?  Commercial or home made ?

 

If home made, can you give details, resistance, ignitor wire, pyrogen etc.

 

With igniters, the key is to get enough energy in there to ignite the pyrogen. This is all to do with current.

The energy comes from the current flowing through it & the time it is there for.

The voltage is what is needed to push the current through the ig, providing the supply can give that current.

 

If you are making your own igniters, it may be you need a fair bit of current to generate that energy.

 

Cheers, Phil.

[JamesB]

So, to begin with, this is going to depend very much on what type of ignitors you use. I'll be talking in terms of professional pyrotechnics ones, just because it's what I have most experience with, but the principles are largely the same whatever you use. The three important parameters are:

No-go current - this is the maximum amount of current that can be flowed through the ignitor to test continuity whilst guaranteeing the ignitor will not fire, this is not so important to your use case.

Firing current - the minimum current flow required to guarantee ignition

Firing current application time - the duration this current must flow for to ensure ignition. 

 

So in simple terms, you need to ensure that the supply you build can supply at least the required firing current for a minimum of the firing current application time.

 

The problem you are experiencing is most likely related to internal resistance, there is no such thing a perfect voltage supply, all have a certain amount of resistance at the output, which reduces the maximum amount of current that can flow. Small batteries can be particularly bad for this, but all sources of power are affected to some degree.

 

Regarding capacitors, they do not increase the voltage output, merely store energy, and in this case, they could enable sufficient energy to be stored, to allow a large amount of current to flow, firing the ignitor. So using a large capacitor would certainly contribute towards solving your problem, the larger the capacitor, the longer it will maintain this current flow for (I'm talking about capacitance in F rather than physical size). It is also perfectly possible to use various other methods to increase the supply voltage (e.g. a boost converter), then accumulate energy at this higher voltage in a capacitor, and use this for firing. 

 

To light an LED when the capacitor has charged, the best method would be to use a simple op amp circuit set up as a comparator, apply desired voltage to the inverting input, capacitor voltage to the non-inverting input, connect a power supply, and once capacitor voltage > desired voltage, the output would go high, which would be used to light an LED. There would need to be a bit of extra work in terms of applying the two input voltages via voltage dividers etc. but it's doable, and provided you use your original power supply, before it's stepped up, there should be minimal drain on the capacitor. 

 

The simple but less accurate way to do it, is to look at the amount of current required to light an LED e.g. I = 20mA, Supply voltage, e.g. Vs = 5V, Volt drop over LED, Vd = 0.7V, so resistance to allow current to flow at desired (supply) voltage = (Vs - Vd)/ I  = 215 ohms. Obviousl the LED will start off dim and brighter when the capacitor charges, so not a binary indication, but it'll work, an assuming the power source supplies more power that the LED draws, the effect on the capacitor will be fairly small.

[megabusa]

Yep, I was thinking about the igniter characteristics ....

 

If home made, it could be they take a few amps to fire, rather than the mA we see in pro igniters.

 

The simplest solution (assuming it is to be portable) could be SLA battery or 2.

[JamesB]

Yeah, very fair point, I definitely remember being very young and using model rockets with my dad, having no luck with the 2*AA battery launch controller and resorting to 12V motorcycle batteries!

[megabusa]

Well a 12 lead acid will fire just about anything !

 

Jamesgr86  -  Another thing to consider is the length & resistance of the wires to the igniter from the launch controller.

 

You can have the best power source in the world but if the cables have high resistance that will limit the current supplied to the igniter.

 

Lets start by looking at the igniter specs ....

 

[whoof]

Capacitive discharge systems have their place but for a one off ignitor an small LA is simplest.

Or even a lipo if you have one.

 

One of these will set off just about anything

 

http://www.maplin.co...ah-1-pack-mm22y

 

Despite the cost it is likely to be cheaper and less hassle than anything else.

 

They can be had for half that price if you look although shipping could be a problem.

Edited by whoof, 15 April 2016 - 09:25 PM.

[cooperman435]

Being a Yorkshireman Id go here :-)

https://www.ebay.co....tm/121556653067

[Arthur Brown]

Currently Tool Station is my supplier for cheap Lead Acid cells.

 

The OP's issue is most likely that the igniters are insensitive and that the stand off wire is thin and resistive -both limiting the current available.